In some insects, a female's egg-laying choices completely determine the quality of food available to her sedentary offspring. Female seed beetles, for example, glue eggs to legume seeds, and the hatching larva burrows into the seed directly beneath the egg-laying site. Not surprisingly, females avoid adding eggs to seeds that already bear eggs, and thus minimize the degree of competition that their larval progeny encounter within seeds. We have been interested in understanding how such nonrandom behavior evolves.

In one project, Geoff Morse helped determine the mechanisms underlying the rather different egg-laying behaviors observed in populations of the seed beetle Callosobruchus maculatus from India and Brazil. Previous work had shown that Indian females were extremely adept at detecting and avoiding egg-laden seeds, while Brazilian females laid eggs in a nearly random manner. We proposed three mechanisms to account for this difference: 1) Indian females are simply more sensitive to the presence of eggs on seeds; 2) eggs from the Indian population are more deterrent to further egg-laying; or 3) both. We presented females with a choice between egg-laden seeds and egg-free seeds and varied both the source of the eggs and the source of test females. The results supported hypothesis 3; females from the Indian population showed a keener response to presence of eggs no matter which population supplied the eggs, and, eggs from Indian females were more deterrent to further egg-laying no matter which population supplied the test females. These experiments address proximate (mechanistic) reasons for the different egg-laying patterns but do not consider ultimate (evolutionary) explanations. Further work suggests that the strong avoidance of occupied seeds by Indian females is related to the extreme competitiveness of their larvae.

A second project asked, can females also distinguish between seeds bearing their own eggs and seeds bearing eggs laid by other females? If egg-free seeds are no longer available, a female with this ability can preferentially place her eggs on seeds bearing unrelated eggs and thus avoid competition between her own offspring. Discrimination between hosts bearing "self" or "nonself" eggs has been demonstrated in some parasitic wasps, but the trait may be rare in other insects. Tiffany Tinney helped tackle this question by presenting a female with a mixture of seeds bearing either her own eggs or unrelated eggs. We found that a female was not able to recognize which seeds bore her own eggs, and thus was unable to reduce the chances that her offspring would compete with each other within seeds.

Although the egg-laying choices of seed beetles tend to be highly nonrandom, there may be limits to their degree of "sophistication." Further research on egg-laying behavior in insects must consider how variation in this trait is related to other aspects of the insect's life cycle.