Release strategies in weed biocontrol: how well are we doing and is there room for improvement?

A large number of factors associated with the agent and habitat can be manipulated when making a biocontrol release. Yet, it is only recently that an experimental approach has been taken, particularly with the release of arthropod weed biocontrol agents, for example examining factors such as release size to test ideas originating from theoretical or retrospective studies. For release size, both retrospective analyses and experimental studies with arthropod agents usually show that larger releases have a higher probability of establishing. However, a limited number of large releases at a few sites also run the risk of chance extinction from locally major environmental effects. These risks can be minimized by releasing at many widespread sites, so there is often a trade-off between a few large releases and many smaller one. Unless these risks and relationships are known, a mixed strategy with arthropods, using a range of release sizes, is likely to be optimal at least at the start of a release programme. Recent theoretical attention has been applied to specific mechanisms creating lower individual fitness in small populations, termed Allee effects. It appears that Allee effects, including genetic inbreeding and problems in finding mates, may have been underestimated: They can be powerful, as witnessed by deliberate extinctions using sterile male release. Genetic issue with biocontrol releases, such as strain selection, inbreeding depression, drift and creation of laboratory-selected strains, have also received considerable, mostly theoretical, analysis. However, the actual rate of establishment success per agent species in weed biocontrol programmes is now 80–100%. These overall analyses do hide problems in establishing particular agent species: Programmes in New Zealand targeting ‘recalcitrant’ environmental weeds have been hampered by partial or complete failure to get potentially critical agent species established. A case study of heather beetle in New Zealand is used to illustrate the frustrations and research challenges in a release programme.