Containment testing

Host range testing methods

Choice tests

In choice tests, two or more plant or host species are presented to the biocontrol agent simultaneously, and the comparable response is a measure of preference between the two options (. The target species is often, but not always, one of the choices offered. This method is most commonly used to compare oviposition preferences but can also be used for comparing feeding preferences of adults and sometimes of larvae that are sufficiently mobile to move between hosts.

Choice tests rose to a height of popularity in the 70's and 80's because practitioners felt that the no-choice test (especially larval starvation tests) were resulting in too many cases in which a promising agent was rejected when a wider than expected physiological host range was being predicted in the laboratory. Hence there was a time when testing for weed agents relied on the results of choice tests alone. Sometimes choice tests were only conducted on those species that had revealed positive results in no-choice tests, apparently in the mistaken belief that subsequent lack of attack in choice tests would identify which of the positive responses in the no-choice data set were "erroneous" (Fornasari et al. 1991). This line of reasoning has since been shown to be inappropriate as research has now revealed many cases where choice tests failed to reveal a lower but significant host association (Marohasy 1998).

Choice tests have often been used to compare oviposition responses in weed biocontrol agents, parasitoids and predators. In some cases the polyphagy of a species is clearly revealed when all non-target species are attacked at rates similar to the known field host. However, in choice tests, sometimes agents will show strong oviposition preferences towards the target pest or weed, or even fail to attack non-target hosts at all. From such results it is tempting to conclude that the agent is host specific, whereas a more accurate interpretation may be that the preference ranking (or hierarchy) of the agent is that the target host sits most highly ranked out of those test species presented to it.

The greatest concern with choice tests is that they have clearly been shown (Withers et al. 2000, Barton-Browne and Withers 2002) to carry an unsatisfactorily high risk that there will be no attack on less stimulating hosts (less preferred or lower ranked) in choice tests that are run for too short a time or when the insect is introduced to the test in an un-stimulated (such as satiated in relation to food, or not ready to oviposit in relation to reproductive) state. Consequently, such tests can fail to reveal the fundamental host range, and there is a risk of a false negative result. On this basis Barton-Browne and Withers (2002) strongly recommended that choice tests should never be used as the only type of test for assessing risk to non-target hosts, and that they should not be used for a 'screening' phase of host specificity testing programs, if the intention is the use the results to reduce the host list for further testing.

Another way of expressing this, is that for the results of a choice oviposition test to be predictive of field events, (1) the agent must experience the choice in the field – that is, the non-target (lower ranked) host must not be the only possible host encountered, or (2) the non-target (low ranked) host must be so non-preferred that even agents deprived of their preferred hosts for considerable periods will keep searching rather than attack the low ranked species. Since these conditions may not always be met, inferring that a species not attacked in a choice oviposition test is not a host will lead to some unpredicted impacts (van Driesche and Murray 2004). Despite these concerns choice tests remain valuable, and in some cases, absolutely crucial for helping to assess the likely risk of non-target attack in the field, when we are reliant on laboratory testing methods alone.

References

Barton-Browne L. and Withers T.M. (2002). Time-dependent changes in the host-acceptance threshold of insects: implications for host specificity testing of candidate biological control agents. Biocontrol Science and Technology 12: 677-693.

Fornasari L., Turner C.E. and Andres L.A. (1991). Eustenopus villosus (Coleoptera: Curculionidae) for biological control of yellow starthistle (Asteraceae: cardueae) in north America. Environmental Entomology 20: 1187-1194.

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.

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.

Withers T.M., Barton-Browne L. and Stanley J. (2000). How time-dependent processes can affect the outcome of assays. Pp. 27-41 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.