Host range testing methods
A variation on the no-choice and choice test design is called the sequential test, in which the natural enemy is exposed to a series of test species, one at a time. These were called "sequential no-choice tests" by (Withers and Mansfield 2005, but "sequential choice tests" by van Driesche and Murray (2004). The sequence in which species are presented in such tests can vary. For example, the presumably high ranking target species can be presented before or after the presentation of one or a sequence of non-target species, or both. Another possibility is that insects are given access to the target species between each test with a different non-target species. Furthermore the duration of the experimental regime, times during presentations and between them, can vary (Barton-Browne and Withers 2002). All these factors can potentially have a significant effect on the outcome of the test.
A theoretical analysis of the potential outcomes of some sequential no-choice test types (Barton-Browne and Withers 2002) concluded that the outcome of the test varied according to the period of time for which the insects were given no-choice access, particularly access to host B. For instance, if the parasitoids oviposited during the first access to the target host (which was often the aim – to ensure the parasitoid was physiologically and behaviourally ready to oviposit), it may not accept the non-target host when it was first presented with it (note the generality of this outcome will depend upon the species reproductive biology and behaviour, and what latency period occurs between ovipositions). Generally we can conclude that the probability the parasitoid will accept the non-target host during the test will depend on whether the test was run for sufficient length of time for time-dependent processes (Barton-Browne and Withers 2002) to act upon the parasitoid to lower its acceptance threshold to a level whereby the non-target host stimulated attack behaviour (Withers and Mansfield 2005).
The general conclusion around sequential no-choice tests is that they should only be utilised cautiously when the physiology and behaviour of the parasitoid or weed biocontrol agent is very well documented and understood in terms of its temporal pattern of oviposition. Otherwise the validity of the results will be questionable (van Driesche and Murray 2004).
A complicated test design for phytophagous insects was used to establish the rank order (or preference rank) of plant species for oviposition in butterflies (Wiklund 1981). The insects were confined in a large field cage and each day the plant array was altered to ascertain the preference ranking. A similar design has been demonstrated where tethered lepidopterans have been successfully manipulated to ascertain the length of their discrimination phase with regards to different host species (Singer 1982, Jallow and Zalucki 1996) for quantifying the host preferences. However such experiments are relatively technically complicated and not all insects are suited to this type of experimental manipulation or to longer running assays. Marohasy (1998) suggests a similar method of sequential choice test where the plant species that receives the most eggs progressively is removed until there is not oviposition on any plant species. This has been referred to as the Preference Ranking Test (van Driesche and Murray 2004). This type of method is designed to overcome the problem of false negative results owing to unresponsiveness to lower ranked plants species in choice tests. This method has been trialled with some success (Solarz and Newman 1996, Withers et al. 2000) but whether such complicated sequential designs to reveal host preference rankings are worth the effort and can be safely interpreted will depend upon the circumstances and biology of each agent.
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.
Jallow M.F.A. and Zalucki M.P. (1996). Within- and between-population variation in host-plant preference and specificity in Australian Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae). Australian Journal of Zoology 44: 503-519.
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.
Singer M.C. (1982). Quantification of host preference by manipulation of oviposition behaviour in the butterfly Euphydryas editha. Oecologia 52: 224-229.
Solarz S.L. and Newman R.M. (1996). Oviposition specificity and behavior of the watermilfoil specialist Euhrychiopsis lecontei. Oecologia 106: 337-344.
Wiklund C. (1981). Generalist vs. specialist oviposition behaviour in Papilio machaon (Lepidoptera) and functional aspects on the hierachy of oviposition preferences. Oikos 36: 163-170.
Withers T. and Mansfield S. (2005). Choice or no-choice tests? Effects of experimental design on the expression of host range. Pp. 620-633 In: Second International Symposium on Biological Control of Arthropods, Davos, Switzerland, 12-16 September, 2005, M.S. Hoddle (Ed.) United States Department of Agriculture, Forest Service, Washington.
Withers T.M., McFadyen R.E. and Marohasy J. (2000). Importation protocols and risk assessment for weed biological control agents in Australia: The example of Carmenta nr ithacae. Pp. 195-214 In: Nontarget Effects of Biological Control, P.A. Follett and J.J. Duan (Ed.) Kluwer Academic Publishers, Norwell, MA.
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.
Parameters that can be measured in host range tests