Selecting biological control agents
Factors influencing host selection in the target range
Habitat specificity, habitat suitability, and agent mobility
Natural enemies do not co-occur universally with their hosts. The distribution of agents is mediated not only by the presence of the host but also by a wide range of physical and biological requirements. The likelihood of non-target effects is therefore dependent on not only the host-specificity of the natural enemy, but also its ability to persist alongside susceptible hosts. Identification of well-documented requirements that limit distribution might allow an applicant to argue that the ecological host range of a proposed control agent was smaller than the fundamental host range. This would occur if the distribution of at risk non-target species did not overlap with the predicted distribution of the control agent, or that agents may perform poorly in that environment. Any claim that a host within the fundamental host range of an agent was excluded from the ecological host range on the grounds of habitat specificity would need to be well-supported.
Many factors other than the presence of a host limit the distribution of parasitoids and herbivores, but the most obvious is climate. Insects are poikilothermic, and absolutely dependent on ambient temperature for their continued development and existence. No biocontrol agent can permanently colonise a habitat outside the lethal temperature range of any of its life stages. Distribution of populations may therefore be limited by elevation or latitude, but predicting distribution accurately is difficult without adequate information about the climate preferences of agents and potential non-target hosts. When Microctonus hyperodae was introduced to New Zealand for the control of the lucerne pest Listronotus bonariensis, it was considered unlikely that the parasitoid would move from the lowland disturbed cropping environment to higher elevation native grasslands. However, parasitoids have colonised such areas and limited attack on non-target weevils has been observed in this cold habitat (Barratt et al. 1997, Barratt 2004).
Distribution can be mediated by rainfall or aridity, crop systems, forest type, aspect, plant species, plant community, and even physical characteristics such as nest hole size (Sands and Van Driesche 2004). Benson et al. (2003) found that Cotesia glomerata foraged in sunny open areas but not in the shade. This is why the brassica crop pest Pieris rapae is attacked in NE U.S.A. but the related forest species P. virginiana is not. Beard and Walter (2001) found that a species of a predatory phytoseid mite was associated with one particular tree species. Kitt and Keller (1998) showed by wind tunnel experiments that only aphids found on rose bushes would be at risk from Aphidius rosae Haliday.
Romeis et al. (2005) reported that parasitism levels by Trichogramma varied greatly among different habitats, plants or plant structures on which the host eggs were located. The mechanisms for this variation included plant spacing, plant structure, plant surface structure and chemistry, plant volatiles and plant colour. In addition, plants can affect parasitoid behaviour and activity by providing carbohydrate food sources such as nectar to the adult wasps, and by affecting the nutritional quality of the host eggs for progeny development. Rutledge and Wiedenmann (1999) tested the responses of three Cotesia species (Braconidae) that parasitise stem-boring Lepidoptera to volatiles from a range of host and non-host grasses and volatiles from non-grass, non-host plants. The three species were differentially attracted to grasses in a pattern consistent with field host records. They suggested that testing preference for habitat plants could help to determine the ecological host range of some parasitoids, assist in matching these to target crops, as well as streamlining testing for impacts against non-target species. Specificity can even be restricted to microsites such as plant structures. Duan and Messing (1998) showed that opiine braconids introduced to Hawaii for fruit fly control only attacked tephritid larvae in fruit or fruit-like structures, and caused no non-target effects on those in flowerhead-gall infesting tephritids. Parasitoids of leafmining species often attack a wide range of species, but the realised host range can be limited by the host plants on which those leafminers feed (Askew 1994).
Habitat specificity does not allow control agent populations to persist. However, susceptible non-target plants may still be at temporary risk if the control agent can disperse out of acceptable habitats in sufficient numbers to cause damage to non-target hosts (spill over). Withers et al. (2008) detected huge populations of the moth Cydia succedana in a valley 4 km from the nearest stand of known hosts. There was no evidence of damage to local non-target plants that might have generated this population. Dispersal monitoring eventually indicated that this large population had accumulated in the valley passively on the prevailing wind. The valley was not a suitable permanent habitat for the moth, but had a susceptible non-target species been present in the valley, significant 'spill over' damage may have resulted as a result of this dispersal.
Even when habitat characteristics allow persistence, population growth and hence any potential risk to non-target plants is affected by environmental conditions. In marginal habitats annual climate patterns might determine oviposition rates and lifetime fecundity, the number of generations in a year, and the onset and cessation of facultative diapause or aestivation. Julien et al. (1995) compared the potential ranges of both alligator weed Alternanthera philoxeroides and its chrysomelid control agent Agasicles hygrophila. The model predicted where the agent would have some impact on the weed and those areas where beetle damage would be little or absent. Where agent and host have different temperature thresholds for development and activity the effect of climate on relative growth rates and reproductive rates may well determine whether non-target impacts will be significant or not.
Askew R.R. (1994). Parasitoids of leaf-mining Lepidoptera: what determines their host ranges? Pp. 177-202 In: Parasitoid community ecology, B.A. Hawkins and W. Sheehan (Ed.) Oxford University Press, Oxford
Barratt B.I.P. (2004). Microctonus parasitoids and New Zealand weevils: comparing laboratory estimates of host ranges to realized host ranges. Pp. 103-120 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.
Barratt B.I.P., Evans A.A., Ferguson C.M., Barker G.M., McNeill M.R. and Phillips C.B. (1997). Laboratory nontarget host range of the introduced parasitoids Microctonus aethiopoides and Microctonus hyperodae (Hymenoptera: Braconidae) compared with field parasitism in New Zealand. Environmental Entomology 26: 694-702.
Beard J.J. and Walter G.H. (2001). Host plant specificity in several species of generalist mite predators. Ecological Entomology 26: 562-570
Benson J., Pasquale A., Van Driesche R. and Elkinton J.S. (2003). Assessment of risk posed by introduced braconid wasps to Pieris virginiensis, a native woodland butterfly in New England. Biological Control 26: 83-93
Duan J.J. and Messing R.H. (1998). Effect of Tetrastichus giffardianus (Hymenoptera: Eulophidae) on nontarget flowerhead-feeding tephritids (Diptera: Tephritidae). Biological Control 27: 1022-1028.
Julien M.H., Skarratt B. and Maywald G.F. (1995). Potential geographical distribution of alligator weed and its biological control by Agasicles hygrophila. Journal of Aquatic Plant Management 2: 55-60
Kitt J.T. and Keller M.A. (1998). Host selection by Aphidius rosae Haliday (Hym., Braconidae) with respect to assessment of host specificity in biological control. Journal of Applied Entomology 122: 57-63
Romeis J., Babendreier D., Wackers F.L. and Shanower T.G. (2005). Habitat and plant specificity of Trichogramma egg parasitoids - underlying mechanisms and implications. Basic and Applied Ecology 6: 215-236
Rutledge C.E. and Wiedenmann R.N. (1999). Habitat preferences of three congeneric braconid parasitoids: implications for host-range testing in biological control. Biological Control 16: 144-154
Sands D.P.A. and Van Driesche R.G. (2004). Using the scientific literature to estimate the host range of a biological control agent. Pp. 15-23 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., Hill R.L., Paynter Q., Fowler S.V. and Gourlay A.H. (2008). Post-release investigations into the field host range of the gorse pod moth Cydia succedana Denis & Schiffermuller (Lepidoptera : Tortricidae) in New Zealand. New Zealand Entomologist 31: 67-76
Host finding cues and behaviour
Experimental confirmation of host range