Evolutionarily stable seasonal timing of univoltine and bivoltine insects

Abstract

The evolutionarily stable seasonal timing of larval feeding stages for univoltine and bivoltine insects is analyzed, assuming that hatching and pupation dates are under natural selection. A first case assumes that competition among feeding larvae can be neglected. Under this assumption, bivoltine life cannot be optimal if the growth rate increases in proportion to the body weight. If growth rate increases slower than proportionality, and if the resource availability peaks in the middle of the season, a symmetric bivoltinism can be optimal in which the larval periods of the two generations are equal. When resource availability peaks early in the season and then decreases, the first generation has a shorter larval feeding stage and a larger size at pupation than the second generation. The bivoltine life cycle is more likely to be superior to the univoltine one if (!)growth is fast; (2) suitable growing season is long; (3) biomass loss during nonlarval stages is small; and (4) egg size is small. A second case assumes that the growth rate decreases with the larval biomass in the population because of competition; the evolutionarily stable state (ESS) is analyzed for univoltine insects with this assumption. The population at the ESS is composed of a mixture of phenotypes differing in hatching and pupation, but the pupation interval over which some pupation occurs every day, is much longer than the hatching interval. If the growth rate per unit weight decreases with body weight, large larvae should pupate earlier than small ones.