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Selecting biological control agents

Predicting direct non-target impacts

Modelling non-target impacts

Primary versus secondary attack

In the context of predicting nontarget impacts from candidate BCAs, an important issue is whether the attacked species is a primary or secondary host. Primary hosts are those that can support the BCA even when no other hosts are available, while secondary hosts are those that cannot support the BCA in the absence of other host species. Therefore, impacts on secondary hosts arise largely from "spill-over" attack from primary hosts, and removal of spill-over sources will allow recovery of secondary hosts. BCA population growth will be constrained by the availability of primary hosts, but will be independent of the density of secondary hosts. Therefore, secondary hosts could suffer rates of attack that are sufficient to cause local extinction, but this is unlikely to occur in primary hosts because BCA populations will decline in tandem with reductions in their primary host populations.

Previously published models have been analysed to elucidate the differing drivers for population impact in primary and secondary hosts, and composite models have been developed to explore intermediate scenarios. In addition, these ideas have been applied in the context of nontarget parasitism of native weevils by the Moroccan biotype of Microctonus aethiopoides in mid-altitude tussock grasslands in Otago. M. aethiopoides has been observed to parasitise up to 24% (mean = 2%) of native weevils in the vicinity of populations of the target host, Sitona discoideus (Barratt et al. 2007), but it is not known whether these are primary or secondary hosts. M. aethiopoides can develop through multiple generations on the non-target native weevil Niceana cervina in the laboratory McNeill et al. 2009, but experimental releases of the parasitoid on the Lammermoor-Rock and Pillar Range failed to augment parasitism rates in native weevils.


Moroccan Microctonus aethiopoides on Sitona discoideus.

© Copyright AgResearch, used with permission.

The ability of the target host S. discoideus to reproduce on host plants other than its preferred host, lucerne, is confirmed by its establishment on Norfolk Island where lucerne is absent (Vink and Phillips 2007). In the laboratory, M. aethiopoides survives and develops equally well in S. discoideus adults fed on clover (present as a weed in tussock grasslands) as in those fed on lucerne (absent from tussock grasslands), suggesting a pathway for nontarget parasitism via spill-over immigration of S. discoideus to tussock grasslands. In addition, the potential for S. discoideus to survive on clover in these environments raises the possibility of resident S. discoideus populations acting as a local source of nontarget parasitism. However, it is not yet known whether such resident S. discoideus populations do persist in native tussock grasslands. An ad hoc survey failed to detect and S. discoideus larvae on the roots of white clover on the Lammermoor-Rock and Pillar Range (B. Barratt, pers. comm.). In addition, field experiments have shown heavy mortality of M. aethiopoides pupae and parasitised native weevils over the winter, but sufficient survival of the latter to allow a few parasitoid adults to emerge in spring. On current evidence, it seems most likely that the primary source of nontarget parasitism by M. aethiopoides in mid-altitude tussock grasslands is spill-over from the target host, perhaps slightly augmented by local population cycling within native weevils.

The modelling analyses of the impacts of BCAs on their primary and secondary hosts are improving our ability to define risks to nontarget species, but further development and validation is required. The role of spill-over effects is often addressed in applications to the EPA, but these are usually hypothetical with no supporting data or analytical framework. The modelling work described above has potential both to provide the needed analytical framework, and to clarify the types of supporting data that applicants must present to the EPA to ensure robust decisions can be made.


Barratt B.I.P., Ferguson C.M., Bixley A.S., Crook K.E., Barton D.M. and Johnstone P.D. (2007). Field parasitism of nontarget weevil species (Coleoptera : Curculionidae) by the introduced biological control agent Microctonus aethiopoides Loan (Hymenoptera : Braconidae) over an altitude gradient. Environmental Entomology 36: 826-839

McNeill M.R., Ferguson C.M., Bixley A.S. and Barratt B.I.P. (2009). Development of Microctonus aethiopoides Loan through multiple generations of novel weevil hosts in the laboratory. Pp. 617-618 in: Proceedings of the 3rd International Symposium on Biological Control of Arthropods, P.G. Mason, D.R. Gillespie and C. Vincent (Ed.) Christchurch, New Zealand.

Vink C.J. and Phillips C.B. (2007). First record of Sitona discoideus Gyllenhal 1834 (Coleoptera: Curculionidae) on Norfolk Island. New Zealand Journal of Zoology 34: 283-287.