Selecting biological control agents
Experimental confirmation of host range
The styles of tests that can be employed are well reviewed elsewhere in BIREA. van Driesche and Reardon (2004) edited a guide to best practice for assessing the host range of predators and parasitoids. That publication is not reviewed here. Heard and Van Klinken (1998), Sheppard (1999) and Spafford-Jacob and Briese (2003) have reviewed the use of different designs for host range testing of phytophages for weed control. van Driesche and Murray (2004) have reviewed the strengths and weaknesses of the designs that can be employed in host range testing parasitoids and reached the general conclusion that as far as possible the test regime adopted should model the ecological context in which the agents will interact with potential hosts. This view is also implicit in the selection criteria suggested by Kuhlmann et al. (2006).
In the view of Babendreier et al. (2006), the purpose of host range testing is to determine the ecological host range of the natural enemy. This seems ambitious, and they acknowledge that designing tests for this purpose is difficult. An alternative view is that host range testing provides an opportunity to define the physiological or fundamental host range, and by the use of a range of test designs, to provide additional information with which to predict the likely field host range. This approach seems more likely to be successful. A test design array should take into account the following principles:
- Factors within test designs, such as the size of the arena or the presence of plant material, may either reduce or increase the acceptability of hosts to natural enemies (Babendreier et al. 2005). At least one test (usually a simple, well controlled 'no choice' test in conditions that are highly suitable for the agent) must create a 'maximum challenge' to the susceptibility of the host to create a reliable physiological host range (Hill 1999, Withers and Barton-Browne 2004). However, it is important for reviewers to distinguish the physiological host range revealed by such tests from the likely field host range.
- 'Choice' tests are often used because this design represents field conditions more realistically than 'no choice' tests. The value of 'choice' tests is debated (Barratt et al. 2006), and depending on factors such as agent size and mobility, 'no choice' may well be the more realistic design (Hill 1999).
- Natural enemies can respond differently to their host depending on age and physiological state. A newly emerged natural enemy may have a different intensity of response than an aged individual. For example, Kitt and Keller (1998) found that older Aphidius rosae adults were more selective than newly emerged individuals. A satiated agent may attack fewer hosts than a deprived agent. The duration of a test may therefore influence its outcome. Test designs must consider these possibilities. The issues posed by time and behaviour-dependent effects are reviewed by Withers and Barton-Browne (2004).
- Marohasy (1998) reviewed the design and interpretation of host-specificity tests for weed biological control agents and recommended that tests should be designed to take more account of agent behaviour. She suggested methods for measuring the rank order of acceptability of test species, and the time taken before an agent will accept a lower-ranked plant species as additional measures of host acceptance. These novel approaches may well have application in the testing of arthropod control agents.
- Hosts must be presented in tests in the correct or most acceptable stage, and it is important that the target host is represented in each design (Barratt et al. 1999, Babendreier et al. 2005).
- In their native range the host range of parasitoids and phytophages can vary between populations (see discussion about biotypic and geographic variation above). Best practice in New Zealand now dictates that only those populations that have been tested should be released. Later releases of populations from other sources should be subject to confirmation of that host range by limited testing (Withers et al. 2008).
van Lenteren et al. (2006) have summarised the issues that should be considered when designing host specificity tests on arthropod natural enemies, and propose a methodology for addressing those issues. Their methodology is very clear and well-annotated, and provides a sound baseline for the development of a testing regime. However, they acknowledge that a theoretical and methodological background for such a testing regime for arthropod natural enemies is not yet mature. They summarise what information should be brought to bear on a test design:
- know the foraging behaviour of the natural enemy;
- know the influence of quality and rearing conditions of the host plant, host and natural enemy. These should be described in detail to trace the effects of conditioning, learning and multitrophic communication on results;
- understand the effect of diapause state of both host and parasitoid;
- avoid hosts with infections;
- recognise behavioural variation in natural enemies and identify the source (genetic, phenotypic, physiological);
- understand the genetic characteristics of the parasitoid and match strains to the host;
- consider the environment of the test and how this might affect associative learning leading to changes in relative behaviours of host and parasitoid;
- be aware of the influence of biotypes and multitrophic stimuli on design (van Lenteren et al. 2006, van Lenteren et al. 2006).
Berndt et al. (2007) provide an example of how to develop a test design.
Babendreier D., Bigler F. and Kuhlmann U. (2005). Methods Used to Assess Non-target Effects of Invertebrate Biological Control Agents of Arthropod Pests. BioControl 50: 821-870.
Babendreier D., Bigler F. and Kuhlmann U. (2006). Current status and constraints in the assessment of non-target effects. Pp. 1-13 In: Environmental impact of invertebrates for biological control of arthropods – methods and risk assessment, F. , F. Bigler, D. Babendreier and U. Kuhlmann (Ed.) CABI Publishing, Wallingford, UK
Barratt B.I.P., Blossey B. and Hokkanen H.M.T. (2006). Post-release evaluation of non-target effects of biological control agents. Pp. 166-186 In: Environmental Impact of Arthropod Biological Control: Methods and Risk Assessment, U. Kuhlmann, F. Bigler and D. Babendreier (Ed.) CABI Bioscience, Delemont, Switzerland.
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.
Berndt L.A., Mansfield S. and Withers T.M. (2007). A method for host range testing of a biological control agent for Uraba lugens. New Zealand Plant Protection 60: 286-290.
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.
Hill R.L. (1999). Minimising uncertainty - in support of no-choice tests. Pp. 1-10 In: Host specificity testing in Australasia: towards improved assays for biological control, W.T.M., L. Barton Browne and J. N. Stanley (Ed.) CRC for Tropical Pest Management, Brisbane, Australia.
Kitt J.T. and Keller M.A. (1998). Host selection by Aphidius rosae Haliday (Hym., Braconidae) with respect to assessment of host specificity in biological control. Journal of Applied Entomology 122: 57-63
Kuhlmann U., Mason P.G., Hinz H.L., Blossey B., De Clerck-Floate R.A., Dosdall L.M., McCaffrey J.P., Schwarzlaender M., Olfert O., Brodeur J., Gassmann A., McClay A.S. and Wiedenmann R.N. (2006). Avoiding conflicts between insect and weed biological control: selection of non-target species to assess host specificity of cabbage seedpod weevil parasitoids. Journal of Applied Entomology 130: 129-141
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.
Spafford-Jacob H. and Briese D.T. (2003). Improving the selection, testing and evaluation of weed biocontrol agents. CRC for Australian Weed Management Technical Series no. 7, Adelaide, Australia.
Withers T.M. and Barton-Browne L. (2004). Behavioral and physiological processes affecting the outcome of host range testing. Pp. 40-55 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.
Withers T.M., Hill R.L., Paynter Q., Fowler S.V. and Gourlay A.H. (2008). Post-release investigations into the field host range of the gorse pod moth Cydia succedana Denis & Schiffermuller (Lepidoptera : Tortricidae) in New Zealand. New Zealand Entomologist 31: 67-76
van Driesche R. and Reardon R. (2004). Assessing host ranges for parasitoids and predators used for classical biological control: a guide to best practice. Pp. 243. USDA Forest Service, Morgantown, West Virginia.
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.
van Lenteren J.C., Bale J., Bigler F., Hokkanen H.M.T. and Loomans A.J.M. (2006). Assessing risks of releasing exotic biological control agents of arthropod pests. Annual Review of Entomology 51: 609-634.
van Lenteren J.C., Cock M.J.W., Hoffmeister T.S. and Sands D.P.A. (2006). Host specificity in arthropod biological control, methods for testing and interpreting the data. Pp. 38-63. CAB Publishing, Delemont.
Selecting test species
What we do know