Insect Eggs

Are openings in stored-product insect eggs related to effectiveness of fumigants?

by S. G. Gautam, Ph.D. G. P. Opit, Ph.D. (Oklahoma State University),and S. Walse, Ph.D. (USDA-ARS)

Fumigants have long been used for effective post harvest management of stored-product insect pests. Coincident with the regulatory phaseout of methyl bromide (MeBr), the use of sulfuryl fluoride (SF) for post harvest disinfestation has increased. However, SF is a species-specific ovicide and eggs of some species require much higher concentration x time exposure or several days of exposure for control (UNEP 2011). The egg stage is the most fumigant-tolerant insect stage and may require higher concentration x time exposure than other life stages. Species-specific response of eggs to SF led us to investigate why some stored-product insect eggs are difficult to kill using fumigants. We compared the egg morphology of driedfruit beetle (DFB) eggs that require relatively higher SF concentrations to kill, to navel orangeworm (NOW), cigarette beetle (CB), and tobacco moth (TM) eggs, which require relatively lower SF concentrations to kill.

Stored-product insect eggs take in gas by diffusion through the chorion (egg wall) and via chorionic respiratory openings, namely, aeropyles and micropyles. Aeropyles (Figure 1) are microscopic holes that open into the inner chorion meshwork and enable continuous gas exchange with the ambient environment. Micropyles are small openings on the surface of the egg wall through which male insect sperms enter the egg.

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Figure 1. SEM pictures of aeropyles. Dried fruit beetle (A), navel orangeworm (B, note the intrachorionic meshwork), tobacco moth
(C, note the intrachorionic meshwork), and cigarette beetle (D, note the many aeropyles with relatively smaller openings).

We compared the abundance, distribution, and location of aeropyles and micropyles on the surface of DFB, NOW, TM, and CB eggs using a scanning electron microscope. We then measured parameters that may influence gas exchange such as, the total aeropylar cross sectional area (i.e., the average cross sectional area of aeropyles multiplied by the average number of aeropyles per egg), the number of aeropyles per square micrometer (μm2), and the surface-to-volume ratio of eggs (Table 1). DFB eggs, which are the most SF-tolerant, had only two aeropyles and the smallest aeropylar area compared to NOW, TM, and CB eggs (Table 1). The movement of gas through egg chorion is slow relative to movement through respiratory openings. Therefore, over an equivalent period of time, eggs with a larger number of aeropyles or micropyles are very likely exposed to more fumigant than eggs with fewer respiratory openings. Since mortality is most often directly related to exposure, eggs with a larger number of aeropyles or micropyles should be easier to kill (Gautam et al. 2014).

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References cited
Gautam, S. G., G. P. Opit, J. S. Tebbets, D. Margosan, and S. Walse. 2014. Egg morphology of key stored-product insect pests of the United States. Annals of the Entomological Society of America: 107: 1-10.
(UNEP) United Nations Environment Programme. 2011. Special review on achieving control of pest eggs by sulfuryl fluoride, pp. 110-136. In Report of the Technology and Economic Assessment Panel. (http://ozone.unep.org/Assessment_Panels/TEAP/Reports/TEAP_Reports/TEAP_Progress_ Report_May_2011.pdf).

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