COMBINING THE CINNABAR MOTH (TYRIA-JACOBAEAE) AND THE RAGWORT FLEA BEETLE (LONGITARSUS-JACOBAEAE) FOR CONTROL OF RAGWORT (SENECIO-JACOBAEA) - AN EXPERIMENTAL-ANALYSIS

1. A field experiment tested the independent and combined effects of the cinnabar moth (Tyria jacobaeae) and the ragwort flea beetle (Longitarsus jacobaeae) on ragwort (Senecio jacobaea). These insect herbivores feed on different stages of the host-plant and at different times of the year and were introduced to North America as biological control agents. 2. Flea beetles alone were found to reduce vegetative ragwort densities by 95%, and flower production by 39%, as compared to plants in control plots. Damage by cinnabar moths was simulated by removing all leaves and capitula from generative plants, but plants were able to regenerate some of the foliage and flowers. The treatment ultimately reduced capitulum production by 77% and the number of achenes per capitulum by 15%. Flea beetle damage was found to reduce the ability of flowering plants to compensate for defoliation and defloration to the extent that capitulum production was reduced by 98% and no viable achenes were produced. 3. These findings support the strategy of introducing complementary enemies which attack different stages and at different times, thereby reducing the number of invulnerable life stage and temporal refuges for the host.