Insect Communication Codes Isolation and Identification of Pheromones "Hey, good looking, I'm over here waiting for you," signals a female moth to her male counterpart. "Come on, guys, the food is over here. Just follow my trail," says one ant to other members of her colony. "We're all getting together on this sumptuous tree over here," announces a bark beetle to his compatriots. This is insect "talk." Insects rely on their keen sense of smell to communicate via a sophisticated chemical language, using as words specific, volatile compounds. Although other modes of communication, such as visual, tactile, or auditory signals, are important for some insect species, olfaction is often the dominant method for stimulating behavioral responses such as sex attraction, aggregation, alarm, and trail finding. Members of an insect species emit a characteristic "vocabulary" of compounds that triggers a response only from individuals of the same species; these odors used in intraspecific communication are known as pheromones, a term originally proposed by Karlson and Butenandt (1). The most well known pheromones are the sex attrac-
tants. Often the female of the species emits a specific chemical or group of chemicals to announce her availability to her male counterparts. Because other species of insects generally do not respond to her coded message, cross-breeding is avoided. An aggregation pheromone, on the other hand, may guide hordes of both sexes of a bark beetle to a single site for a mass attack on a tree. Trail pheromones deposited by foraging ants create a path for other members of an ant colony to follow toward a food source. Although it is an oversimplification to suggest that every insect uses a specific chemical, unique to its species, to communicate each and every behavior, more than 300 chemical compounds have been identified as components of sex attractants alone. Pheromones, composed of single chemicals or mixtures of chemicals, have been identified for more than 400 insect species, and perhaps 80-90% of these fall into the sex attractant category (2-4). Roughly 70% of the species for which these attractants have been identified are Lepidoptera (moths and butterflies) and about 10% are Coleoptera (beetles); the remainder includes Diptera (flies), Homoptera (scales and mealybugs), and Hymenoptera (wasps and sawflies) (2-4).
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The fact that specific compounds attract given insects creates a very practical purpose for unraveling and duplicating the sex pheromone codes. If one can generate the identical pheromone message for a pest species with synthetic chemicals, members of the attracted sex of that species can be drawn into a trap containing an adhesive or a toxicant. Thus, the presence of insects in such a pheromone-baited trap signals the presence of that insect in the area. The number of trapped insects can be related to population level, and the boundaries of the infested area can be delineated by an array of
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Report B. A. Leonhardt Insect Chemical Ecology Laboratory Agricultural Research Service U.S. Department of Agriculture, BARC-West Beltsville, Md. 20705
these monitor traps. T r a p p i n g d a t a tera (2-4). Surveyed insects include can then be used to dictate the time Japanese beetle, Popillia japonica and geographic area for the most efNewman; gypsy moth, Lymantria disfective application of an insect control par (L.); pink bollworm, Pectinophora agent, such as an insecticide. Proper gossypiella (Saunders); codling moth, timing of such insecticide t r e a t m e n t s Cydia pomonella (L.); corn earworm, results in increased efficiency, with far Heliothis zea (Boddie); boll weevil, less applied chemical. Therefore, both Anthonomus grandis Boheman; Calithe cost of insect control and the defornia five-spined ips (a bark beetle), gree of environmental contamination Ips paraconfusus Lanier; tobacco are reduced significantly. Indeed, by budworm, Heliothis virescens (F.); 1982, pheromone-baited traps were and Comstock mealybug, Pseudococused in pest management programs in cus comstocki (Kuwana). infested areas or quarantined locaIf one looks at the chemical structions for the detection of 81 species of tures of a sampling of sex pheromone Lepidoptera and 47 species of Coleop- compounds (Table I), one can get an idea of the diversity and complexity of the problems encountered in structure identification. N o t only are there large variations in functionality and chain length, b u t also many of the pheromones have several components. M a n y of the lepidopteran sex attract a n t s are m a d e u p of aliphatic aldehydes, alcohols, or acetates t h a t have 10-20 carbon atoms and one or more double bonds. Other classes of pheromone compounds include olefins, methylalkanes, epoxides, esters other t h a n acetates, ketones, phenols, alicyclic alcohols or acetates, spiro compounds, and carboxylic acids. Because the quantity of biologically active material available is low (generally nano-
grams per insect), the analytical chemist is challenged to use every trick in the book to decipher the exact identity of the one or more chemicals t h a t make u p the pheromone message for a particular insect.
Specificity T h e compounds shown in Table I illustrate the array of structural information t h a t needs to be ascertained for full diagnosis of the exact pheromone compound or blend. In addition to identifying the functional group (type and position), carbon skeleton, and molecular weight, one needs to determine the position and geometry of any double bonds and t h e relative a m o u n t s of all compounds t h a t comprise the biologically active message. For example, the females of at least 30 insect species include one or more tetradecen-1-ol acetates in their sex pheromone message (2-4), yet species specificity is preserved through isomeric (geometric and positional) variations and blend compositions. Insects cannot be fooled by compounds t h a t are merely close in structure to their natural sex a t t r a c t a n t s . T h e receptor sites on the a n t e n n a e are so specific that, for o p t i m u m response, the synthetic pheromone m u s t m a t c h in every aspect the pheromone code
ANALYTICAL CHEMISTRY, VOL. 57, NO. 12, OCTOBER 1985
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ULTIMATE GC/FT-IR
Table I. Examples of sex attractant pheromones Sex attractant pheromone structure
Insect
Japanese beetle, Popillia japonica Newman (R,Z)-5-(1-(tecenyl)dihydro-2(3H)-furanone(Japonilure)
American cockroach, Periplaneta americana (L.) (1R,2R.7S,10R) enantiomer of (lZ.5£>-l,lO04Hiiepoxy4
