Differences between the external morphology of dried fruit beetle (DFB) and navel orangeworm (NOW) eggs

Dried fruit beetle (DFB; Carpophilus hemipterus) and navel orangeworm (NOW; Amyelois transitella) are endemic to California (CA) and are key economic pests of the dried fruit and nut industry. Post-harvest chamber fumigation is a critical element of the DFB and NOW pest management strategy. This work was conducted in the context of overcoming ovicidal deficiencies of postharvest fumigants in general. We studied external morphology of DFB and NOW eggs using a scanning electron microscope to identify any structural differences that may explain the relative tolerance of DFB eggs and susceptibility of NOW eggs to sulfuryl fluoride. We found that aeropyles, which enable gaseous exchange by the living embryo, are very few (2) in DFB and are restricted to anterior end only. On the other hand, NOW has a higher number of aeropyles (7.2 ± 2.1) distributed on both the anterior and posterior ends of the egg. The micropyle, 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 of fumigant entry may be predominantly diffusion through the chorion. We speculate that the rate of fumigant penetration through chorion is comparatively lower when compared to diffusion through aeropyles and/or micropyle. Our observations could explain the tolerance of DFB eggs and the relative susceptibility of NOW eggs to sulfuryl fluoride.