Movement Patterns and Egg Distribution in Cabbage Butterflies

Abstract

(1) The movement patterns of Canadian (Vancouver) and Australian (Canberra) cabbage butterfly females searching for hosts are described, and the descriptions incorporated in stochastic simulation models. Some of the parameters of the model (those dealing with probabilities of landing and laying) depend on host plant quality and particularly on the age and species of host plant. Other parameters, and particularly the degree of attraction toward hosts, depend on the current fecundity of the butterfly. (2) The models correctly predict observed behaviour patterns, characteristics of Pieris rapae egg distributions described in the literature, and egg distributions produced in field trials during the study. (3) Vancouver females are quite different from Canberra females. The behavioural differences result in less dispersal, a more aggregated distribution of eggs and more efficient search at low plant densities. (4) The two are nonetheless affected in qualitatively similar ways by changes in resource distribution. At any given host density, fewer eggs are laid on more contagiously distributed hosts and egg distributions are more aggregated at low host densities than at high. The effect of contagious host distribution means that more eggs are laid on isolated plants than on plants in small groups, and more on plants in small groups than on plants in large groups. (5) Thus butterfly density tends to vary inversely with host density, despite aggregative behaviour identical to that which, in predator-prey systems, leads to a direct correlation between the densities of searcher and resource. The difference occurs because the butterfly can find the same host repeatedly, and indicates that such a direct correlation (with its stabilizing properties) is much more difficult for a truly non-discriminating parasite to achieve than it is for a predator. (P. rapae can be regarded as a non-discriminating parasite whose prey happens to be a plant.)