Choice or No-choice Tests? Effects of Experimental Design on the Expression of Host Range

Estimation of the host range of entomophagous biological control agents (parasitoids and predators) is complex. It is not always possible to inoculate all test organisms with eggs or neonates to determine “physiological suitability”. We argue that, for the host range testing of parasitoids, it is important to initially employ test procedures that will maximize the probability that the test species will be accepted for oviposition. This is vital to ensure that our testing methods do not generate data with a false impression of host specificity. No-choice tests are generally thought to maximize the expression of host range. The main reason for this may be increases in readiness to oviposit induced by host deprivation per se and/or associated changes in egg load, which has the potential to counteract any effects of prior experience. Sequential no-choice tests should only be used with caution as they have the potential to produce false negative results if the period of access to the lower ranked host is insufficient to allow time dependent changes in responsiveness of the parasitoid to become apparent, or if insufficient controls are utilized. Choice tests including the target host have the potential to mask the acceptability of lower ranked hosts, thereby producing false negative results. Examples where wider host ranges have been expressed in no-choice tests than in choice tests, and vice versa are presented. Sufficient variation exists that we recommend that researchers routinely use both assay methods for host range testing of parasitoids and predators.