Test species selection
Weed biocontrol agents
Evidence is available to support the view that that host range expansion by phytophagous insects is limited to plant species which have a similar composition of phytochemicals (Futuyma 2000). Such plants are usually also close relatives (Berenbaum and Zangerl 1992). One could conclude from this that biocontrol practitioners could eliminate from host range tests any plants with very different phytochemicals from the target host plant. However, there are rare examples in the literature of herbivorous insects that have expanded their host range to genetically and chemically quite different plants (Tallamy 1999). Further to this, the lack of information available to biological control practitioners often precludes this sort of reasoning. To date there is no evidence of evolutionary changes in the fundamental host range of weed biocontrol agents after release (van Klinken and Edwards 2002). This conclusion is backed up by the review of both weed and insect agent non-target impacts (Louda et al. 2003). A more recent review of data on host use by 117 biological control agents released against weeds in the continental USA, Hawaii and the Caribbean since 1902, (Pemberton 2000) found only 15 insect agents attacking 41 native non-target plant species. All but one of these plants was closely related to the target weed, and the potential for attack by the agents could have been predicted from adequate host range testing. Therefore according to Pemberton the majority of non-target impact examples would be placed into this category, and true unpredicted attack from weed biological control agents is relatively uncommon.
Selecting the test plant list
Wapshere (1974) set down some criteria for selecting plants for testing to demonstrate host specificity of biological control agents. Previous to this, a small group of related plants and a large series of unrelated crop plants had been selected for testing against the potential weed biocontrol agents (Zwölfer and Harris 1971). The criteria outlined by Wapshere in 1974 have proven to form a successful foundation for at least three decades of host specificity testing around the world included the following:
- A centrifugal phylogenetic testing method which involves exposing to the organism a sequence of plants from those most closely related to the weed species, progressing to successively more distantly related plants until the host range has been adequately circumscribed.
- As a safeguard against failure of the centrifugal method cultivated plants selected by the following five criteria should be tested:
- Cultivated plants botanically related to the weed.
- Cultivated plants for which there is little or no information about their biological relevance to the test plants.
- Cultivated plants which have evolved apart from, or for geographic or climatic reasons have not been extensively exposed to the organisms under study.
- Cultivated plants known to be attacked by organisms closely related to the biological control agent under investigation.
- Any plants on which the organism has previously been recorded.
As the understanding of host selection behaviour of phytophagous insects improved (Miller and Strickler 1984), Wapshere reconsidered his testing sequence, especially in light of the number of potential weed biocontrol agents being rejected on the basis of apparently greater host range being predicted from host specificity testing in a caged environment, than was observed in field situations (Wapshere 1989). The conclusion was that if any of the caged methods of host specificity testing (generally conducted within quarantine facilities with artificial lighting and climate controls) leads to the by-passing of an important cue within the insects host selection behavioural sequence, that the results may not be predictive. Wapshere considered that since by-passing of cues increased the potential host range of an insect, any plant not attacked or oviposited upon in a cage or after placing the larva directly onto it, could be considered safe from oviposition or feeding. Thus, if it is not possible to test all plants under natural conditions, the most reliable method of estimating host range and reducing the rejection of apparently safe agents was to test selected plants with the agent at the critical host selection phase, and then to test only those affected at the next less restrictive phase, and so on. What should be left after a sequence of such tests are only those few plants that require a field test or the expression of a complete normal sequence of host selection behaviour to indicate the risk to the plant. This was referred to as the "reverse testing sequence" (Wapshere 1989).
For access to current plant species in New Zealand for determining phylogenetic affiliations, the Landcare Research database [http://nzflora.landcareresearch.co.nz] can be used.
Berenbaum M.R. and Zangerl A.R. (1992). Genetics of physiological and behavioral resistance to host furanocoumarins in the parsnip webworm. Evolution 46: 1373-1384.
Futuyma D.J. (2000). Potential evolution of host range in herbivorous insects. Pp. 42-53 In: Host-specificity testing of exotic arthropod biological control agents: the biological basis for improvement in safety, R.G. Van Driesche, T. Heard, A.S. McClay and R. Reardon (Ed.) USDA Forest Service Bulletin, Morgantown, West Virginia, USA.
Louda S.M., Pemberton R.W., Johnson M.T. and Follett P.A. (2003). Nontarget effects - the Achilles' heel of biological control? Retrospective analyses to reduce risk associated with biocontrol introductions. Annual Review of Entomology 48: 365-396.
Miller J.R. and Strickler K.L. (1984). Finding and accepting host plants. Pp. 127-157 In: Chemical Ecology of Insects, W.J. Bell and R.T. Cardé (Ed.) Chapman & Hall, London.
Pemberton R.W. (2000). Predictable risk to native plants in weed biological control. Oecologia 125: 489-494.
Tallamy D.W. (1999). Physiological issues in host range expansion. Pp. 11-26 In: Proceedings: Host specificity testing of exotic arthropod biological control agents: The biological basis for improvement in safety, N.R. Spencer (Ed.) Bozeman, Montana.
van Klinken R.D. and Edwards O.R. (2002). Is host specificity of weed biological control agents likely to evolve rapidly following establishment? Ecology Letters 5: 590-596.
Wapshere A.J. (1974). A strategy for evaluating the safety of organisms for biological weed control. Annals of Applied Biology 77: 201-211.
Wapshere A.J. (1989). A testing sequence for reducing rejection of potential biological control agents for weeds. Annals of Applied Biology 114: 515-526.
Zwölfer H. and Harris P. (1971). Host specificity determination of insects for biological control of weeds. Annual Review of Entomology 16: 159-178.
Test species selection
Arthropod biological control agents