Host-Regulating Factors Associated with Parasitic Hymenoptera - ACS

Chapter 4

Host-Regulating Factors Associated with Parasitic Hymenoptera Thomas A. Coudron

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Agricultural Research Service, U.S. Department of Agriculture, P.O. Box 7629, Research Park, Columbia, MO 65205-5001

Parasitic Hymenoptera produce natural regulatory factors that influence the physiology, biochemistry, and behavior of the host. Several paralyzing venoms have been characterized. Recently, interest has been expressed in factors that produce nonparalytic effects. These arise from several sites in the reproductive tract of the adult female parasitoid. Endoparasitoids are known to produce teratocytes, viruses, and secretions from glands and other specialized tissues which have effects on the development and behavior of the host. Nonparalytic venoms of endoparasitoids cause delayed development, precocious molt, supernumerary molt, and arrest of embryonic development. Ectoparasitoids rely on venoms to regulate host development. The most studied nonparalyzing venom is produced by the ectoparasitoid Euplectrus sp. Its venom has a unique effect on the host and is responsible for arresting larval-larval ecdysis.

There are estimated to be over 100,000 species of p a r a s i t i c Hymenoptera worldwide (1_). Though only a small percentage has been described (2), i t i s evident that these species have a variety of methods of coping with the physiological ecology of the host. Some parasitoids conform t h e i r development with the growth of the host, causing only moderate changes i n the host. Lawrence (J3) f i r s t used the term host "conformers" when r e f e r r i n g to parasitoids whose eggs, deposited i n the host, hatch but the f i r s t instars remained inactive while allowing the host to develop. Later certain host conditions triggered the f i n a l development of the parasitoid. Other parasitoids regulate host growth to benefit their development, causing major changes i n the host (j4). "Conformers" and "regulators" may represent two points of a continuum of interactions between parasitoids and their hosts (_5, 6). Some parasitoids are regulators i n certain environments and conformers i n other environments. Microctonus aethiopoids, the This chapter not subject to U.S. copyright Published 1991 American Chemical Society Hedin; Naturally Occurring Pest Bioregulators ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


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NATURALLY OCCURRING PEST BIOREGULATORS

adult parasitoid of the a l f a l f a weevil, Hypera postica, i s a good example. M. aethiopoids i s an endoparasitoid that rapidly completes i t s development early i n the season i n i t s non-diapausing adult host. Later i n the season M. aethiopoids enter diapause with the adult j i . postica. In the spring, when the diapause of the host comes to an end, the parasitoid resumes development and eventually k i l l s the host (Puttier B., University of Missouri, personal communication, 1990.). Biosteres longicaudatus i s a larval-pupal parasitoid, of the Caribbean f r u i t f l y Anastrepha suspensa, that allows the host to develop to the pupal stage before actively developing i t s e l f (3). The influence of the host hormones upon parasitoid development may be the primary mechanism of parasitoid-host synchrony. An interesting delayed e f f e c t i s also seen with several encyrtids belonging to the genus Copidosoma. These encyrtids allow their hosts to grow to accommodate their polyembryonic development (7-9). Although some species do act as conformers during part of the interaction with their host, ultimately they spend part of that time as regulators and eventually cause incomplete development and premature death of the host. Regulation does not imply t o t a l domination of the host biology by the parasitoid. Certain host resources and physiological processes place constraints on the parasitoids. Parasitoids that paralyze their hosts, and egg and pupal parasitoids experience f i n i t e resources. These parasitoids tend to conform to the l i m i t a t i o n s of the host, often adjusting their size, number or sex depending on the extent of the n u t r i t i o n a l resource (6). Ultimately, the success of a parasitoid-host interaction depends on the a b i l i t y of the parasitoid to a f f e c t certain biochemical processes within the constraints imposed by the host. Many factors contribute to the interactions of parasitoids and t h e i r host insects (10). Table I summarizes data of p a r a s i t i c Hymenoptera that regulate the biochemistry, development, and behavior of t h e i r host. This i s an attempt to l i s t those species that have been subjected to thorough studies and i s not intended to be a complete l i s t . The presence of the immature stage of an endoparasitoid can a f f e c t the host i n several ways. Besides competing for host nutrients, they e l i c i t responses i n the host due both to their presence and due to substances they secrete. The adult female parasitoid may transfer one or more type(s) of regulatory factor(s) to the host during parasitism. Some genera of braconid and s c e l i o n i d families produce teratocytes. These c e l l s originate from an embryonic membrane i n the parasitoid egg and are released into the host hemocoel. Certain parasitoid females of the families Ichneumonidae and Braconidae contain symbiotic viruses that are injected into the host with the egg. Diverse functions have been attributed to the teratocytes and the symbiotic viruses. Both play a major role i n the interaction of the parasitoid with t h e i r respective hosts. The ovipositor of most p a r a s i t i c hymenopteran i s also used as a ductus venatus for the i n j e c t i o n of material from within glandular structures and specialized epithelium c e l l s attached to the common oviduct (11). Two common glandular structures are the Dufour's gland (= alkaline gland) and the venom gland (= poison gland, =

Hedin; Naturally Occurring Pest Bioregulators ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


4. COUDRON

Host-Regulating Factors and Parasitic Hymenoptera

43

a c i d i c gland). O r i g i n a l l y , the Dufour's gland was postulated to be part of the venom system. That idea i s now thought to be inaccurate (12) . The o i l y secretions produced by the Dufour's gland have been shown to function as host marking compounds and sex pheromones (13) . The venom gland i s usually paired and i n some species i t i s extensively branched. An unpaired segment of the gland, common to both pairs of the gland, can be partly swollen and serves as a reservoir for the venom produced by the gland (11). The contents of the venom gland, reservoir, and the material secreted by specialized epithelium c e l l s affect the physiology, development, and immune systems of the host. In some instances the venomous material interacts with other injected factors that regulate the host biology (10, 14-16). A survey of Table I allows for several general comparisons. Many p a r a s i t i c Hymenoptera have adapted to developing i n s p e c i f i c stages of the host. The development of that host stage i s c r i t i c a l i n determining host s u i t a b i l i t y . Egg, egg-larval, l a r v a l , and larval-pupal parasitoids have d i f f e r e n t mechanisms of host regulation. Endoparasitoids seldom cause permanent paralysis of t h e i r hosts and many allow some development of the host to continue. Larval endoparasitoids must contend with the competent immune system, hormones and metabolic changes of the host. In order to develop i n such an environment, often highly specialized t a c t i c s are used and a variety of host pathologies r e s u l t . There are several reviews on the physiological interactions of endoparasitoids with their hosts (3^ 17-20). There are two possible ways f o r endoparasitoids to contend with their hosts' immune system: (a) the egg and larva stages of some parasitoids appear to have surface properties that r e s i s t encapsulation or avoid s e n s i t i z i n g the host (21, 22); (b) the adult female parasitoid transfers regulatory factors to the host that control the host response and development. The r o l e of regulatory factors has been established f o r several endoparasitoids and frequently a combination of factors i s used. Ectoparasitism i s an e f f e c t i v e strategy for circumventing some problems encountered by the endoparasitoids. Ectoparasitoids appear to r e l y on venoms as the sole source of host regulatory factors. In these instances the venoms arrest development by causing paralysis or by arresting the molting process. Regardless of whether a parasitoid i s an endo- or ectoparasitoid, or s o l i t a r y or gregarious, most species use regulatory factors to adapt the host environment to their needs. The following i s a review of what we presently know of these factors. Factors Associated with the Developing Parasitoid E f f e c t on the Composition of the Host. The primary purpose f o r the association of the parasitoid and the host i s for the parasitoid to use the host as a source of nutrients (for reviews of parasitoid n u t r i t i o n see 23, 24). However, the e f f e c t of parasitism extends beyond the depletion of host nutrients by the feeding parasitoid. Clearly the interaction between the parasitoid and the host i s b i d i r e c t i o n a l , with the parasitoid responding to host substances and the host responding to the parasitoid.


44


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H v p o s o t e r exiguae Hvposoter f u g i t i v u s V e n t u r i a canescens Scelionidae Telenomus h e l i o t h i d i s Trichogrammatidae Trichogramma p r e t i o s u m

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Relatively l i t t l e i s known of the physiology and metabolism of insect parasitoids. Though i t i s generally assumed that the maturing p a r a s i t i c hymenopteran eggs take up host hemolymph components and ions through i t s t h i n wall (17, 25) i t has been shown that some parasitoids rely on t h e i r protein synthetic machinery f o r growth (25, 26). Much of t h i s synthesis occurs a f t e r the parasitoid i s associated with the host (27). Parasitism has resulted i n changes i n the s p e c i f i c gravity, freezing point depression and dry weight of the hemolymph of the host. This correlates with a l t e r a t i o n s i n the concentration and composition i n the host hemolymph (for review see 18). However, the change i n host protein concentration or composition i s not l i k e l y to be due to excretion of waste material by the parasitoid. P a r a s i t i c hymenopteran larvae have an imperforate gut, with a midgut that i s not joined to the hindgut. Much of the waste generated by the parasitoid i s stored i n t e r n a l l y u n t i l pupation when the waste i s released as meconium outside the host. The developing parasitoid may secrete material into the host that a f f e c t s the composition of the host t i s s u e . Such secretions are l i k e l y to originate from the s a l i v a r y glands of the parasitoid. Phenoloxidase a c t i v i t y i s secreted from the salivary gland of Exeristes roborator and i s thought to help i n the preservation of the host tissue (28). Cotesia (= Apanteles) congregata secreted proteins i n v i t r o that arose from de novo synthesis by the wasp (29). This i s not unique to hymenopteran parasitoids. The dipteran Blepharipa s e r i c a r i a e , parasitoid of the silkworm Philosamia synthia, secretes a small peptide into i t s host that i n h i b i t s l i p i d transport by lipophorin (30). A possible b i o l o g i c a l significance of t h i s peptide may be to divert l i p i d consumption by the host during diapause. This type of secretion may be directed at providing nutrients f o r the parasitoid. Parasitism also causes a l t e r a t i o n s i n the constitutive hemolymph proteins of the host. Hemolymph storage proteins i n the cabbage b u t t e r f l y , P i e r i s rapae, decreased i n concentration following parasitism by Cotesia glomerata (31). This decrease may have been due to an observed concurrent uptake of the same proteins by the parasitoid (32). A reduced synthesis of arylphorin by the tobacco hornworm, Manduca sexta, parasitized by Ç. congregata was hypothesized to be due to i n h i b i t o r y e f f e c t s of parasitism on host fat body, food consumption and growth (33). Arylphorin concentrations increased precociously i n larvae of the cabbage looper, Trichoplusia n i , parasitized by Chelonus sp. (Kunkel, J . G.; Grossniklaus, C ; Karpells, S. T.; Lanzrein, C. Arch. Insect Biochem. Physiol., i n press). Concentrations of several hemolymph proteins decreased i n parasitized T. n i , during development of the parasitoid Hyposoter exiguae (34, 35). Several high molecular weight host proteins were detected 40 hours e a r l i e r i n hemolymph from larvae of the f a l l arrayworm, Spodoptera frugiperda, parasitized by Cotesia marginiventris than i n hemolymph of control larvae (36). Parasitism has also been reported to cause the synthesis of proteins that were not present i n unparasitized hosts (18, 37, 38). Two predominant and one minor proteins were found i n the hemolymph of M. sexta larvae parasitized by l a r v a l parasitoid, C, congregata. These proteins were apparently produced by the host during the f i n a l


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stages of the parasitoid*s development i n d i c a t i n g their synthesis was activated i n response to the development of the parasitoid (33). In comparison, one minor high molecular weight protein was found i n the hemolymph of T. n i parasitized by the egg-larval parasitoid, Chelonus near curvimaculatus (39). I t was not determined whether the protein was encoded by the host, the parasitoid, or a factor derived from the parasitoid. However, the protein was only found i n hosts containing a developing p a r a s i t o i d . The cause of these changes i n host proteins i s unclear and i t i s uncertain what, i f any, regulatory role these changes play. Host Response to the Developing P a r a s i t o i d . Sometimes a l t e r a t i o n s i n the composition of the host tissues are part of the response of the host defense reaction. An i n i t i a l hemolymph response to foreign material i s through the hemocytes (plasmatocytes and granulocytes), produced i n hemopoietic organs near the dorsal diaphragm (40). Part of the humoral defense response i s the synthesis of several enzymic, l e c t i n , and b a c t e r i c i d a l proteins (41-43). I t i s also probable that some parasitoids e l i c i t unique humoral proteins, other than the b a c t e r i c i d a l proteins (29). Some p a r a s i t i c Hymenoptera appear equipped to protect themselves against the host defense responses. Early studies by Salt (44, 45) suggested d i f f e r e n t methods existed for parasitoid resistance to the host immune system. More recent studies indicate i n some cases the a b i l i t y of the parasitoid to overcome the host defenses i s associated with factors (e.g., teratocytes, viruses and venoms) that originate from the adult female parasitoid. These factors w i l l be discussed i n the subsequent sections of t h i s chapter. The texture and composition of the outer surface of the parasitoid egg can be a passive means of protection against the host defense reactions. Fibrous layers on the surface of Cardiochiles nigriceps (46), Campoletis sonorensis (47) and C. glomerata (48) eggs have been implicated i n the protection of the egg from encapsulation by hemocytes of their hosts. Histochemical studies showed the fibrous material was composed of neutral glycoproteins and neutral and a c i d i c raucoproteins. The mechanism by which these complex proteins prevent haemocyte adhesion i s unclear. This type of passive defense i s proposed to delay encapsulation u n t i l more permanent means of host immunosuppression ( i . e . , teratocytes, parasitoid-derived viruses or venoms) i s established (46). I t i s unlikety that the parasitoid egg secretes haemocyte-repelling substances, since i n some cases dormant and dead eggs evade encapsulation. I t should be noted that protection from host immune responses i s an important issue i n l a r v a l parasitism. However, protection may not be important i n true egg parasitoids since insect eggs apparently lack the a b i l i t y to encapsulate foreign objects (44). E f f e c t on the Host Endocrine System. Several publications document an endocrine basis of several parasitoid-host interactions (18, 20, 49, 50). However, most of the mechanisms involving the endocrine interactions remain to be c l a r i f i e d . At best, we can describe the q u a l i t a t i v e e f f e c t s but not the regulatory factors responsible for the interactions. U n t i l the characterization of

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regulatory factors responsible f o r these e f f e c t s , i t i s perhaps most f i t t i n g to discuss these e f f e c t s i n t h i s section. Several systems have been studied and although each i s unique, some common features e x i s t . Chelonus sp. stimulate precocious spinning of a cocoon i n their lepidopteran hosts (51). Presumably t h i s i s a r e s u l t of a premature decline i n the juvenile hormone (JH) l e v e l i n the host. In contrast, other genera (e.g., gregarious Cotesia sp. and Copidosoma sp.) cause a delay or suppression of metamorphosis of the host, sometimes causing supernumerary or intercalated molts into immature stages. Presumably t h i s i s a result of a decline i n the JH s p e c i f i c esterase a c t i v i t y ( i . e . , causing a higher than normal l e v e l of JH) (52), and of a disruption of the ecdysteroid l e v e l s i n the host (33). B. longicaudatus superparasitism of the larvae of A^. suspensa causes an increase i n host JH l e v e l s and a decrease i n JH esterase a c t i v i t y that apparently i s the cause i n delayed host larval-pupal metamorphosis (53). These r e s u l t s could be due to the presence of the parasitoid but also could be a result of the extra-embryonic serosa o r i g i n a t i n g from t h i s p a r a s i t o i d . This i s addressed i n a subsequent section. There i s increasing evidence that the braconid parasitoids, Apanteles k a r i y a i (54) and M i c r o p l i t i s croceipes (55), cause a decrease i n the l e v e l s of ecdysone, v i a a mechanism other than a d i r e c t e f f e c t on the prothoracic gland, the s i t e of synthesis of ecdysone. The braconid endoparasitoids complete t h e i r development without extensive destruction of host t i s s u e s . However, redirection of metabolism i n the f a t body tissue within the host could account for an ultimate reduction i n ecdysone (55, 56). In contrast, ichneumonid parasitoids may regulate the host ecdysteroid l e v e l s by a f f e c t i n g the prothoracic gland tissue (57-59). This regulation i s accomplished through the symbiotic viruses that are the subject of a subsequent section. Indirect evidence implies that some parasitoids produce and release hormones into the host that act as regulatory factors that change host events under endocrine c o n t r o l . The homologue JH III was found i n larvae of M. sexta parasitized by C. congregata (52), though only JH I and I I are present i n unparasitized M. sexta (60). In larvae of the large white cabbage b u t t e r f l y , P i e r i s brassicae, parasitized by C. glomerata, the JH t i t e r increased a f t e r neck-ligation of the host. Both cases suggest a possible source of the hormone was the parasitoid (61). This hypothesis i s further supported by recent findings. Larvae of T. ni, parasitized by Chelonus sp. showed reduced l e v e l s of JH I I , but contained high l e v e l s of JH I I I , compared to unparasitized larvae (Jones, G.; Hanzlik, T.; Hammock, B. D.; Schooley, D. Α.; M i l l e r , C. Α.; T s a i , L. W.; Baker, F. C. J . Insect Physiol., i n press). The presence of JH I I I , together with JH I and I I , was detected i n parasitized eggs, but only JH I and I I were present i n unparasitized eggs (Grossniklaus-Burgin, C ; Lanzrein, B. Arch. Insect Biochem. Physiol., i n press). These findings suggest that JH I I I originated from the p a r a s i t o i d . In addition, a comparison revealed that JH t i t e r fluctuations i n the parasitoid were independent of changes i n the JH t i t e r i n the host, which supports the idea that the parasitoid can produce i t s own JH (Grossniklaus-Burgin, C ; Lanzrein, B. Arch. Insect Bioch. Physiol., i n press).


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Levels of JH I I I i n suspense superparasitized by B. longicaudatus were considerably higher than i n unparasitized A. suspensa (53). JH I I I was also found i n f i r s t instars of the parasitoid. These results suggested that the elevated JH I I I l e v e l s i n the superparasitized host may r e s u l t from a decrease i n JH esterase a c t i v i t y with a continued or elevated synthesis of the hormone i n the host or from a secretion of JH I I I by the parasitoids. Parasitoid Response to the Host. Simultaneous with the e f f e c t s of parasitism on the host, are the e f f e c t s of the host on the parasitoid. The response of the parasitoid to host parameters influences the way the parasitoid interacts with the host. This creates a continuum between the "conformers" and the "regulators." Endo- and ectoparasitoids feed i n part on the hemolymph of the host and many endoparasitoids develop within the haemocoel of the host. Both ingestion and cross-cuticular transport of host humoral material by the parasitoid are l i k e l y to occur. Apparently, parasitoids are p a r t i c u l a r l y sensitive to the endocrine milieu i n the host hemolymph (_3, 18, 20) and capable of taking up hormones from the host (Grossniklaus-Burgin, C.; Lanzrein, B. Arch. Insect Biochem. Physiol., i n press). The "hormonal hypothesis" proposed that the growth of many parasitoids was affected and possibly controlled by the hormones of t h e i r hosts (62, 63). M e l l i n i thought t h i s hypothesis was more applicable to dipteran parasitoids. However, several studies have confirmed the application of the hypothesis to p a r a s i t i c Hymenoptera as well. Many parasitoids are sensitive to disturbances of the host endocrine milieu caused by application of insect growth regulators (64-71), and by administering exogenous hormone to the host (72-74). The f i r s t l a r v a l molt of Opius concolor i s regulated by endogenous release of ecdysteroids by i t s host, the Mediterranean f r u i t f l y , C e r a t i t i s capitata, during puparium formation (73, 74). The l a r v a l ecdysis of C. congregata at emergence from the tobacco hornworra, M. sexta, i s regulated by host hemolymph ecdysteroid l e v e l s (52) and affected by topical application of JH or the JH analogue, raethoprene (69). The larval-pupal parasitoid B. longicaudatus deposits an egg i n the f i r s t , second or t h i r d instar larvae of A^. suspensa. The p a r a s i t o i d s f i r s t instar remains inactive (conformer), and l a t e r coordinates i t s molt with host pupation (75). This obligatory synchrony of the early l a r v a l molt of the parasitoid with the host's metamorphosis i s i n response to the ecdysteroid levels i n the host hemolymph (76). In addition to these cases where a d i r e c t association or response has been shown between the host hormonal milieu and the development of the parasitoid, there are other cases where the developmental synchrony of both the parasitoid and i t s host constitutes i n d i r e c t evidence of t h i s relationship (for a review see 20). 1

Teratocytes and Serosa. Teratocytes, which originate from the disintegration of an embryonic membrane or serosa (=* trophamnion) of some p a r a s i t i c Hymenoptera, appear to have a s i g n i f i c a n t role i n parasitoid-host interactions (8, 77-80, Dahlman, D. L. Arch. Insect Biochem. Physiol., i n press). A discussion of teratocytes i s



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included i n t h i s section since these c e l l s are derived from the developing parasitoid embryo and are released into the host when the embryo hatches. However, teratocytes can cause c e r t a i n e f f e c t s on the host, independent of the developing parasitoid. Though teratocytes may increase i n s i z e when they are released into the host, they do not multiply within the host (18). Secretory organelles have been observed i n teratocytes that may be important to the function of these c e l l s (80, 81). I t has been proposed that these c e l l s serve n u t r i t i o n a l and gaseous exchange functions that benefit the parasitoid (80, 82, 83). In theory nutrients could be sequestered from the host hemolymph or produced by de novo synthesis within the c e l l s . Several studies have investigated the production and secretion of regulator molecules by teratocytes. The presence of rough endoplasmic r e t i c u l a and other organelles i n the teratocytes suggests these c e l l s are a c t i v e l y metabolizing and synthesizing material (8). Substances released by teratocytes have been shown to a f f e c t phenoloxidase a c t i v i t y (9_, Dahlman, D. L. Arch. Insect Biochem. Physiol., i n press). An endocrine r o l e has also been postulated i n some species (84). Injection of teratocytes from the braconid parasitoid M. croceipes into larvae of the tobacco budworm, H e l i o t h i s virescens, caused an elevation of the ecdysteroids, an increase i n JH l e v e l s and a decrease i n the JH esterase a c t i v i t y i n the host (85, Dahlman, D. L. Arch. Insect Biochem. Physiol., i n press). Results from recent studies show that the presence of teratocytes causes a disruption of the c e l l u l a r defense system of the host (Tanaka, T.; Wago, H. Arch. Insect Biochem. Physiol., i n press, Kitano, H.; Wago, H.; Arakawa, T. Arch. Insect Biochem. Physiol. i n press). This supports e a r l i e r speculation that encapsulation was suppressed by the teratocytes (80) or substances produced by the teratocytes that i n h i b i t the host immune response, (86-88). In most p a r a s i t i c Hymenoptera studied the serosa degenerates a f t e r hatching. However, the serosa of IS. longicaudatus remains intact (Lawrence, P. 0. Arch. Insect Biochem. Physiol., i n press). Though there are no other reports of serosas that remain i n t a c t , i t i s possible that some species not recorded as having teratocytes (see Table I) may maintain an i n t a c t serosa. C e l l s of the serosa of B. longicaudatus are secretory and appear to use large coated v e s i c l e s to transport molecules from the host, and m i c r o v i l l i to release materials into the host (Lawrence, P. 0. Arch. Insect Biochem. Physiol., i n press). A polypeptide, approximately 24 Kd, was found i n the hemolymph of the host and apparently was produced i n the serosa c e l l s . Injection of the protein into healthy _A. suspensa prepupae i n h i b i t s metamorphosis (Lawrence, P. 0., University of F l o r i d a , personal communication, 1990.). The e f f e c t of 13. longicaudatus on _A. suspensa i s s i m i l a r to the e f f e c t of teratocytes of M. croceipes injected into larvae of H. virescens reviewed above (Lawrence, P. 0. Arch. Insect Biochem. Physiol., i n press). The sequestering of host material through coated v e s i c l e s could support a nutrient role of the serosa. However, d e f i n i t i v e evidence i s s t i l l lacking.


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The proximity of the serosa and teratocytes with the developing parasitoid, and their simultaneous presence i n the host, make i t d i f f i c u l t to determine which i s the source of the parasitoid regulatory f a c t o r s . I t i s also possible that regulatory factors are synthesized i n one tissue or location and released to the host at another location v i a a d i f f e r e n t tissue. These points remain to be determined i n most cases.


Factors Associated with the Adult Female Paxasitoid Parasitoid-Derived Viruses. Symbiotic viruses found i n some members of the Ichneumonidae and Braconidae families are assembled i n the nuclei of the c e l l s of the calyx epithelium (89). C e l l s that produce the viruses may form a layer around the calyx lumen near the ovariole end of the calyx (90). The viruses are secreted into the lumina of the l a t e r a l oviduct and injected into the host with the egg at the time of o v i p o s i t i o n . Once i n the host the virus material i s expressed (91, 92). At present there i s no unequivocal evidence of r e p l i c a t i o n of these viruses i n the host (93), though t h i s continues to be an active area of i n v e s t i g a t i o n . V i r a l genomes consist of separate, multiple heterologous, double-stranded, c i r c u l a r DNA of various lengths. Viruses isolated from ichneumonids have a fusiform nucleocapsid surrounded by two unit-membrane envelopes. This i s c h a r a c t e r i s t i c of the genus Polydnavirus (94). Viruses isolated from braconids have c y l i n d r i c a l nucleocapsids of variable lengths surrounded by a single unit membrane, and morphologically have some resemblance to baculoviruses (89). These symbiotic viruses from both the ichneumonids and the braconids, have been assigned to the new virus family, polydnaviridae (95). Reports of the secretory nature of the calyx region date back over twenty years. Salt (44) recognized that secretions of the calyx of the ichneumonid Venturia (= Nemeritis) canescens affected the host immune system. Studies on the ichneumonid C. sonorensis revealed the presence of nuclear secretory p a r t i c l e s associated with the calyx c e l l s (96). Those studies were followed by the confirmation of DNA i n virus p a r t i c l e s associated with the oocytes of the braconid C. nigriceps (97). Several reports now corroborate the role of the symbiotic viruses i n inactivating defense mechanisms of the host. Washed eggs of the ichneumonid parasitoids C. sonorensis (98) and Hyposoter f u g i t i v u s (99) are encapsulated and do not develop when introduced a r t i f i c i a l l y into their hosts JH. virescens and the forest tent c a t e r p i l l a r , Malacosoma d i s s t r i a , respectively. However, when virus isolated from the calyx of the parasitoid accompanies the egg, viable parasitoid progeny develop, suggesting a role for the virus i n protecting the parasitoid egg from the immune response system of the host. In a cross-protection experiment, washed eggs of C. sonorensis and H. exiguae developed i n H. virescens larvae when heterologous combinations of eggs and symbiotic virus were used (100). These results suggest that the viruses from the two ichneumonids act i n a similar manner to promote successful parasitism. The mechanism by which the virus of Ç, sonorensis suppresses the host's a b i l i t y to encapsulate the parasitoid egg remains



52


unknown. Davies et a l . (101) demonstrated that within 8 hours of i n j e c t i n g calyx f l u i d , 75% of the c i r c u l a t i n g capsule-forming hemocytes (plasmatocytes) were removed from the host hemolymph. Such an a l t e r a t i o n i n the host plasmatocytes could account for the suppression of encapsulation, though the fate of the l o s t plasmatocytes remains unknown. The a b i l i t y of the virus from the ichneumonid, H. exiguae to i n h i b i t phenoloxidase a c t i v i t y i n the host T. n i , may provide another mechanism of suppression of encapsulation (102). A similar decline i n the monophenoloxidase a c t i v i t y i n the hemolymph of M. sexta occurred after i n j e c t i o n of virus from the braconid C. congregata (Beckage, N. E.; Metcalf, J . S.; Nesbit, D. J . ; S c h l e i f e r , K. W.; Zetlan, S. R.; de Buron, I. Insect Biochem., i n press). The phenoloxidase-tyrosine enzyme system i s also thought to be part of the host immune system (103, 104). Suppression of the host immune system, observed following parasitism by two braconid parasitoids, has also been attributed, i n part, to the presence of a symbiotic v i r u s . Long v i r u s - l i k e filaments from the calyx region of the reproductive t r a c t of the braconid parasitoid M i c r o p l i t i s mediator were found attached to the surface of the chorion of the oviposited egg (105). These filaments appeared to prevent encapsulation i n half the eggs tested i n the host, the common armyworm, Pseudaletia separata. There was also evidence that the f i l o p o d i a l elongation of the host hemocytes was strongly suppressed (106). Also, half of the sephadex p a r t i c l e s , injected into P. separata together with calyx f l u i d from the braconid endoparasitoid A. k a r i y a i , were not encapsulated (15). It should be noted that a mixture of calyx f l u i d and venom material for both braconids, was more e f f e c t i v e i n suppressing encapsulation than the calyx f l u i d alone. Apparently, calyx f l u i d (containing symbiotic virus) and venom act s y n e r g i s t i c a l l y and both were essential for complete evasion of the host defense reactions. There was some evidence suggesting the calyx material affected the capsule-forming c e l l s of the host, while the venom material provided a non-antigenic protective covering over the egg (106). A similar e f f e c t was observed with a mixture of calyx f l u i d and venom material for the braconid C. nigriceps (14). Another explanation for the synergism observed between symbiotic viruses and venomous material may involve tissue i n f e c t i o n and expression of the v i r u s . Venom promoted uncoating of a braconid virus i n v i t r o and, also promoted persistence i n vivo of DNA from the virus (16). Injection of virus material has also been associated with host synthesis of new proteins. Parasitism by C. sonorensis resulted i n the synthesis of a new glycoprotein i n several of i t s habitual hosts (107). The appearance of the glycoprotein was duplicated by the i n j e c t i o n of either calyx f l u i d or p u r i f i e d v i r u s . The glycoprotein has been correlated with suppression of encapsulation by the host. It i s interesting that parasitism by C. sonorensis of two non-permissive hosts, the velvetbean c a t e r p i l l a r , Anticarsia gemmatalis, and the bertha armyworm, Mamestra configurata, did not stimulate the production of the glycoprotein by those hosts (107). D e f i n i t i v e indication of t r a n s c r i p t i o n of parasitoid-derived viruses i n the host has thus f a r eluded researchers. Parasitism by C. congregata induced synthesis of new hemolymph proteins i n the



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host M. sexta (108). Synthesis of one of the major induced proteins occurred within a few hours of parasitism. Induction was also caused by the i n j e c t i o n of ovarian calyx f l u i d from the parasitoid. Exposure of the calyx f l u i d to psoralen and UV l i g h t destroyed i t s capacity to induce synthesis of the major new protein. This suggested that the synthesis may be mediated by v i r a l nucleic acid, though i t was not determined i f i t represented a v i r a l gene product or a host protein induced by v i r u s - s p e c i f i c a c t i v i t y i n the host. Another type of i n t e r a c t i o n of the parasitoid-derived viruses with the host i s based on the endocrine regulation of development. Injections of the calyx f l u i d or i s o l a t e d virus material of C. sonorensis arrested development of 40% of the ÏÏ. virescens larvae tested (58, 109). Injections into i s o l a t e d thoraces were most e f f e c t i v e . Ecdysteroid production by the prothoracic gland of the host ceased f o r ca. 10 days following the i n j e c t i o n . Arrested development was reversed by i n j e c t i o n s of ecdysone and 20-hydroxyecdysone. The prothoracic glands of injected host larvae were p a r t i a l l y degenerated. In contrast other tissues associated with the endocrine system of the host appeared normal (59). These observations suggested that the virus material induced tissue degeneration that was s p e c i f i c to the prothoracic gland. S i m i l a r l y , parasitism of P. separata, by A. k a r i y a i or i n j e c t i o n of the calyx f l u i d or virus material from the parasitoid, caused a decline i n the host ecdysteroid t i t e r and arrested metamorphosis of the host (54). Again, i n j e c t i o n of exogenous 20-hydroxyecdysone reversed the developmental a r r e s t . In t h i s case, administration of prothoracicotropic hormone caused a reactivation of the prothoracic glands i n the treated host. These r e s u l t s showed that the virus material i n h i b i t e d the synthesis or secretion of prothoracicotropic hormone and also lowered the ecdysteroid l e v e l i n the host. Also i n t h i s case, a mixture of both venom and calyx f l u i d were needed to obtain the f u l l prolongation of the l a r v a l stage i n the host (110). Other developmental e f f e c t s caused by a symbiotic virus from a parasitoid may involve the n u t r i t i o n a l physiology of the host. An increase i n trehalose l e v e l s i n the hemolymph of Η· virescens parasitized by M. croceipes could be duplicated by the i n j e c t i o n of the calyx f l u i d from the parasitoid (111). Though there i s no known explanation f o r the increase i n trehalose, i t i s proposed to r e s u l t from the release of glucose that i s catabolized to trehalose i n the host (13). An increase i n the host hemolymph trehalose l e v e l may have a n u t r i t i o n a l benefit f o r M. croceipes which i s a hemolymph feeder. From t h i s review we see that symbiotic viruses from the braconids and the ichneumonids have some differences i n their physical structures and their apparent interaction with the host. Some variations noted i n the action of these viruses may eventually be correlated with differences i n v i r a l structures and the interaction of the viruses with venom components. Certain host tissues may be susceptible to one virus-type and r e f r a c t i v e to the other virus-type. I t i s possible that ichneumonids, which are commonly known as tissue feeders, contain symbiotic viruses that have a more prominent e f f e c t on the endocrine system of the host, thereby preserving the host t i s s u e . In comparison, braconids, which



54


are commonly known as hemolymph feeders, may contain symbiotic viruses that contribute more to a l t e r a t i o n s i n the composition of the host hemolymph that would benefit the parasitoid. Other types of virus material have been recorded from p a r a s i t i c hymenopteran species. V i r a l p a r t i c l e s of parasitoid o r i g i n were reported i n tissues of the /L. suspensa superparasitized by the s o l i t a r y endoparasitoid B. longicaudatus (112). Rhabdoviruses were found i n vesicles near the basement membrane of the host epidermal c e l l s and pox viruses were found i n hemocytes adjacent to the epidermis (Lawrence, P. 0.; Akin, D. Canad. J . Zool., i n press). The rhabdoviruses were proposed to a f f e c t migration of vesicles to the c e l l apices and the a c t i v a t i o n of the molting f l u i d . The function of the pox virus i s not yet known but could relate to the changes i n the hemolymph l e v e l s of ecdysteroids, JH, and JH esterases (53). A long nonoccluded filamentous virus, morphologically d i s t i n c t from the polydnaviruses, was characterized from the reproductive t r a c t of the parasitoid C. marginiventris. Though the role of the filamentous virus i n the parasitoid-host interaction i s unknown, the virus was reported to r e p l i c a t e i n hypodermal and tracheal matrix c e l l s of the host larvae (113). Secretions from Glands and Other Specialized Tissues. In general, p a r a s i t i c Hymenoptera are known to i n j e c t secretions from glands and specialized tissues into their hosts. Those secretions, except f o r the symbiotic viruses and virus material or calyx f l u i d , are c o l l e c t i v e l y presented i n the next two sections as venomous substances. Paralyzing Venomous Substances. The best characterized regulatory factors at present are a group of substances isolated from the venom of the wasps of the p a r a s i t i c genus Bracon (= Haborbracon, = Microbracon), and the predatory sphecids Philanthus trangulum and Sceliphron caemantarium (114), and s c o l i i d Megascolia f l a v i f r o n s (115). These species cause paralysis i n their hosts or prey that varies from complete and permanent paralysis, which causes immediate cessation of feeding, to a transient or an incomplete paralysis. Though these venoms act on the nervous system of the host or prey, the active ingredients vary considerably. The chemical nature of the active ingredients range from large highly l a b i l e proteins with molecular weights of 43 kd to 56 kd found i n the braconid species, and short chain peptides (kinins) found i n the s c o l i i d species, to low molecular weight substances l i k e the polyamines found i n the sphecid species (116). Piek and Spanjer (114) l i s t members of the families Braconidae and Ichneumonidae having paralyzing venoms. Current a r t i c l e s are available on t h i s area (12, 115, 117, 118) and no further d e t a i l w i l l be given i n t h i s report. Nonparalyzing Venomous Substances. Often the i n j e c t i o n of the virus material and nonparalyzing venomous substances occur simultaneously so that the e f f e c t s are not e a s i l y separated. Reference i s made i n t h i s section to reports where venomous substances were shown to perform a function independent of symbiotic



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viruses, or no reference was made i n the report to the presence of virus material. A d i v e r s i t y of functions and a variety of developmental abnormalities have been associated with nonparalyzing venomous substances. Of the four sources of regulatory factors reviewed i n Table I, venomous substances are common to species i n a l l the families represented, by ecto- and endoparasitoids a l i k e , and the only source of regulatory substances i n ectoparasitoids known to a f f e c t the development of the host. Growth of H. virescens was arrested following parasitism by C. sonorensis (88). The mechanism for the i n h i b i t i o n i n host weight gain was unknown, but the e f f e c t could be reproduced by i n j e c t i o n s of aqueous extracts of the l a t e r a l oviducts of the female parasitoid. These contained protein but no RNA or DNA material and was inactivated by heat and exposure to organic solutions or protease action. This evidence strongly suggests the arrest of weight gain i n the host was i n response to the i n j e c t i o n of a proteinacious substance secreted by the l a t e r a l oviducts. Vinson (88) proposed the primary purpose f o r arrested weight gain was to reverse the flow of nutrients from organs of the host to the hemolymph that a i d the n u t r i t i o n of the developing p a r a s i t o i d . The scelionid egg parasitoid Telenomus h e l i o t h i d i s interrupts egg development i n i t s host R. virescens. A regulatory substance, apparently produced and secreted by the exocrine c e l l s of the common oviducts of the parasitoid, was responsible f o r the rapid arrestment of host embryogensis (77). Necrosis of host tissue i s also associated with parasitism. However, i f oviposition was interrupted prior to egg deposition, the r e s u l t i n g pseudoparasitized host exhibited arrestment of embryogenesis but no tissue necrosis. Apparently i n t h i s case the secreted substance was not associated with restructuring of host tissue f o r the n u t r i t i o n of the parasitoid. Also, the mode-of-action and the nature of t h i s regulatory substance i s unknown. Results from an early study by Shaw (119) indicated that stinging by the s o l i t a r y endoparasitic braconid Clinocentrus g r a c i l i p e s of i t s host, Anthophila f a b r i c i a n a was separable from oviposition. Stinging alone caused temporary paralysis followed by reduced feeding, precocious molting, incomplete pupal formation and death of the host. The e f f e c t s were attributed to the presence of a nonparalyzing venom from the p a r a s i t i c wasp. Proteins from the venom gland of the egg-larval parasitoid, C. near curvimaculatus are injected into the host within the f i r s t eight seconds of the oviposition period (120). These proteins remain intact i n the host f o r two days at which time they are rapidly degraded prior to hatch of the parasitoid egg. The exact role of these proteins remains unknown. Interesting i s the finding that these proteins did not cause the precocious metamorphosis followed by arrested development observed i n parasitized T. n i (120, 121). Another regulatory substance, injected a f t e r the venom proteins but before the parasitoid egg, caused the developmental redirection of host t i s s u e . The source and nature of t h i s substance i s unknown but i t i s proposed to be d i f f e r e n t from calyx f l u i d and virus material, also associated with t h i s parasitoid, and injected



56


at a time d i s t i n c t from the i n j e c t i o n of other regulatory substances. In some parasitoid-host systems certain material, which would come under the heading of venomous substances as used i n t h i s chapter, may play a role i n suppression of the immunological response of the host against the parasitoid by acting as a nonreactive egg coating. Examples of these are the mucinous accessory gland secretions of the ichneumonid, Pimpla t u r i o n e l l a (122-124), and material from the venom gland of C. glomerata. This material protects the parasitoid egg without i n h i b i t i n g the encapsulation a b i l i t y of the host (125). The parthenogenetic ichneumonid V^. canescens secretes a particulate material onto the surface of the egg as i t passes through the calyx. The material, which contained antigenic determinants similar to a protein component of the host Ephestia kuehniella, formed a physical covering for the egg (126, 127). The observed s i m i l a r i t y between proteins of the secreted material and a host protein suggested the secreted protein could simply mask the egg surface and prevent the egg from being recognized as a foreign body. In t h i s way the parasitoid protein provided passive protection from the host immune system similar to the fibrous layers found on eggs of Ç. nigriceps. Though the protein occurred i n a particulate structure that gave i t the appearance of the virus material i t was found to contain glycoprotein but no detectable amount of nucleic acids. In other parasitoid-host systems certain material apparently interferes with the a c t i v i t y of the host hemocytes. Examples of these are the accessory genital gland secretions of P. t u r i o n e l l a , which i n h i b i t s capsule-forming hemocytes (128), and accessory gland secretions of the cynipid Leptopilina heterotoma, which causes deterioration of the hemocytes (129). Careful documenting of the i n h i b i t i o n of molting ( i . e . , arrest of apolysis and ecdysis) i n parasitized lepidopteran hosts i s an increasingly important area of study (114, 119, 130-133). Members of the Eulophidae family that are i n the genus Euplectrus and some Eulophus species produce a venom that causes t h i s s p e c i f i c type of response i n their hosts. In contrast, most a l l other hymenopteran ectoparasitoids (e.g., Braconidae, Ichneumonidae, Eulophidae, and Bethylidae) of lepidopteran and coleopteran larvae paralyze t h e i r hosts (134-136). The development of Orthosia s t a b i l i s was arrested after stinging by the gregarious ectoparasitoid Eulophus larvarum. During the period of arrested development, the host was recorded as mobile and continued to feed for a period of time but f a i l e d to produce a new c u t i c l e or i n i t i a t e apolysis (119). This e f f e c t was attributed to a venomous substance, presumably transferred from the parasitoid to the host at the time of stinging, and was shown to be independent of oviposition or the developing parasitoid. Euplectrus sp. are naturally gregarious ectoparasitoids that develop on noctuid and geometrid larvae (137-139). Euplectrus p u t t l e r i and Euplectrus kuwanae are host s p e c i f i c (140, 141). In contrast, Euplectrus plathypenae has several lepidopteran host species (142, 143). A l l three species arrest the development of t h e i r hosts. Recent studies have shown that Euplectrus comstockii also arrests the development of i t ' s hosts (Coudron, Τ. Α., USDA,



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ARS, unpublished data). Euplectrus sp. i n h i b i t host ecdysis once stinging ( i . e . , puncture with the ovipositor), oviposition ( i . e . , egg deposition), or parasitism ( i . e . , stinging and oviposition) has occurred (132). Molt i n h i b i t i o n usually occurs without host paralysis and prevents shedding of the c u t i c l e , which i s the s i t e of attachment of the parasitoid*s eggs. Contact with the host by parasitoid eggs or developing parasitoid larvae i s not necessary f o r the expression of the molt arrest e f f e c t . Instead, Euplectrus sp. transfer an arrestment substance to the host at the time of stinging that acts as a potent bioregulator (132, 133). The recording of superparasitism by the s o l i t a r y endoparasitoid 15. longicaudatus i n puparia of A^. suspensa (112, 144) i s the only other instance of arrestment of apolysis and ecdysis of host larvae following parasitism by an entomophagous insect i n a family other than Eulophidae. The presence of viable parasitoids was required f o r t h i s effect to be manifested and the added stress of the additional parasitoid load ( i . e . , superparasitism) was thought to cause the observed arrestment and not a venom from the parasitoid alone. Venom from E. plathypenae arrested molting i n 44 species of Lepidoptera, plus 19 species i n s i x other orders. The arrestment was expressed i n a l l natural hosts and i n most, but not a l l , insects outside the natural host range of the parasitoid (132). The active substance has been located i n hemolymph of parasitized hosts, i n a gland/reservoir complex isolated from the female ectoparasitoid, and found associated with the f l u i d and c r y s t a l l i n e structures within the gland/reservoir complex (133, 145). The developmental arrest could result from disrupting events normally under endocrine control (130, 131, 146, Kelly, T. J . ; Coudron, T. A. J . Insect Physiol., i n press). However, treatment of parasitized larvae with exogenous JH, 20-hydroxyecdysone, and several hormone analogs did not restore the normal molting process i n the parasitized host (133, 145). Molting was arrested even when parasitism occurred after the release of endogenous molting hormone (133). Clearly, t h i s system offers a new and i n t r i g u i n g area f o r research on the regulation of insect development and parasitoid-host interactions. Future Exploitation C l a s s i c a l b i o l o g i c a l control involves the action of b e n e f i c i a l organisms i n the suppression of pest insects. A wider view of b i o l o g i c a l control has evolved to embrace attempts to develop improved strains of b e n e f i c i a l organisms through controlled selection and genetic manipulation of species. Improved strains are targeted at increasing the suppression phenomenon, widening the host range, and reducing the lapse of time between parasitism, or i n f e c t i o n , and cessation of host feeding. This has enhanced our e f f o r t s to understand desirable attributes of bénéficiais and has brought on a search to i d e n t i f y the b i o l o g i c a l substances ( i . e . , bioregulators) that are responsible f o r the regulation of molecular and c e l l u l a r processes involved i n growth, development, and behavior of beneficial and harmful insects. Understanding the mechanisms used by p a r a s i t i c Hymenoptera i n regulating their host physiology may provide valuable information i n t h i s direction (147).



58


The f i r s t step i s the determination of desired t r a i t s controlled by bioregulators. Second i s the chemical characterization and elucidation of the mode-of-action of the natural substances that regulate the development of pest insects. Thirdly i s the refinement of techniques to modify and regulate the expression of the substance at the c e l l u l a r l e v e l . Over the past decade genetic engineering has developed from the advances made i n molecular biology and biotechnology areas. This technology has been applied to the production of transgenic plants that are r e f r a c t i v e to extensive insect damage, and to transgenic microbials that are designed to produce desirable b i o l o g i c a l materials. These powerful tools of the molecular g e n e t i c i s t s could make substantial and rapid contributions to b i o l o g i c a l pest c o n t r o l . Applications of such contributions may include the transfer of desirable bioregulators to other b e n e f i c i a l organisms to enhance effectiveness or improve desirable q u a l i t i e s ( i . e . , speed of k i l l ) and may lead to the production of new substances that influence unique insect biochemical processes. Such contributions may take the form of insect transformations through the use of P-elements (148) and plasmids (149), or the incorporation of bioregulators into the genome of plants consumed by phytophagous insects. Refer to Kuhlemeier et a l . (150), McCabe et a l . (151) and Benfey and Chua (152) for recent reviews on the incorporation and regulation of foreign genes i n plant tissues. I t may be possible to use vectors to produce large quantities of desirable substances that can then be formulated for d i r e c t use as naturally derived pesticides. Refer to Luckow and Summers (153) and Maeda (154) for recent reviews on the use of insect baculoviruses as vectors for expression of foreign DNA. Presently, concerns over insect resistance to synthetic i n s e c t i c i d e s (155) has grown to include concerns over resistance to transgenic organisms that are designed s p e c i f i c a l l y to replace the use of synthetic chemicals (156). One potential solution for avoiding resistance to transgenic organisms i s the use of many gene t r a i t s , each of which encodes for a natural bioregulator. The r a t e - l i m i t i n g step to t h i s solution i s the determination of characters that are desirable for manipulation and the i d e n t i f i c a t i o n of the genes that express for those substances with desirable t r a i t s . The regulatory factors produced by p a r a s i t i c Hymenoptera have promising applications i n t h i s area. Though p a r t i a l descriptions can be given for the e f f e c t s of these regulatory factors on the host insect, i t i s apparent that much of t h e i r function remains unclear. Chemical characterization i s incomplete for most of these factors and a thorough understanding of their e f f e c t s , i n d i v i d u a l l y and i n combination, has yet to be detailed. However, the protein nature and independent action of several venomous substances make them amenable to approaches that involve genetic engineering technology. The synthesis and expression of genes encoding i n s e c t i c i d a l proteins derived from scorpion venoms have been reported (157, 158). Enriched genetic material (RNA and DNA) coding for these substances can be i s o l a t e d from s p e c i f i c tissues known to produce the active substances or cDNA can be constructed that encodes for the active substance. The genomic material can be transferred into microbials



4. COUDRON


59

(both symbiotic or pathogenic bacteria and viruses) which can be used as vectors to transport and express the substances i n pest insects. Transgenic plants also could be constructed to produce the active substances d i r e c t l y within the s p e c i f i c plant tissue that would be consumed by the pest insect. The potential f o r constructing a r t i f i c i a l pathogens to act as delivery systems of these active substances i s also under consideration. These approaches create a basis f o r the construction of an i n s e c t i c i d a l method f o r the use of naturally occurring substances with desirable a t t r i b u t e s . Genes encoding f o r fast-acting venoms could greatly diminish the lapse of time between parasitism by a b i o l o g i c a l control agent and the r e s u l t i n g cessation of host feeding. Gene products should be selected that have minimal mammalian t o x i c i t y . Certain paralyzing toxins from the venoms of the s c o l i i d wasps are now known to act as blockers of the synthesis of acetylcholine by preventing the access of choline to the presynaptic terminals (115, 116, 118). Unfortunately i t i s generally accepted that t h i s system i s s i m i l a r i n insects and vertebrates. In contrast, an i n s e c t i c i d a l neurotoxin, found i n the venom of the scorpion Androctonus a u s t r a l i s , i s s e l e c t i v e l y toxic to several species of insects but harmless to crustaceans, arachnids and mammals (159). The venom from the Eulophid species i s also fast-acting (133) and i t appears to function on a physiological process ( i . e . , ecdysis) that i s more unique to arthropods. These q u a l i t i e s may make the active toxin i n the Eulophid venom an a t t r a c t i v e alternative to the paralyzing toxins. The delicate interplay between parasitoid and host development o f f e r s a wealth of opportunities f o r future e x p l o i t a t i o n . Avenues for exploitation w i l l expand as we continue to characterize the biochemical parameters contributing to the parasitoid-host relationships. A strategy of examining and exploiting these s p e c i f i c interactions could lead to s i g n i f i c a n t new advances i n p r a c t i c a l b i o l o g i c a l control measures. Such advances may include new pest control measures targeted at physiological and metabolic pathways s p e c i f i c to insects. An understanding of these interactions also may a i d i n the structuring of more e f f e c t i v e compounds that mimic or antagonize the e f f e c t s of the b i o l o g i c a l compounds produced by b e n e f i c i a l agents and a i d i n the production of pharmacologically active agents that are environmentally acceptable for use to control pest insects. Acknowledgments I sincerely thank Mr. Benjamin Puttier f o r h i s encouragement and many helpful comments, and Drs. N. Beckage, D. Dahlman, B. Lanzrein, P. Lawrence and M. Strand f o r providing material i n press and their discussions on t h i s subject. The author g r a t e f u l l y recognizes Drs. B. Hammock, D. Jones, P. Lawrence and D. S t o l t z f o r t h e i r review of the text, Karen Ridgwell f o r her assistance i n preparing the manuscript, and USDA, ARS f o r supporting the preparation of t h i s document.



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