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Test species selection

It is important to choose an appropriate range of species for use in host range testing experiments. Some of the most important issues are discussed below.

A general problem that is gradually becoming much better understood is the existence of previously unrecognised biotypes. Either the weed, the agent or the natural enemy may actually consist of several distinct populations that vary biologically but are lumped under a single taxonomic name. For example, a parasitoid may only attack some of the populations of a host. Conversely, a parasitoid may exist as several distinct biotypes, each of which shows behavioural or physiological adaptation to different hosts.

Because of the far-reaching consequences of biotype specificity and effectiveness, host range testing should be done with the exact geographic population of parasitoid that will be released. This important issue needs to also underline all the considerations listed below. top

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 (Zw�lfer 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:

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. top

Arthropod biological agents

The number of insect species which could be considered for host range testing will always far out-number those for plants, and our knowledge of insect phylogeny is often far less comprehensive than that for plants. Indeed, in New Zealand many insect species remain undescribed, and undiscovered. This makes the adoption of the test species selection method for host specificity testing far much more difficult than it is for weed biological control agents. Furthermore, rearing insects in sufficient numbers for robust tests is often more challenging than for plants. Nevertheless, attempts to develop test insect selection methods have been made.

Selecting the test insect list

Some factors to be considered in selecting test species for host specificity testing have been discussed elsewhere in detail (Goldson and Phillips 1990, Goldson et al. 1992, Barratt et al. 2000). Depending upon the nature of the insect pest that is the proposed target of biological control, sometimes significant amounts of preliminary information can be gleaned on a proposed biological control agent from various sources in the public arena (Hoddle 2004, Sands and Van Driesche 2004). Many potential pitfalls such as taxonomic synonymy and misidentifications can be avoided if awareness of the possibilities is high (Sands and Van Driesche 2004). One process that could be followed in obtaining a suitable host testing list is (Barratt et al. 1999):

Another approach similar to that described above has been published recently where the list compiled using phylogenetic and ecological criteria is filtered by eliminating species which have different spatial, temporal and morphological characteristics which do not coincide with the target species (Kuhlmann et al. 2006). top

References

Barratt B.I.P., Ferguson C.M., McNeill M.R. and Goldson S.L. (1999). Parasitoid host specificity testing to predict host range. Pp. 70-83 In: Host specificity testing in Australasia: towards improved assays for biological control, T.M. Withers, L. Barton-Browne and J.N. Stanley (Ed.) CRC for Tropical Pest Management, Brisbane, Australia.

Barratt B.I.P., Goldson S.L., Ferguson C.M., Phillips C.B. and Hannah D.J. (2000). Predicting the risk from biological control agent introductions: A New Zealand approach. Pp. 59-75 In: Nontarget effects of biological control introductions, P.A. Follett and J.J. Duan (Ed.) Kluwer Academic Publishers, Norwell, Massachusetts, USA.

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.

Goldson S.L. and Phillips C.B. (1990). Biological control in pasture and lucerne and the requirements for futher responsible introduction of entomophagous insects. Bulletin of the Entomological Society of New Zealand 10: 63-74.

Goldson S.L., McNeill M.R., Phillips C.B. and Proffitt J.R. (1992). Host specificity testing and suitability of the parasitoid Microctonus hyperodae (Hym.: Braconidae, Euphorinae) as a biological control agent of Listronotus bonariensis (Col.: Curculionidae) in New Zealand. Entomophaga 37: 483-498.

Hoddle M.S. (2004). Analysis of fauna in the receiving area for the purpose of identifying native species that exotic natural enemies may potentially attack. Pp. 24-39 In: Assessing host ranges for parasitoids and predators used for classical biological control: a guide to best practice, R.G. Van Driesche and R. Reardon (Ed.) USDA Forest Service, Morgantown, West Virginia.

Kuhlmann U., Schaffner U. and Mason P.G. (2006). Selection of non-target species for host specificity testing. Pp. 15-37 In: Environmental impact of invertebrates for biological control of arthropods: methods and risk assessment F. Bigler, D. Babendreier and U. Kuhlmann (Ed.) CABI Publishing, Wallinford, Oxford.

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. Card� (Ed.) Chapman & Hall, London.

Pemberton R.W. (2000). Predictable risk to native plants in weed biological control. Oecologia 125: 489-494.

Sands D.P.A. and Van Driesche R.G. (2000). Evaluating the host range of agents for biological control of arthropods: rationale, methodology and interpretation. Pp. 69-83 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.

Sands D.P.A. and Van Driesche R.G. (2004). Using the scientific literature to estimate the host range of a biological control agent. Pp. 15-23 In: Assessing host ranges for parasitoids and predators used for classical biological control: a guide to best practice, R.G. Van Driesche and R. Reardon (Ed.) USDA Forest Service, Morgantown, West Virginia.

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.

Zw�lfer H. and Harris P. (1971). Host specificity determination of insects for biological control of weeds. Annual Review of Entomology 16: 159-178.

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