Use of Pheromones in Insect Control Programs: Slow Release

22 Use of Pheromones in Insect Control Programs: Slow Release Formulations B. A. BIERL, E. D.

DEVILBISS,

and

J.

R. PLIMMER

Downloaded by FUDAN UNIV on November 20, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0033.ch022

Agric. Environ. Qual. Inst., USDA, Beltsville, Md. 20705

Among the newer strategies for insect pest management are techniques that make use of naturally occurring compounds that affect the behavior of insects. These compounds are extremely specific in their action and have minimal impact on non-target species. They must be protected during the period required for activity because they are usually readily degradable by oxidants or sunlight. For this purpose, synthetic or natural polymers may be used as permeable coatings to protect labile materials and permit their controlled release. The U.S. Department of Agriculture supports a number of pest control programs involving the use of insect attractants. Such compounds are ideally suited for incorporation in controlled-release formulations. We have examined a number of these formulations and studied some properties that affect the practical application of polymeric formulations for release of attractants, including the emission rate and i t s dependence on temperature and other parameters. It i s to be anticipated that more sophisticated theoretical treatment can ultimately provide a direction for improved technology. During the past decade, increased knowledge of the chemical basis of sensory communication among insects has contributed to the science of pest management. Mating behavior, aggregation attraction to sites for feeding or oviposition, and other forms of insect behavior are mediated by chemicals known as "pheromones". Such chemicals are used for communication among members of the same species. In particular, some lepidopteran species are equipped with well-developed antennae which can detect pheromones at extremely low concentrations. Often a specific blend of two or three chemicals i s used by a particular insect species. However, i n the case of the gypsy moth, Lymantria dispar (L.), evidence to date indicates that a single compound, identified as cis-7,8-epoxy-2-methyloctadecane. disparlure, i s the sex pheromone emitted by the virgin female to attract the male (1). The last decade has witnessed considerable advancement i n our knowledge of the chemistry of natural pheromones, largely because instrumental techniques have improved to the extent that microgram 265 Paul and Harris; Controlled Release Polymeric Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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quantities of material can be used for structural studies. The minute amounts of pheromones that are secreted by the insect may be obtained by extraction or by collection of volatiles emitted by caged insects. Sex attractant pheromones are emitted by i n sects of one sex to attract the other and stimulate mating. The value of virgin female moths as baits for trapping was recognized during the eighteenth century. The use of synthetic chemicals as attractants for trapping insects i s a well-established practice and the identification of pheromones has supplemented and extended the value of this technique. Insect traps containing chemical baits are used for surveying or monitoring insect infestations. In some cases, grids of traps may be adequate to achieve some degree of population control by mass trapping. Damage to ornamental or economic plants may thus be reduced. Trapping for survey purposes i s important i f i n secticide applications are to be used effectively. Reductions i n cost and i n chemical contamination of the environment can be achieved i f insecticides are applied on a local basis only when tie need exists. The use of attractant traps for proper timing of insecticidal spray applications has achieved considerable savings i n the cost of insecticides used to control f r u i t f l i e s (2). Other benefits may be cited. For example, regular applications of orchard sprays to control the codling moth, Laspeyresia pomonella (L.), have affected the balance of population of other insects with the result that some of these have now become major pests. Traps baited with the sex attractant pheromone, trans,trans-8,10-dodecadien-lo l (codlelure) (3,4) have been used to improve timing of pesticide applications and to reduce their frequency. Thus, savings i n costs have also been accompanied by benefits i n pest management practices (5^,7), A promising technique for control of some insect species i s the use of pheromones for reduction of mating. Permeation of the infested area with sex attractant pheromones or related compounds may lead to disorientation of the male and a reduction i n frequency of mating. Effective reduction of mating should result i n a decline of the pest population. Experimental evaluation of this technique has demonstrated i t s a b i l i t y to reduce mating of the gypsy moth, the pink bollworm, Pectinophora gossypiella (Saunders), and other insect pests. In one experiment (&), the sex pheromone of the pink bollworm, gossyplure, was evaporated into the a i r from evaporators spaced at 40 m. intervals i n a l l cotton fields i n the Coachella Valley of California. The amount of larval b o l l infestation was comparable to that provided by conventional insecticide applications and there was about a month delay i n the onset of larval infestations i n the b o l l s , as compared to previous years. Mating reduction of the gypsy moth has been successfully achieved using formulations of the sex pheromone, disparlure (£, 10). Disparlure i s a stable l i q u i d of relatively low v o l a t i l i t y

Paul and Harris; Controlled Release Polymeric Formulations ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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that can be evaporated slowly into the a i r from a wick or reservoir. However, consistently successful application of a control technique based on a i r permeation must depend on formulations of 'delivery systems' that can be used under a variety of environmental conditions. Demonstration of the value of a particular chemical i n pest control i s of great s c i e n t i f i c interest, but practical application i s necessary to prove i t s worth. The use of pheromones has great promise since they are species-specific and w i l l not adversely affect predators or beneficial insects. Environmentally, they present a few problems since they are effective at application rates of few grams per acre (2 to 16 grams per acre i n the case of disparlure). Chemicals that are readily degradable in the environment and that can be used at such extremely low rates of application offer valuable alternatives to conventional pest control chemicals. Therefore, research efforts to demonstrate the use and practical application of pheromones as alternative techniques of pest control are being undertaken i n many parts of the U.S. Success depends i n part on the use of the correct pheromone or blend, combined with a reliable technique for release into the environment. Our research on formulation of insect pheromones had the following two major objectives. The f i r s t was to achieve optimum rates of release from a dispenser that could be used i n a survey trap. The second was to provide a formulation that would permeate the forest atmosphere with the desired concentration of pheromone throughout the mating period of the insect. The principal topic of the discussion to follow i s the gypsy moth and i t s attractant pheromone, disparlure. The gypsy moth is a serious pest of forest, shade, and orchard trees i n the northeastern U. S. and i n Europe. Larvae emerge i n early May from eggs laid i n the previous year. Leaves are consumed by the larvae and the resulting defoliation k i l l s many trees. The larvae pupate and emerge as adult moths i n early summer; a f l i g h t period of six to eight weeks follows. The male i s a strong f l i e r but i n North America, at least, the female does not normally f l y . The female emits the pheromone to attract the male for mating. For effective use of disparlure i n traps, the rate of emission of disparlure from the source must be similar to that from the female moth and should remain at that level throughout the mating period. Several types of dispensers have been used. -, The one i n current use (the Hereon® dispenser, manufactured by — the Hereon Div. of Health-Chem Corp., New York, N.Y., 10010) has proved effective for the controlled emission of disparlure and a number of other insect pheromones (11). It consists of a plastic laminate fabricated i n three layers; two outer plastic 1/ Mention of a proprietary product does not imply endorsement by the USDA.


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layers cover an inner layer impregnated with disparlure. The rate of emission of lure i s controlled by i t s concentration and by the thickness of the outer layers (Table I ) . Table I.


% Lure 2.4 2.0 2.0 5.0 6.0 3.9

Emission Rate of Hereon Dispenser

mils

Per Dispensers yg/hr * mg lure

7 12 21 7 13 6

3.4 4.8 7.4 7.0 14.4 6.2

0.10 0.09 0.08 0.24 0.22 0.22

Season Used

-

1974

1975

* Emission rate at room temperature By using a simple technique for trapping and measurement of released pheromone (12) , wejiave shown that the emission rate of disparlure from a Hereon © dispenser under controlled conditions is relatively constant (13, 14). The emission rate of disparlure from the dispensers used i n 1974 f i e l d tests was very low (0.1 yg/hr) i n proportion to the amount of material originally present. Although the rate of emission remained constant over extremely long periods, the loss of disparlure was low. Approximately 90% of the lure remained i n dispensers that had hung i n traps for the entire season. Current studies of emission rates have taken into account not only concentration and thickness of outer layers of the dispenser but also factors such as temperature and a i r movement. The dispensers used i n 1975 f i e l d tests, manufactured i n 1975,again contained 6 mg disparlure but the thickness of the laminate was only 6 mils. From such a dispenser, the rate of emission was 0.24 yg/hr at 80°F i n a constant flow of dry a i r at 100 ml/min (Table I I ) . Table I I . Temperature Dependence of 1975 Hereon Dispensers Temp

yg/hr *

80° F 90° F 100° F

0.24 0.45 0.83

Rate of Increase:

1.8 yg/hr per 10° F

* Emission rate for dispenser containing 6.2 mg of lure The rate approximately doubled with each increment of 10°F., and at 100°F, the rate was 0.83 yg/hr. Under our experimental condi-



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tions, the increase i n emission rate with flow rate was linear. Emission rates for efficient trapping cannot be arrived at by precise laboratory measurement. Biological and atmospheric variations w i l l be responsible for wide fluctuations i n the concentration of the natural pheromone. Overall constancy of emission rate from the dispenser must be maintained throughout the f l i g h t period, but emission must also be maintained at an adequate rate over the range of environmental conditions encountered i n the f i e l d . Hereon © dispensers appear to satisfy these conditions. Thus, current dispenser technology appears adequate but there are other subtle factors which may enter into the consideration of insect trapping efficiency. Mating disruption techniques that rely on permeation of the forest environment by pheromones are subject to similar limitations. The technical problem i s that of providing a uniform concentration of pheromone i n the forest atmosphere throughout the f l i g h t period of the gypsy moth. Conventional formulations, such as those used for pesticides would require application at frequent intervals. Since this i s impractical, a controlled-release formulation i s essential. The effects of environmental conditions on such formulations would be expected^to be even more important than those observed with the Hereon® dispensers. It i s important that an effective level of pheromone be maintained i n the forest atmosphere during the entire f l i g h t period of the moth. One of the most promising methods of controlled release of insect pheromones i s microencapsulation. Although various methods of encapsulation are available, our own research has been primari l y directed toward the study of the behavior of gelatin-walled microcapsules containing a 2% solution of disparlure i n xylene/ amyl acetate. The capsules range from 25 to 250 microns i n size and are formulated by the National Cash Register Corp., Dayton, Ohio. The formulation was applied as an aerial spray at the rate of 2 to 16 grams disparlure per acre. To improve the performance of the microencapsulated material during application and in the f i e l d , thickeners and stickers were added. Such additives w i l l affect rates of emission and our laboratory studies were aimed at accumulating comparative data on formulations of varying composition. Ultimately we hope to correlate laboratory data with f i e l d behavior so that the performance of candidate formulations may be predicted from the laboratory results. During the 1975 f i e l d season, microencapsulated disparlure was applied by a i r over large acreages i n Pennsylvania, Massachusetts, New Jersey, and Maryland; by taking f i e l d samples, we had an opportunity to observe the behavior of microcapsules under practical conditions. Subsequently, these observations were supplemented by laboratory measurements. However, our ultimate goal i s to study the distribution of disparlure under forest conditions. Experiments to measure disparlure concentrations i n a i r have been successful on experimental plots and further experiments along these lines w i l l be continued during


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the 1976 f i e l d season. Emission rates of the microcapsules produced i n 1975 were determined for 250 mg of capsules containing 4.2 mg of disparlure. The capsules were mixed with 2 other ingredients of the formulation, hydroxyethyl cellulose and Rhoplex B-15, and were then allowed to dry overnight on a microscope slide. Dry a i r was continuously passed over the capsules for 672 hours (28 days) at 80°F. Over this period there was no reduction i n emission rate, although some fluctuations were observed. The average rate of emission was approximately 2 ug per hour and at this rate, the lure content would be reduced to 50 percent of i t s original value after 43 days. The experiment did not take into account factors such as humidity, temperature variations, sun, etc. In another experiment, microscope slides coated with capsules were maintained indoors at room temperature; outdoors,inverted, covered; outdoors, inverted, uncovered; and outdoors, totally exposed to the effects of sunlight, a i r , etc. Under these conditions, the time required for the disparlure content of the capsules to f a l l to half of i t s i n i t i a l value was 123 days, 34 days, 15 days and 10 days respectively (Table I I I ) . Table III. Aging Effects of Microencapsulated Disparlure

Location

Rain

Indoors Outdoors Outdoors Outdoors

No No No Yes

Exposed to Sun Wind No No Yes Yes

No Yes Yes Yes

H/2 * (days) 123 34 15 10

* Based on residual lure contents Many capsules, especially those of larger diameter, appeared to lose their spherical shape; this collapse of the capsule wall seemed to be related to high humidity. The effects of humidity and temperature on the 1975 NCR capsules were determined i n the laboratory. Increasing temperature had a greater effect on the emission rate of disparlure from microcapsules than i t did with the Hereon© dispensers; the rate from capsules was 0.59 yg/hr at 100°F (Table IV).


22.

BiERL E T A L .

Table IV.

Pheromones

80°F 90°F 100°F Rate of Increase:


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NCR Microencapsulated Disparlure

Temp.

*

in Insect

Ug/hr 0.59 1.6 4.2 2.7 ug/hr per 10°F

Emission rate for 4.2 mg of lure encapsulated i n 250 mg of wet capsules

The rate had therefore increased by a factor of 2.7 per 10°F compared with a factor of 1.8 for the Hereon® dispensers. I f the a i r passing over the capsules was saturated with moisture there was also an increase i n emission rate; the effect of passing humidified a i r over capsules for one week was to i n crease the rate of emission by 13-26%. However after 20 days of aging there was a 30 to 40% reduction of emission rate i n moist air compared with that i n dry air. ^ The rate of emission of disparlure from Hereon dispensers or NCR microcapsules was affected by environmental parameters i n a predictable manner. Although minor modifications of the formu lations affected the rate of emission, the observed changes were not comparable i n magnitude to those resulting from temperature variations or fluctuations i n other environmental parameters. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8.

9.

B i e r l , Β. Α., Beroza, Μ., and Collier, C. W. Science (1970) 170, 87-9. Chambers, D. L., Cunningham, R. T., Lichty, R. W., and T h r a i l k i l l , R. B. Bioscience (1974) 24, 150-2. Roelofs, W., Comeau, Α., Hill, Α., and Milicevic, C. Science (1971) 174, 297-9. Beroza, Μ., B i e r l , Β. Α., and Moffitt, H. R. Science (1974) 183, 89-90. Batiste, W. C. Environ. Entomol. (1972) 1, 213-8. Batiste, W. C., Berlowitz, Α., Olson, W. Η., DeTar, J. Ε., and Loos, J. L. Environ. Entomol. (1973) 2, 387-91. Madsen, H. F., and Vakenti, J. M. Environ. Entomol. (1973) 2, 677-9. Shorey, Η. Η., Gaston, L. Κ., and Kaae, R. S., i n "Pest Management with Insect Sex Attractants and Other Be havior-Controlling Chemicals", M. Beroza, ed. ACS Symposium Series, Number 23. American Chemical Society, Washington, D.C. 1976. pp. 67-74. Beroza, Μ., Hood, C. S., Trefrey, D., Leonard, D. E., Knipling, E. F., Klassen, W., and Stevens, L. J. J. Econ. Entomol.


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10. 11.

12. 13.

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(1974) 67, 659-64. Cameron, Ε. Α., Schwalbe, C. P., Beroza, Μ., and Knipling, E. F. Science (1974) 183, 972-3. Beroza, M., Paszek, E. C., Mitchell, E. R., B i e r l , Β. Α., McLaughlin, J . R. and Chambers, D. L. Environ. Entomol. (1974) 3, 926-8. Beroza, Μ., B i e r l , Β. Α., James, P, and DeVilbiss, D. J . Econ. Entomol. (1975) 68, 369-72. Beroza, Μ., Paszek, E. C., DeVilbiss, D., B i e r l , Β. Α., and Tardif, J. G. R. Environ. Entomol. (1975) 4, 712-4. B i e r l , Β. Α., and DeVilbiss, D., Proceedings of the Inter national Controlled Release Pesticide Symposium, Dayton, Ohio; September, 1975. PP. 230-46.


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Use of Pheromones in Insect Control Programs: Slow Release

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