Genetic and phenotypic variation in diet breadth in a generalist herbivore

The genetic and plastic components of polyphagy were investigated in a population of Lymantria dispar, the gypsy moth. A simple genetic experiment assessed the expression of (1) genetic variability in life history traits within each of four environments, (2) genetic variability in diet breadth, expressed as a change in the ranks of family performance across hosts, and (3) homeostasis (equivalent performance by a family across hosts) versus phenotypic plasticity (variable performance by a family across hosts). Sibs from each of 14 families, randomly selected from a single population, were reared on four diets: two natural hosts - chestnut and red oak, and two synthetic hosts - a standard laboratory diet and a low-protein version of this diet. Average population performance, measured in terms of development time and pupal weight, was better on standard laboratory diet than on low-protein diet, and was equal on chestnut and red oak for pupal weight, but better on chestnut oak for development time. Average population performance provided no information about the genetic component of host use ability. The gypsy moth expressed genetic variation in development time within each host environment and in pupal weight within natural host environments. Phenotypic plasticity was expressed by a significant number of families in development time and pupal weight across synthetic hosts and, to a lesser extent, across natural hosts. It was only across natural hosts that genetic variation in diet breadth was expressed, and this was confined to females. Genetic variability in diet breadth may be maintained in this species as a consequence of the unpredictability of its food sources.