Host range testing methods
Positive controls are essential for oviposition and feeding trials, to validate negative responses by showing that the group from which the test biocontrol agents were drawn, had the capacity for oviposition or feeding. Individuals used in controls should either be:
- different insects from the same rearing batch that are exposed to the target pest simultaneously with the main experiment, or
- the same insects used in the main experiment that, after exposure to a non-target species, are re-used by exposing them to the target host to demonstrate their physiological readiness to respond positively to a highly ranked host (van Driesche and Murray 2004).
Negative controls, in which test species are not exposed to parasitoids, or plants not exposed to weed agents, are needed to detect background mortality of test species that is unrelated to the biocontrol agent. For feeding tests with predators, controls in which only water is provided show whether the test prey species provides any nutritional benefit to the predator by assessing survival time with prey versus water alone. For tests measuring survival and development, performance on a host for survival and development of known suitability or an artificial diet could be used as a standard against which reduced survival in a poorer host can be gauged (van Driesche and Murray 2004).
Rigorous positive controls are absolutely critical for the reliable host range testing of parasitoids and predators, particularly when no-choice tests are used. Parasitoids can differ in the way they mature their eggs prior to oviposition. This can be expressed according to the "ovigeny index" (Withers and Barton-Browne 2004). The main implication of the ovigeny index on host range testing methods is for those species that emerge with no or very few mature eggs. There is a risk that young females may not oviposit in any host because of a lack of mature eggs. Futhermore, even young females carrying some mature eggs might not oviposit in lower ranked hosts because of a low egg load. Positive controls needs to be females from the same rearing group exposed to the target pest to confirm egg laying ability of the parasitoid cohort used.
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. 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.