Invasion by the Variegated Leafhopper and Biotic Interactions: Parasitism, Competition, and Apparent Competition

Abstract

In 1980 the variegated leafhopper (VLH), Erythroneura variabilis, invaded California's San Joaquin Valley, and has since rapidly increased in numbers to become a major pest in vineyards. Correlated with the invasion by VLH are declining populations of congeneric grape leafhopper (GLH), E. elegantula. This paper explores interspecific competition and shared parasitism as possible reasons for this decline in GLH. Although interspecific competition between VLH and GLH was evident in field—cage experiments, its effects were equivalent to instraspecific competition. Instead of simple direct competition explaining the replacement of GLH by the invading VLH, the key appears to be parasitoid that is shared by two leafhoppers. In particular, the native GLH experiences higher attack rates from the shared parasitoid (Angarus epos) than does the invading VLH. This differential parasitism apparently shifts the competitive balance from one of equality to a strong disadvantage for the native relative to the invader. This is the first study, to our knowledge, to document the importance of parasitoid—mediated indirect effects in an invasion.