Selecting biological control agents
Predicting direct non-target impacts
Exploration in the target home range
Natural host range
The host range of a biological control agent in the area of origin is potentially useful as a guide to likely host range in new areas of introduction. If available, this information can assist in risk assessment and prediction of post-release impacts. Regulators considering applications for the release of new biological control agents often request information about host range in the country of origin and in other areas where introductions have occurred, for example, the FAO 'Code of Conduct' for import and release of biological control agents (FAO 1996). Aquiring information about natural host range is considered to be an important first step in the process of biological control agent host specificity testing, particularly selection of test species (Kuhlmann et al. 2005). This can help substantiate the interpretation of host range tests in containment, and provide support to the hypothesis that host range tests in containment have over-estimated field host range, if data from the natural range indicate that a particular species has not been attacked in the field. Several studies have shown that fundamental host range is greater than ecological host range (e.g. Cameron and Walker 1997). Furthermore, a comparison of the value of laboratory tests as a predictor of ecological host range in the area of origin with a proposed area of release might also be of assistance in interpreting containment host range data.
The approach to this work is likely to be different for weed compared with insect biological control agents. In some ways initial information for the latter is more easily achieved because while collecting specimens of the target to rear out parasitoids, or culture pathogens, other species are likely to be collected from the same habitat. The challenge then becomes rearing a number of different species, although for parasitoids, dissections can be carried out to determine parasitism. For weed biological control agents, it is necessary either to observe impacts on other plants and determine whether the candidate biological control agent was responsible, or to carry out laboratory or field tests confining the proposed agent on a range of plants.
During the exploration phase of a biological control programme, target host populations are naturally the focus of investigation, and related or co-existing species are often not examined, especially in the case of entomophagous species. Weed biocontrol programmes do sometimes include examination of other plants in the area, and plants which are phylogenetically related.
While there are many examples of this approach having been taken during biological control programmes, few studies have been undertaken either retrospectively or post-release to test the accuracy of predictions made from native and novel host range studies. Those that have been carried out, show in general that research conducted in the natural geographical range of a proposed biological control agent has or could have assisted in predicting the likely breadth of post-release host range of biological control agents. Some examples are given in the next section.
Cameron P.J. and Walker G.P. (1997). Host specificity of Cotesia rubecula and Cotesia plutellae, parasitoids of white butterfly and diamondback moth. Pp. 236-241 In: Proceedings of the 50th New Zealand Plant Protection Conference, M. O'Callaghan (Ed.) NZ Plant Protection Society Inc.
FAO (1996). Code of conduct for the import and release of exotic biological control agents. Food and Agriculture Organisation. International Standards for Phytosanitary Measures No 3.
Kuhlmann U., Schaffner U. and Mason P.G. (2005). Selection of non-target species for host specificity testing of entomophagous biological control agents. Pp. 566-583 In: Second International Symposium on Biological Control of Arthropods, Davos, Switzerland, 12-16 September, 2005, M.S. Hoddle (Ed.) United States Department of Agriculture, Forest Service, Washington.
Some case studies