There are other considerations in addition to commodity losses caused by insects which justify the development of improved monitoring systems for stored-product infestations. One is the adverse publicity that may befall a business that delivers to the consumer a product which is contaminated with insects. Another is the need for more sensitive and efficient insect detection methods for the various State and Federal regulatory agencies. These agencies are responsible for preventing the introduction of harmful insects from abroad, and for ensuring that processed food is free of insect filth.
Various techniques are often used to monitor for storage insects. These include light, food, and physical traps. Also various methods of examining the commodity for insects, insect parts, or insect fecal material may be employed. Each of these techniques has drawbacks that limit its usefulness. A new and powerful tool that overcomes many of the limitations of other insect monitoring methods, particularly in the area of sensitivity, is the sex pheromone-baited trap.
The number of sex pheromones identified from postharvest insect pests has increased greatly in recent years and pheromones are now known for more than 15 species (see Levinson and Levinson 1979 for recent listings). One might ask whether 15 different sex pheromones would be necessary to monitor these 15 species. Fortunately, the answer is no. Several species share common pheromone components. For example, in addition to various secondary compounds, the almond moth, Ephestia cautella (Walker) [now Cadra cautella 9/97]; warehouse moth, E. elutella (Hübner); Mediterranean flour moth, E. kuehniella (Zeller); raisin moth, E. figulella (Gregson); and Indian meal moth, Plodia interpunctella (Hübner), have (Z,E)-9,12-tetradecadien-1-ol acetate as the major component of their sex pheromone. Traps baited with this chemical are attractive to each of these species. Likewise, a component of the sex pheromone of several species of Trogoderma (dermestid beetle), (Z)-15-methyl-8-hexadecenal, attracts 5 species of Trogoderma to traps containing it as a bait. The use of sex pheromone-baited traps may be further simplified by releasing several sex pheromone baits from the same trap as demonstrated by Vick et al. (1979) for the Angoumois grain moth, Sitotroga cerealella (Oliver) and P. interpunctella.
Reichmuth et al. (1976, 1978) showed that weekly catches of E. elutella in pheromone-baited traps clearly reflected the seasonal population fluctuations in grain warehouses. Levinson and Levinson (1979) surveyed a flour mill in Greece simultaneously for E. kuehniella, E. cautella, and P. interpunctella over an 11-month period using (Z,E)-9,12-tetradecadien-1-ol acetate as a trap bait. Here again, seasonal population fluctuations were clearly shown.
The following results from work by Vick and Coffelt (unpublished data) illustrates the usefulness of pheromone-baited traps to detect hidden insect infestations in packaged food stores. Six pheromone-baited traps were placed in a military food warehouse and distribution center which had no evidence or history of insect infestation. The traps were placed in areas of the building where commodities most likely to be infested were stored. Approximately 100 insects, primarily P. interpunctella and E. cautella, were captured per trap per week for the 17 weeks of the test. Some traps consistently captured many more insects than others, suggesting that the traps can be valuable in pinpointing the location of infestations within a single building. Likewise, Barak and Burkholder (1976) discovered the existence of a previously unknown infestation of T. variable in a warehouse using sex pheromone-baited traps.
Another important role for sex pheromone-baited traps is in helping decide when to apply chemical controls, and then in determining the effectiveness of the control method. Vick and Coffelt (unpublished data0 monitored for 2 years E. cautella populations with sex pheromone-baited traps in large warehouses filled with farmers' stock (unshelled) peanuts. This is one situation where no attempt is made to keep the stored commodity free of insects. Peanut warehouses are not insect tight and to attempt to exclude all insects would be impossible. Rather the warehouseman attempts to keep insect infestation levels below an economic threshold primarily by the use of fumigants. Unfortunately, warehousemen usually rely on guess-work to determine when to fumigate, and they use no monitoring tools after fumigation to determine the success of the treatment. Our monitoring traps indicated that the fumigant treatments as applied in the warehouses under study were effective only against flying adults and insect population levels generally reached their highest levels of the storage season about 4 weeks after a fumigation.
Some practical considerations are summarized from the work of Mankin et al. (1980a, b) to show how the use of monitoring traps can be optimized in a given application. In a confined space such as a warehouse, the effectiveness of a pheromone trapping system for surveying insect populations can be optimized by the proper choice of trap location and pheromone release rate. The active space of a trap, defined as the area within which the trap is attractive to insects, is determined by the pheromone release rate and the position of boundaries. If the release rate is less than about 0.001 µg/h, the pheromone concentration falls below the threshold of detection within a short distance of the trap, and the active space is small. The active space may extend to a radius of 10 m or more when the release rate is increased to 0.1 µg/h. Release rates above this level may be counterproductive for several reasons. First, regardless of the release rate, the active space of a trap located within a sheltered area or cul de sac tends to be confined within that area. Also, the location of a highly circumscribed insect population can be detected more readily when the traps have small active spaces. At release rates higher than about 10 µg/hr the pheromone concentration near the trap is so high that the insect alters its searching behavior in a manner that decreases the probability of the insect being captured. Finally, an insect habituates quickly to high concentrations of pheromone and less quickly to low concentrations. On the basis of these considerations, the optimal system for insect surveys may be a series of traps, spaced 5-10 m apart, releasing 0.01-0.1 µg/hr. The dispensers described by Vick et al. ( 1979) for (Z,E)-9,12-tetradecadien-1-ol acetate fall within this release rate range.
In conclusion, synthetic sex pheromone for most of the important insect pests are available commercially in formulations that permit the optimization of trapping effectiveness. Sex pheromone-baited traps have been applied successfully to several monitoring problems already, and they appear to hold considerable promise as monitoring tools for regulatory agencies and industry.
Barak, A. V. and W. E. Burkholder. 1976. Trapping studies with dermestid sex pheromones. Environ. Entomol. 5: 111-114.
Levinson, H. Z. and A. R. Levinson. 1979. Trapping of storage insects by sex and food attractants as a tool of integrated control. In F. J. Ritter, ed. Chemical Ecology: Odor Communication in Animals. Elsevier/North Holland Biomedical Press, Amsterdam.
Mankin, R. W., K. W. Vick, M. S. Mayer, J. A. Coffelt, and P. S. Callahan. 1980a. Models for dispersal of vapors in open and confined spaces with applications to sex pheromone trapping in a warehouse. J. Chem. Ecol. 6: 929-950.
Mankin, R. W., K. W. Vick, M. S. Mayer, and J. A. Coffelt. 1980b. Anemotactic response threshold of the Indian meal moth, Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae), to its sex pheromone. J. Chem. Ecol. 6: 919-929.
Reichmuth, von Ch. H. V. Schmidt, A. R. Levinson, and H. Z. Levinson. 1978. Die Fängikeit Pherombeködertes Klebefallen für Speichmotten (Ephestia elutella Hbn.) In unterschiedlich dicht befallenen Getreidelogern. Z. Angew. Entomol. 86: 205-212.
Reichmuth, von Ch. R. Wohlgemuth, A. R. Levinson, and H. Z. Levinson. 1976. Untersuchchungen über den Einfatz von pheromonbeköderten Klebefallen zur Bekampfung von Motter im Vorratschutz. Z. Angew Entomol. 82: 95-102.
Vick, K. W., K. Kvenberg, J. A. Coffelt, and C. Steward. 1979. Investigation of sex pheromone traps for simultaneous detection of Indian meal moths and Angoumois grain moths. J. Econ. Entomol. 72: 245-249.
2Postdoctoral fellow, employed through a cooperative agreement with the Insect Attractants, Behavior, and Basic Biology Research Laboratory and the Department of Entomology and Nematology, University of Florida, Gainesville, Florida 32611
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Last modified September 23, 1997