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Background information

Biological control best practice

Biological control agent selection

Selection of appropriate biological control agents is obviously critical to any biological control programme, and justification of this selection will be expected by the EPA in the application. Some of the factors that will be considered, and the type of information that should be provided, are discussed below.

Biological control agent selection usually follows a detailed investigation or 'exploration' in the country of origin of the target pest. Here natural enemies are observed, collected, and ideally tested for host range. These studies are likely to be essential in assessing efficacy of the biological control agent in relation to the target, but might also signal the likely breadth of host range.

Increasing emphasis on target and potential non-target effects is putting pressure on biological control practitioners to improve the selection methods for biological control agents, with one of the aims being to reduce the number required to achieve successful control. An effective agent has to establish in the new country, and reach and maintain a sufficient population size to cause significant damage to critical stages of the target pest's life cycle. Knowledge of the ecology of the target host, whether it is a weed or insect pest, and of the potential agents can help in the selection of the most effective agents.

The degree of host specificity is clearly very important for environmental safety determination, so characteristics of biological control agents that have a bearing on host specificity are worth examining. Table 1 categorizes some features of predators and parasitoids that might affect host specificity. A parasitoid is likely to have a narrower host range than a predator because its more intimate relationship with its host generally demands greater specialization. Similarly, an endoparasitoid has to adapt to the physiology of its host, thus requiring additional specialization and further limiting host range. For the same reason, koinobionts generally have a narrower host range than idiobionts (Askew and Shaw 1986). Linked with this is the mechanism by which the host immune response is overcome. This often requires the injection of venom or other parasitoid-derived proteins during oviposition, but some braconids and ichneumonids deliver polydnavirus or other virus-like particles (Vinson 1990), some of which transcribe proteins in the host to bring about host immunosuppression (Webb and Summers 1990). Since the symbiont is a genetically variable organism with the potential to modify host physiology (Stoltz and Xu 1990), this could provide a mechanism for host range expansion (Whitfield 1994). Interestingly, M. aethiopoides, which attacks several non-target weevils in New Zealand, has virus-like particles (VLPs) which are structurally similar to polydnavirus (Barratt et al. 1999), but no VLPs have been found in M. hyperodae, which has a very narrow host range (Barratt et al. 2006).

Host characteristics that influence host range should also be considered. For example, a mobile, dispersive host with a broad plant host range could transport or attract a parasitoid into contact with a greater diversity of potential non-target hosts than a sedentary host restricted to a specific crop plant, or a plant with a limited distribution. It has been suggested there is more environmental constancy in late succession plants and therefore more opportunity for specialization by parasitoids attacking herbivores of late successional plants (Godfray 1994). There are undoubtedly other characteristics that could be added to this list, but this type of information alone cannot be regarded as a guide to the suitability of biological control agents. Hawkins and Marino (1997) analysed a number of variables which might help explain parasitoid host range expansion in North America, including some of those in Table 1, and found very little correlation. They concluded that either their data were inadequate, or that the processes determining host range are extremely complex and unpredictable.

Table 1. Characteristics of parasitoids believed to be associated with either polyphagy or oligophagy.
More polyphagousMore oligophagous
Host immuno-suppression by symbionts e.g. polydnavirusHost immunosuppression by non-symbionts
Mobile, dispersive hostSedentary host
Host generalist feeder or on widely distributed plantsHost crop-specific or restricted to plants with limited distribution
Host on early successional plantsHosts on late successional plants

For pathogens, Knudsen et al. (1997) have provided a useful review of the types of screening protocols available, but they stress importance of a good understanding of the life cycle of the pest organism, particularly inoculum transfer and critical inoculum threshold levels under field conditions rather than depending upon laboratory methods.

Host specificity characteristics of weed biological control agents is discussed in the section on containment testing.


Askew R.R. and Shaw M.R. (1986). Parasitoid communities: their size, structure and development. Pp. 225-264 In: Insect parasitoids, J.K. Waage and D.J. Greathead (Ed.) Academic Press, London.

Barratt B.I.P., Evans A.A., Stoltz D.B., Vinson S.B. and Easingwood R. (1999). Virus-like particles in the ovaries of Microctonus aethiopoides Loan (Hymenoptera: Braconidae), a parasitoid of adult weevils (Coleoptera: Curculionidae). Journal of Invertebrate Pathology 73: 182-188.

Barratt B.I.P., Murney R., Easingwood R., Ward V.K. (2006). Virus-like particles in the ovaries of Microctonus aethiopoides Loan (Hymenoptera: Braconidae): comparison of biotypes from Morocco and Europe. Journal of Invertebrate Pathology 91: 13-18.

Godfray H.C.J. (1994). Parasitoids: Behavioural and Evolutionary Ecology. Princeton University Press, Princeton. 473 pp.

Hawkins B.A. and Marino P.C. (1997). The colonization of native phytophagous insects in North America by exotic parasitoids. Oecologia 112: 566.

Knudsen I.M.B., Hockenhull J., Jensen D.F., Gerhardson B., Hokeberg M., Tahvonen R., Teperi E., Sundheim L. and Henriksen B. (1997). Selection of biological control agents for controlling soil and seed-borne diseases in the field. European Journal of Plant Pathology 103: 775.

Stoltz D.B. and Xu D. (1990). Polymorphism in polydnavirus genomes. Canadian Journal of Microbiology 36: 538-543.

Vinson S.B. (1990). How parasitoids deal with the immune system of their host: an overview. Archives of Insect Biochemistry and Physiology 13: 3-27.

Webb B.A. and Summers M.D. (1990). Venom and viral expression products of the endoparasitic wasp Campoletis sonorensis share epitopes and related sequences. Proceedings of the National Academy of Sciences of the United States of America 87: 4961-4965.

Whitfield J.B. (1994). Mutualistic viruses and the evolution of host ranges in endoparasitoid Hymenoptera. Pp. 163-176 In: Parasitoid community ecology, B.A. Hawkins and W. Sheehan (Ed.) Oxford University Press, Oxford.