Differences in egg morphology between coleopteran and lepidopteran stored-product insect pests in relation to tolerance to fumigants

Driedfruit beetle (DFB, Carpophilus hemipterus, Coleoptera: Nitidulidae) and navel orangeworm (NOW, Amyelois transitella, Lepidoptera: Pyralidae) are endemic to California and pose a threat to the California dried fruit and nut industry. Post-harvest chamber fumigation is a critical element of the pest management strategy. However, eggs of DFB are tolerant to fumigants and are difficult to kill using sulfuryl fluoride and propylene oxide, whereas NOW eggs are relatively susceptible. Therefore, we studied the external morphology of DFB and NOW eggs using a scanning electron microscope to identify any structural differences that may explain their differential response to sulfuryl fluoride. We found that aeropyles, which enable gaseous exchange in eggs, are few (2) in DFB and restricted to only one end of the egg. On the other hand, NOW has a greater number of aeropyles (7) distributed on both ends of the egg. The micropyle, which is an opening for sperm entry that also facilitates gaseous exchange, is absent in DFB but present in NOW. It is possible NOW eggs with a micropyle and several aeropyles may have a higher rate of fumigant penetration than DFB. In DFB eggs with only 2 aeropyles, the route for fumigant entry may predominantly be diffusion through the chorion.  However, fumigant entry into the egg by diffusion through the chorion may be comparatively slower compared to entry through aeropyles and/or micropyle. Our findings provide a possible explanation for why DFB eggs are more tolerant to fumigants than NOW eggs.