Predicting parasitoid accumulation on biological control agents of weeds.
Natural enemies may reduce the effectiveness of weed biocontrol agents and can also cause environmental damage, for example to a shared native insect host through apparent competition. Indeed, successful biocontrol may rely on enemy-free space and avoidance of apparent competition in the area where the biocontrol agent is introduced. We surveyed parasitism in 28 insects released for weed biocontrol in New Zealand (NZ). We reviewed the global literature and databases to complement this survey, and to collate records of these insects being parasitized in their area of origin. We also collated records of native insects that feed on weeds targeted for biocontrol in NZ to test Lawton's (1985) hypothesis that, to find enemy-free space, selected agents should 'feed in a way that is different' and 'be taxonomically distinct' from native herbivores in the introduced range. We found that 19, mostly native, parasitoid species attack 10 weed biocontrol agents in NZ, of which 15 were confined to five agents that possessed 'ecological analogues', defined as a native NZ insect that belongs to the same superfamily as the agent and occupies a similar niche on the target weed. Parasitoid species richness in NZ was positively correlated to richness in the area of origin. However, only agents with ecological analogues contributed significantly to this pattern. A review of NZ weed biocontrol programmes indicated that parasitism is significantly associated with the failure of agents to suppress weed populations. Synthesis and applications. Although our conclusions are based on an unavoidably limited data set, we conclude that biocontrol agents that escape attack from parasitoids are more likely to suppress weed populations and should be less likely to have significant indirect non-target effects in food webs. Biocontrol practitioners can reduce the chance of weed biocontrol agents attracting species-rich parasitoid faunas after introduction by (i) selecting agents that have species-poor parasitoid faunas in their area of origin, and/or (ii) avoiding agents that have 'ecological analogues' awaiting them in the introduced range.