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Selecting biological control agents

Experimental confirmation of host range

Predicting indirect non-target effects of agents

What we do know

The introduction of exotic biological control agents is not an intrinsically safe operation, but one that is made safe by the way it is practiced (McClay and Balciunas 2005). This is understood by every responsible biological control practitioner, and is reflected in the wording of the HSNO Act, and in the Methodology Order that interprets it. One of the guiding principles of the Act is the safeguarding of the life-supporting capacity of ecosystems, and in their deliberations the EPA must take account of the inherent value of ecosystems. The Act requires the EPA to reject an application if it would result in significant deterioration of a natural habitat or significant displacement of a native species in its natural habitat. Applicants must supply sufficient information for the EPA to be able to make these judgements.

A new organism will always modify the ecosystem into which it is introduced, however slightly. It is therefore wrong to suggest that biological control has no effect on communities and ecosystem processes. Hoddle (2004) concluded that adding exotic organisms to an ecosystem has three effects: the links in food webs that connect populations are permanently changed; pest density population dynamics are altered; and changes in the relative dynamics of populations cause changes in community structure. van Lenteren et al. (2006) review the range of potential non-target effects that might be modified by the introduction of an exotic biological control agent. The control agent may attack a species other than the target pest and affect its abundance. The possible consequences of direct non-target attack have already been reviewed. Indirect non-target effects occur when the presence of the agent changes community structure by means other than consumption. Resource competition occurs when two organisms exploit and divide a single resource. This may reduce the population of the displaced species, sometimes to local extinction, but it has rarely if ever led to species extinction (van Lenteren et al. 2006). Parasitoids and predators may be able to attack other natural enemies of the target; this is termed intraguild predation or facultative hyperparasitism (van Veen et al. 2006). The natural enemy may itself be food for resident natural enemies. This is especially so for biological control agents for weeds. This intrusion into the ecosystem is called enrichment, and may provide a food subsidy for parasitoids and predators within the ecosystem. If this leads to increased natural enemy populations, and increased mortality of another resident host species, then the introduction has caused apparent competition between the introduced and the resident hosts. These indirect effects can be either via a host (bottom-up, McEvoy and Coombs 1999), or top-down via a higher trophic level (Holt and Hochberg 2001). The density dependent regulation of ineffectual generalists does not occur under apparent competition and non-target populations can decline (Hoddle 2004).

Messing et al. (2006) reviewed the literature on indirect non-target effects, and attempted to synthesise the styles of interaction described. Pearson and Callaway (2005) have also reviewed the literature on indirect non-target effects.


Hoddle M. (2004). Restoring balance using exotic species to control invasive exotic species. Conservation Biology 18: 38-49

Holt R.D. and Hochberg M.E. (2001). Indirect interactions, community modules and biological control: a theoretical perspective. Pp. 13-37 In: Evaluating indirect ecological effects of biological control, E. Wajnberg, J. K. Scott and P. C. Quimby (Ed.) CABI Publishing, Wallingford, Oxon., UK

McClay A.S. and Balciunas J.K. (2005). The role of pre-release efficacy assessment in selecting classical biological control agents for weeds - applying the Anna Karenina principle. Biological Control 35: 197-207

McEvoy P.B. and Coombs E.M. (1999). Biological control of plant invaders: regional patterns, field experiment and structured population models. Ecological Applications 9: 387-401

Messing R.H., Roitberg B.D. and Brodeur J. (2006). Measuring and predicting indirect impacts of biological control, competition, displacement and secondary interactions. Pp. 64-77 In: Environmental impact of invertebrates for biological control of arthropods - methods and risk assessment, F. Bigler, D. Babendreier and U. Kuhlmann (Ed.) CABI Publishing, Wallingford, UK

Pearson D.E. and Callaway R.M. (2005). Indirect nontarget effects of host-specific biological control agents: Implications for biological control. Biological Control 35: 288-298.

van Lenteren J.C., Bale J., Bigler F., Hokkanen H.M.T. and Loomans A.J.M. (2006). Assessing risks of releasing exotic biological control agents of arthropod pests. Annual Review of Entomology 51: 609-634.

van Veen F.J.F., Morris R.J. and Godfray H.C.J. (2006). Apparent competition, quantitative food webs, and the structure of phytophagous insect communities. Annual Review of Entomology 51: 187-208