Some consequences for a parasitic herbivore, the milkweed longhorn beetle, Tetraopes tetrophthalmus, of a host-plant shift from Asdepias syriaca to A. verticillata.

The impact of differences in host plants on individuals and populations of insect herbivores was investigated using the milkweed longhorn beetle, Tetraopes tetrophthalmus (Forster), larvae of which feed parasitically on host rhizomes. One host, Asclepias syriaca L., was larger in stem and rhizome diameter and grew in cooler soil than the other host, A. verticillata L. The major effects on beetles were retarded phenology at the cooler site and reduced size on the smaller host. Reduced size of beetles was correlated with several important individual attributes: reduced length of life, number of ovarioles, egg size, and a reduced probability of mating with large beetles. The population consequences of these characteristics, largely inferred from these observations, were little or no outbreeding in the small population on A. verticillata.