Effects of Vegetation Disturbances on Insect Biological Control of Tansy Ragwort, Senecio Jacobaea.

Abstract

Our study had two major objectives: (1) to clarify the roles of buried seed and different types of localized disturbance in activating outbreaks of a pasture weed (tansy ragwort, Senecio jacobaea), and (2) to measure the effectiveness of two natural enemies (the cinnabar moth Tyria jacobaeae and a ragwort flea beetle, Longitarsus jacobaeae) in inhibiting weed population increase and spread. We conducted a 5-yr field experiment on the coast of Oregon using a randomized-block design with four blocks x three levels of disturbance (background vegetation was Tilled, Clipped, Unaltered) x two levels of cinnabar moth (Exposed, Protected) x two levels of flea beetle (Exposed, Protected) = 48 plots (each plot was 0.25 m^2). Disturbance consistently increased ragwort abundance (measured as density of juveniles, adults, and their offspring; cover; and biomass); the effect was generally greater in Tilled compared to Clipped disturbance treatments. We also found striking differences in the contribution of each natural enemy to ragwort control. The flea beetle quickly reduced ragwort survival, and this led to a strong and rapid reduction in ragwort abundance. The cinnabar moth reduced ragwort fecundity, but this did not translate into reductions in ragwort cover (measured in 1986 and 1987) or biomass (measured annually from 1986 through 1990). These results establish that (1) ragwort populations were limited more by availability of microsites for germination and establishment than by availability of seed, (2) the ragwort flea beetle was the key factor regulating ragwort abundance, and (3) reduction in ragwort fecundity by the cinnabar moth had little effect on the dynamics of ragwort populations on local scales of space and time. These findings underscore the value of field experiments for investigating the dynamics of biological control systems, the manner in which they are regulated, and their response to perturbation. They further establish how colonization and invasion by ragwort depend on attributes of the disturbance and of natural enemy regimes.