Containment testing

Overseas field surveys or field tests

The most significant non-containment research that can assist with a biological control application involves research on the host range of the proposed agent in the country of origin, and open field or large cage testing in another country where the proposed agent is either endemic or established. Open-field tests obtain host range data on agents under more natural conditions than those obtained via cage tests. The results are more reliable and more likely to reflect that which will occur should the agent be introduced to New Zealand. It has been argued that such field tests are necessary or highly desirable to ascertain the potential risk to those probably closely related plants whose status as a host cannot be discounted even after the "reverse testing sequence" method has been followed in the laboratory (Wapshere 1989).

Nonetheless, open-field tests have been criticised on several grounds. It has been argued that agent densities in such situations may be much lower than potential populations in the country of introduction and any changes in host choice induced by conditions of extreme intra-specific competition and/or starvation would not be replicated (McFadyen 1998). This is a valid criticism considering that temporary spill-over feeding as a result of large population densities and/or rapid defoliation of target weeds has been responsible for a significant number of reported non-target attacks in weed biocontrol (Louda et al. 2003). Such conditions would virtually never be recreated either in the country of origin or in a field test.

So for the EPA to consider data reliable open field tests would have to be considered sufficiently robust including evidence that:

the agents used in field tests are genetically equivalent to the agent to be imported (same source population/ biotype and species);
the genetic source of the weed in the field test and in New Zealand are the same, at least in a significant portion of the test;
NZ native plants growing in the field test are representative of their natural populations in New Zealand and are in appropriate physiological state/ age/ maturity for the field test to be reliable;
damage or insects collected as part of the results can be reliably attributed to the agent compared to other closely related insects present in that area;
a sufficient density of the proposed agent was present over the duration of the field test to ensure all test plants were exposed to agents searching for host plants;
environmental and climatic conditions, including factors capable of influencing plant palatability, such as soil type or soil moisture, were similar to those likely to be experienced in New Zealand;
for tests that were carried out primarily for the benefit of another country (e.g. Australia or Hawaii), that all the above remain fully applicable to the unique New Zealand situation (i.e., that all identified potentially at risk New Zealand plants were included in at least one field array);
the design of field trials was robust and included sufficient replication over space or time to allow valid statistical analysis.

Briese (1999) has reviewed the potential types of experimental design that can be used in field tests, including when augmentation of background populations are made to enhance the robustness of the data, and identifies a number of issues critical to the success of open field tests. Furthermore he has suggested a new design of open-field test called the "two-phase set design" that will encourage spill-over feeding in a field test, if this is ever likely to occur as a result of the rapid defoliation or death of the target weed in the field (Briese 1999).

In conclusion, open field tests have not become widely used, in part because they must be done in the country of origin of the agent, in part because of the potential quarantine problems of moving test organisms to that region, and in part because they are often seen as a final step, not a first step (due to cost). Applying open field tests to insect biocontrol agents is even less feasible because of quarantine concerns with the test insect species. It is difficult to conceive of a separate country existing where all the same relevant non-target insects are also pre-existing. Conceivably, simulated open field tests could be constructed using walk-in cages within naturally-lighted, quarantine glasshouses, which would provide large enough spaces for natural insect behaviours to be displayed (van Driesche and Murray 2004).

References

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.

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.

McFadyen R.E.C. (1998). Biological control of weeds. Annual Review of Entomology 43: 369-393.

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.