Two-thirds of weed biocontrol agents that establish fail to suppress their target weed (McFadyen 1998), and this failure can be a result of natural enemies (Goeden & Louda 1976). These natural enemies, such as parasitoids may cause environmental damage, for example to a shared native insect host through apparent competition (Carvalheiro et al. 2008).
Concerns for environmental safety have triggered tighter regulation of biocontrol world-wide (Sheppard et al. 2003). In New Zealand (NZ), biocontrol introductions are administered by the Environmental Risk Management Authority, which evaluates the risks, costs and benefits of introducing biocontrol agents (Barratt & Moeed 2005), including their potential impacts on food webs. Information regarding the potential parasitoids of a candidate biocontrol agent is, however, often unreliable. For example, the parasitic Hymenoptera of the NZ biogeographic region are poorly known (Berry 2007). Biocontrol practitioners have, therefore, sought to make predictions by identifying factors that influence parasitism. For example, Harris (1991) argued that gall midges should be given a low priority as biocontrol agents because they readily acquire parasitoids, and Hill & Hulley (1995) noted that parasitism varied according to level of concealment. A broad analysis showed that the numbers of primary parasitoid species acquired by insect herbivores in their introduced range are correlated with the numbers of primary parasitoid species attacking them in their area of origin (Cornell & Hawkins 1993). So, should herbivorous insects with a high diversity of natural enemies in their area of origin be passed over as potential weed biocontrol agents? Paradoxically, weed biocontrol practitioners argue that many herbivorous insects are kept rare by natural enemies in their areas of origin and can become excellent weed biocontrol agents in the ‘enemy-free space’ of the introduced range (Strong, Lawton & Southwood 1984; Hunt-Joshi, Root & Blossey 2005). Furthermore, agents that are kept rare in their area of origin may be more effective because their target weeds have had little selection to evolve resistance to them (Myers, Higgins & Kovacs 1989).
Lawton (1985) stated that enemy-free space and avoidance of apparent competition in the introduced range may be critical for weed biocontrol agent success and recommended that selected agents should ‘feed in a way that is different’ and ‘be taxonomically distinct’ from native herbivores in the introduced range.
We addressed these issues by surveying the parasitoid fauna of weed biocontrol agent species in NZ and testing the following hypotheses:
- 1 Parasitoid richness increases with the time that an agent has been present in the introduced region.
- 2 Weed biocontrol agents that have rich parasitoid complexes in their area of origin should accumulate more parasitoid species in the introduced range than species which have no or few parasitoids that attack them in their area of origin.
- 3 Agents that ‘feed in a way that is similar’ and are ‘taxonomically close’ to the native fauna should be susceptible to parasitism.
- 4 Parasitism has reduced the effectiveness of weed biocontrol agents in NZ.