Background/Question/Methods We used meta-analysis of the effects of single species of insect herbivores on their host plants to investigate whether insect herbivores released for plant biocontrol have stronger effects than other insect herbivores and if so whether it was due to the enemy release hypothesis, the new associations hypothesis, or the lottery hypothesis. Herbivory was measured as reductions in the numbers of plant flowers, leaves, seeds or stems between treatments and controls or biomass, length or diameter reduction in leaves, roots, seeds, stems and whole plants.
Results/Conclusions Insect herbivores in studies classified as biocontrol studies by the authors had greater effects on plant morphometrics than insect herbivores in studies not considered as biocontrol studies by the authors. The enemy release hypothesis was not supported as a cause for this pattern since the strength of herbivory in traditional biocontrol releases, where natural enemies of the insect biocontrol agent were absent, was not different to biocontrol investigations for potential agents in native areas, where natural enemies of the potential biocontrol agent were present. The new associations or neoclassical control hypothesis was not supported because there was no significant difference in the strength of herbivory between either native insects with non-native plants vs. native insects with native plants, or between non-native insects and native plants vs. native insects with native plants. The lottery hypothesis was also not supported because native insects with native plants that were part of a biocontrol investigation looking for potential biocontrol agents had stronger effects than native insects had on native plants that were not considered part of a biocontrol investigation. Our results support the idea that biocontrol workers are able to use their expertise to select insects that have stronger than average effects on plants.