Natural enemies may be a cause of discrete generations in tropical insects

Abstract

It is commonly believed that the marked seasonally of temperate regions results in the insects that live there often having discrete generations, whereas in the tropics, generations are more likely to overlap because of the more-or-less continuously favourable conditions. These sweeping generalizations, however, are misleading. Several examples are now known of tropical insect populations that tend to show discrete generations, both under equilibrium^1–5 (Fig. la) and non-equilibrium conditions^6–9, which are not seasonally determined. We are also struck by the frequency with which laboratory insect populations cultured under constant conditions show discrete generations^10–12 (Fig. 1b). A common factor in these examples is the presence of insect parasitoids as a major mortality factor. Here we demonstrate, using theoretical population models, one way in which parasitism can be the cause of approximately discrete generations. In particular, we show that the ratio of the lengths of the host and parasitoid life cycles is of prime importance in determining whether generations tend to be discrete or continuous and discuss the importance of this in biological control.