Host range testing methods
Test designs for weed biocontrol
As mentioned above, the assessment of plants as potential hosts for herbivorous insects began over 70 years ago. A variety of tests have been developed, although those most in favour have changed over time due to changes in the perspective of biocontrol practitioners and developments in the study of insect behaviour. Tests used have focused on oviposition, adult feeding, larval development and survival, oogenesis and multi-generation population persistence, and host preference. There have been a number of thorough reviews that describe the different test method types and discuss and detail the appropriateness of each test type for different types of weed biocontrol agents (Harris and Zwölfer 1968, Cullen 1989, Wapshere 1989, Heard and Van Klinken 1998, Marohasy 1998, Briese 1999, Sheppard 1999, Heard 2000, van Driesche and Murray 2004). Each of these papers makes a valuable contribution to the appropriateness of different types of test design. For this reason a thorough debate of the pros and cons of various testing designs according to the biology and behaviour of the individual weed biocontrol agent is beyond the scope of this website. Instead, bullet points will be used to summarise main points below and for more details you should consult the above papers.
What we do need to emphasise in applications to the EPA is that the applicant is open and clear in justifying both the way they arrived at the test species selection list as well as the reasoning for approaching the testing methodology in terms of the host selection behaviour and biology of the potential agent. Wapshere (1989) provides a very clear and logical explanation of how both could be approached, and applicants are urged to read that paper.
The possible agents that may be chosen as potential weed biocontrol agents is very large, so there are no strict guidelines on how to undertake testing. Careful consideration of the behavioural characteristics of both the:
- damaging life stage/s - responsible for the physical damage to the plant;
- dispersive life stage/s - responsible for selecting which plant will or will not be attacked.
The importance of testing the host specificity of both life stages separately is that the dispersive life stage has the ability to choose the plant species that will be contacted by the other life stages. For instance, in many cases it is the adult female insect that is mobile and actively undertakes host searching and host choice for oviposition. Often internally feeding or virtually immobile larvae do not have the ability to move far enough to choose a different, perhaps more palatable species of plant. In such cases, host specificity testing that establishes both the host preference of the adult female, and then the survival and feeding ability of the larva, will capture all the vital information required to make an accurate risk assessment.
There are examples where the reproducing female may be immobile on a plant part, yet produces highly dispersive crawlers or ballooning larvae that themselves move according to weather conditions such as wind and the proximity of other plant species will have a significant influence on what plants they contact. In such a case, host specificity testing that establishes the preference of the neonate larvae to settle on the different plants, and then the survival and reproductive potential of the resultant individuals, will capture all the information required to make an accurate risk assessment.
In other cases the reproductive female may be only loosely selective in choosing a certain habitat in which to oviposit, and the larvae may be highly mobile with the ability to locate different individuals and species of plants through some or all of their damaging life stages. In such a case, host specificity testing will need to both establish the feeding preferences of the mobile life stages of the larvae between different plants, and the oviposition preferences of the adults, as well as the survival and reproductive potential of individuals on individual plant species.
A flow chart generalising decisions involved in the selection of an initial host specificity testing procedure for weed biocontrol agents has been presented by Sheppard (1999). However, Withers (1997) strongly insists that the results of initial screening trials undertaken should not be used to reduce the plant list used for testing in subsequent trial types. This is because host specificity testing on both the dispersive and damaging life stages need to be gathered independently for a quantitative risk analysis to be undertaken.
Briese D.T. (1999). Open field host-specificity tests: is "natural" good enough for risk assessment? Pp. 44-59 In: Host specificity testing in Australasia: towards improved assays for biological control, T.M. Withers, L. Barton-Browne and J. Stanley (Ed.) Scientific Publishing, Department of Natural Resources, Brisbane.
Cullen J.M. (1989). Current problems in host-specificity screening. Pp. 27-36 In: Proceedings of the VII International Symposium on Biological Control of Weeds, E.S. Delfosse (Ed.) CSIRO Publications, Melbourne.
Harris P. and Zwölfer H. (1968). Screening of phytophagous insects for biological control of weeds. Canadian Entomologist 100: 295-303.
Heard T.A. (2000). Concepts in insect host-plant selection behavior and their application to host specificity testing. Pp. 1-10 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.
Heard T.A. and Van Klinken R.D. (1998). An analysis of test designs for host range determination of insects for biological control of weeds. Pp. 539-546 In: Proceedings of the Sixth Australasian Applied Entomological Research Conference, M.P. Zalucki, R.A.I. Drew and G.G. White (Ed.) Brisbane, University of Queensland.
Marohasy J. (1998). The design and interpretation of host-specificity tests for weed biological control with particular reference to insect behaviour. Biocontrol News and Information 19: 13-20.
Sheppard A.W. (1999). Which test? A mini review of test usage in host specificity testing. Pp. 60-69 In: Host specificity testing in Australasia: towards improved assays for biological control, T.M. Withers, L. Barton-Browne and J. Stanley (Ed.) Scientific Publishing, Department of Natural Resources, Brisbane.
van Driesche R.G. and Murray T.J. (2004). Overview of testing schemes and designs used to estimate host ranges. Pp. 68-89 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.
Wapshere A.J. (1989). A testing sequence for reducing rejection of potential biological control agents for weeds. Annals of Applied Biology 114: 515-526.
Withers T.M. (1997). Changes in plant attack over time in no-choice tests: an indicator of specificity. Pp. 214-217 In: Proceedings of the 50th New Zealand Plant Protection Conference, M. O'Callaghan (Ed.) Lincoln University, NZ., New Zealand Plant Protection Society Inc.
Host range testing methods
Test designs for arthropod biological control