Influence of egg morphology on ovicidal efficacy of fumigants

Abstract: There is great variability in the way eggs of different stored-product insect pests respond to fumigation treatments. Doses of sulfuryl fluoride required to kill Carpophilus hemipterus (L.) (Coleoptera: Nitidulidae) eggs are higher than those required to kill Lasioderma serricorne (Fabricius) (Coleoptera: Anobiidae), Ephestia elutella (Hübner) (Lepidoptera: Pyralidae), and Amyelois transitella (Walker) (Lepidoptera: Pyralidae) eggs. No published explanation for this variability exists. The external morphology of insect eggs has been extensively studied using the scanning electron microscope (SEM). However, these studies have focused on elucidating external morphology of insect eggs in order to develop phylogenetic relationships and to facilitate tracing origin of infestations in domestic and international trade. No studies have addressed the relationship between respiratory structures on the chorion surface that facilitate gaseous exchange and relative tolerance of insect eggs to fumigants. Therefore, the objective of our study was to use a SEM to compare the abundance, distribution, and location of egg respiratory structures of C. hemipterus with those of L. serricorne, E. elutella, and A. transitella in order to determine how respiratory structures of stored-product insect eggs may be related to their tolerance to fumigants. We found that C. hemipterus eggs have only two aeropyles at the anterior pole, and no micropyle(s). E. elutella had many aeropyles and a single micropyle per egg whereas each A. transitella and L. serricorne egg had many aeropyles and several micropyles. In C. hemipterus eggs with only 2 aeropyles, the route for fumigant entry may predominantly be diffusion through the chorion whereas in L. serricorne, E. elutella, and A. transitella entry is mostly through aeropyles and micropyle(s). Fumigant entry into eggs by diffusion through the chorion may be comparatively slower compared to entry through aeropyles and micropyles. Although confirmatory measurements of fumigant diffusion into eggs are needed, our findings provide a possible explanation for why C. hemipterus eggs are more tolerant to fumigants than L. serricorne, E. elutella, and A. transitella eggs.