Comparative Egg Morphology and Chorionic Ultrastructure of Key Stored-Product Insect Pests

Eggs of Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and Plodia interpunctella (Hübner) (Lepidoptera: Pyralidae) were imaged with scanning electron microscopy to explore how respiratory openings on the chorion surface may be related to the efficacy of fumigants. Each P. interpunctella egg had many aeropyles and several micropyles, whereas each T. castaneum egg had neither aeropyles nor micropyles. Transmission electron microscopy was used to obtain cross sectional images of chorions, with eggs of T. castaneum, P. interpunctella, Carpophilus hemipterus (L.) (Coleoptera: Nitidulidae), and Amyelois transitella (Walker) (Lepidoptera: Pyralidae) each having unique chorionic ultrastructure. The types of layers and relative thicknesses of different layers of the chorion varied across species. Exochorion of lepidopteran eggs was a lamellate structure and the thickest layer, whereas crystalline endochorion was the thickest layer in coleopterans. Although quantitative data on fumigant uptake by eggs of different insect species are needed, the findings of the current study suggest that species-specific tolerance of eggs to fumigants may partly be explained by differences in respiratory structures and chorion characteristics.