Larval competition and egg-laying decisions by the bean weevil, Callosobruchus maculatus

Previous studies have shown that female Callosobruchus maculatus (F.) can accurately assess the number of eggs already laid on a bean; an absolute measure of egg load probably determines the female's egg-laying decisions. In this study relative fitness gain is assumed to determine the probability that a female accepts beans with different egg loads. Models were constructed to describe the egg-spacing dynamics of females that follow three decision rules: a random, an absolute and a relative rule. Comparison of the uniformity of egg distribution achieved by the females under different conditions and of four other geographical strains with the predictions of the models leads to the conclusion that a relative rule incorporating the degree of larval competition determines egg-laying decisions for the females of the four geographical strains with strong larval competition, while an absolute rule is used by the strain with weak larval competition. Based on the model, a quantitative relationship between larval competition and the female's egg-spacing behaviour was also constructed. The evolution of egg-laying decision rules is discussed. (C) 1997 The Association for the Study of Animal Behaviour.