Distributions and interactions of insect herbivores as influences on host plant density and performance

Biological control programmes to reduce the density of invasive weeds often introduce multiple species of insect herbivores before reductions of the weed population occur. The factors leading to successful control need to be identified to improve success rates and reduce the number of insect introductions. These factors may be traits of the biocontrol agent-plant system, or may be external influences such as the presence of other biocontrol agents or environmental conditions. A characteristic that varies among insect herbivores is their distribution among plants. I developed a simulation model that demonstrates how insects distributed in direct proportion to plant density cause the fastest rate of host plant population decline. This agrees with observations of a successful species, Larinus minutus, compared to an unsuccessful one, Urophora affinis, in the diffuse knapweed (Centaurea diffusa) biocontrol system. Using the successful biocontrol agent, L. minutus, I experimentally tested whether adult as well as larval feeding increased control of diffuse knapweed. My results suggest that agents with both adult and larval feeding can control plants in a wider range of environments. With a meta-analysis I tested if the presence of other biocontrol agents alters the relationship that a natural enemy has with the host-plant, and showed that the reduction in plant performance caused by one natural enemy is, on average, independent of the second. I also show that the occurrence of non-independent interactions between natural enemies can be predicted by a small range of enemy or plant attributes. (direct interactions and attack to reproductive plant parts). My experiment using the root herbivore, Cyphocleonus achates and the aboveground herbivore L. minutus showed that interactions between the two species can increase the reduction in seed produced by the plant, potentially enhancing biocontrol. My thesis demonstrates that attributes of the biocontrol agents themselves and their interactions with traits of other agents can alter plant performance or plant population decline. Research into attributes associated with successful, and unsuccessful, biocontrol will improve success rates.Science, Faculty ofZoology, Department ofGraduat