Host range testing methods
Test designs for arthropod biological control agents
Similarly to weed biocontrol, there are some extremely valuable publications describing the design of host range tests for parasitoids and predators (van Driesche and Murray 2004). A common generalisation is that estimation of the host range of entomophagous biocontrol agents (parasitoids and predators) is more complex than for phytophagous weed biological control agents. This is primarily because there is an additional trophic layer involved and often an intimate and specific relationship between the target and test organisms and their substrate (usually their food plant). An important consequence of this intimate and specific relationship between the host or prey of entomophagous agents and their substrate is that prior experience of the substrate can affect the organism's responsiveness to cues from this and other substrates.
A second complicating factor for endoparasitoids is that it is not possible, in most cases, to inoculate all test organisms with eggs or neonates to determine "suitability", though exceptions do exist (Morehead and Feener 2000, Fuester et al. 2001). Thus, a program to determine the host range of parasitoids is denied one of the most powerful tools (the so-called physiological host range test on larvae) that is used in determination of host range of phytophagous agents (Hill 1999, van Klinken 2000). This means that, in the host range testing of parasitoids, it is important to employ test procedures that will maximize the probability that the test species will be accepted for oviposition in the first place. This is vital for an accurate risk assessment (Withers and Barton-Browne 2004). It is possible in some cases, however, to distinguish between behavioural host acceptance and physiological compatibility of parasitoids and test species. For example, McNeill et al. (2000) used a pathogen on the ovipositor of parasitoids as an indicator of an oviposition attempt in a test species. Parallel exposure of untreated parasitoids to the test insects gave an estimate of physiological compatibility.
Failing a high tech method such as this, only the careful dissection of all exposed hosts to identify parasitoid ovi-or larvi-position will reliably ascertain whether a failure to develop a parasitoid was the result of a physiological host immune response or a lack of oviposition. This is a laborious and time-consuming exercise but may be necessary, along with carefully chosen controls, to prove that a suitable testing method has been utilised.
Fuester R.W., Kenis M., Swan K.S., Kingsley P.C., López-Vaamonde C. and Hérard F. (2001). Host Range of Aphantorhaphopsis samarensis (Diptera: Tachinidae), a larval parasite of the gypsy moth (Lepidoptera: Lymantriidae). Environmental Entomology 25: 332-340.
Hill R.L. (1999). Minimising uncertainty - in support of no-choice tests. Pp. 1-10 In: Host specificity testing in Australasia: towards improved assays for biological control, W.T.M., L. Barton Browne and J. N. Stanley (Ed.) CRC for Tropical Pest Management, Brisbane, Australia.
McNeill M.R., Barratt B.I.P. and Evans A.A. (2000). Behavioural acceptability of Sitona lepidus (Coleoptera: Curculionidae) to the parasitoid Microctonus aethiopoides (Hymenoptera: Braconidae) using the pathogenic bacterium Serratia marcescens Bizio. Biocontrol Science and Technology 10: 205-214.
Morehead S.A. and Feener D.H. (2000). An experimental test of potential host range in the ant parasitoid Apocephalus paraponerae. Ecological Entomology 25: 332–340.
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.
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.
van Klinken R.D. (2000). Host-specificity testing: why do we do it and how we can do it better. Pp. 54-68 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.
Test designs for weed biological control