Are specialists really safer than generalists for classical biocontrol?

Generalists rarely are considered for classical biocontrol because their broad feeding habits are expected to make non-target impacts inevitable. This assumes an increase in overall ecological risk with increasing number of feeding connections. With the goal of inspiring fresh consideration of the safety of exotic biocontrol agents for classical biocontrol, we present a selective review of the impacts of nine particularly-well-studied exotic natural enemies ranging from relative specialists to generalists. Surprisingly, non-target effects could be particularly strong for relative specialists that attacked just a few natives, but were often widespread but weak for the broadest generalists. This appeared to reflect relatively strong apparent competition and density-dependence for the narrow feeders, versus broadly diffuse net effects for the broader feeders. Overall, we suggest a sole focus on specialists in classical biocontrol might be an unreliable means to reduce ecological risk. Additional research is needed to fully compare the net direct and indirect effects of generalists and specialists across food webs, following classical biocontrol releases.

Automated summary

The impact of exotic natural enemies in classical biocontrol can vary, with relative specialists potentially having strong non-target effects while the broadest generalists have weaker but more widespread impacts. The focus on specialists in classical biocontrol may not be a reliable means to reduce ecological risk, and more research is needed to compare the effects of generalists and specialists across food webs.