Ecological factors promoting the evolution of colony defense in aphids: computer simulations

Summary

Colony defense has been reported in a limited number of species of aphids. This paper examines which life-historical traits have promoted the evolution of colony defense using two kinds of deterministic simulation models. These models postulate that first-instar larvae can counterattack predators and that the duration of this instar stage is a variable, subject to selection. Prolonging the first-instar span increases the proportion of defenders in the colony, while it results in a delay in reproduction. By calculating the optimal first-instar span, the optimal defensive effort of a colony under various ecological conditions could be estimated. Simulations based on the general model, which regards the number of adults maturing in a period as performance, predicted that a lower birthrate leads to a longer first-instar span (larger investment in defense). This condition also allowed the evolution of dimorphism in the first-instar span, which may ultimately result in the appearance of soldiers. Where birthrate declines with time, the first-instar span was predicted to be prolonged in later stages. Colony duration had little influence on the optimal first-instar span if the season is long enough to repeat generations. The galling-aphid model that assumes a fixed number of generations predicted that a longer duration of colonies leads to a longer first-instar span, but that birthrate has little influence on the optimal first-instar span. A tendency in defense reported in pemphigid aphids was consistent with the prediction from the galling-aphid model.