Interspecific Competition Between Folivorous Insects on Erigeron Glaucus

Two abundant herbivorous insects, Philaenus spumarius, the meadow spittlebug (Homoptera: Cercopidae), and Platyptilia williamsii, the calendula plume moth (Lepidoptera: Pterophoridae) both spend their immature development on new leaves of Erigeron glaucus. Field experiments were conducted in the natural environment at Bodega Bay, California, to test the persistence and growth of each of these herbivores when they did and when they did not co—occur with the other species. The presence of spittlebugs had no effect on the persistence of plume moths. However, during each of two years, spittlebugs that co—occurred with plume moth caterpillars had lower persistence times than those without caterpillars, and presumably had lower survival. Spittlebug persistence was correlated with production of new leaves by the host plant; plume moth larvae consumed the terminal bud and greatly reduced new leaf production. Spittlebugs on buds that were experimentally made unavailable to spittlebugs for food and refuge had low persistence times, as did spittlebugs on rosettes that had been damaged by moths during the previous winter. This asymmetric interspecific interaction had significant effects on spittlebug populations. Plume moths and spittlebugs come in contact often under natural conditions, and the number of spittlebugs in one year was influenced by persistence of spittlebug nymphs in the previous year. Plume moths and spittlebugs were also found to co—occur on individual rosettes of the host less often than predicted from their abundances. This study and other examples of interspecific competition between plant—feeding insects indicate that more field experiments are needed before any conclusions can be drawn about the significance of competition for insect herbivores.