Selecting biological control agents
Experimental confirmation of host range
Indicators of effects
Food webs in native and exotic ranges
Food webs provide a description of the components of a community and of the interactions between those components. Binary food webs use the presence or absence of interactions to illustrate linkages between species in a community. Simple food webs might be easily constructed in simple ecosystems such as crops, but in not in more complex environments. In natural habitats for example, not only are the interactions highly complex, but many of the components may be unknown to science. Construction of simple food webs may therefore require considerable research. Fully quantitative food webs provide a measure of the relative frequency of interactions, as well as the relative abundance of the components. All quantitative food webs are data hungry and expensive to prepare. The value and limitations of food webs in describing the processes that structure communities are reviewed by van Veen et al. (2006). Food webs constructed in the native range could help predict the host range and parasitoid susceptibility of the proposed agents. Food webs constructed in the receiving range could help identify potential parasitoids and competitors for the proposed agent.
Willis and Memmott (2005) demonstrated how food webs can be used to show the interactions between plants, seed-feeding insects and their parasitoids, and to generate testable hypotheses regarding indirect interactions between introduced agents and non-target species. Their model was the weed, bitou bush, Chrysanthemoides monilifera ssp. rotundata, and a biocontrol agent for this weed in Australia, the tephritid fly, Mesoclanis polana. The food webs revealed how a highly host-specific biocontrol agent such as M. polana has the potential to change community structure by increasing the abundance of native parasitoids. The webs also suggest that indirect interactions between M. polana and native non-target species were possible, mediated by shared parasitoids.
Hopper and Wajnberg (2006) described the food web based on the Lepidoptera of Alakai Swamp on Kauai, Hawaii. They showed that 83% of parasitoids reared from native moths were biological control agents (all introduced prior to 1945), 14% were accidental immigrants, and 3% were native species. Parasitism by biological control agents reached 28% in some species of moth. However, the authors could not assert what this meant for the population dynamics of the species attacked, and hence for the community dynamics in the swamp, or whether species had already been lost from the community.
Munro and Henderson (2002) used quantitative webs to interpret the complex interactions between native parasitoids of native Lepidoptera and the introduced species Trigonospila brevifacies (Hardy) in New Zealand. Its host range overlaps with 12 native and one introduced parasitoid species, and it parasitised more species of Tortricidae than other parasitoids at the North Island forest sites surveyed. All native parasitoid species were less abundant than T. brevifacies and this species comprised 15.6 - 79.5% of the parasitoid load per species.
Food webs summarise the trophic interactions in a community plus give insights into indirect but trophically-mediated interactions such as resource exploitation and apparent competition. In food webs based on phytophagous insects there is huge scope for trait mediated indirect effects (van Veen et al. 2006). There is increasing awareness that other interactions may be equally important, particularly where behaviour of one species is modified by another. For example, the impact of kairomones on parasitoid behaviour is not captured in food webs. Infochemical interaction webs might be a natural counterpart to trophic webs (Vet and Godfray 2008).
Food webs have value for visualising interactions in communities, but as yet do not have sufficient power to predict how natural enemy interactions structure communities (Hopper and Wajnberg 2006, van Veen et al. 2006).
Lockwood (2000) considered that the conservation of 'species' as a basis for non-target impact assessment lacked depth. He made the case that a population is not a collection of individuals but the energy and nutrient flows that manifest through the individual. For this reason, in assessing impacts the emphasis should be on population and community processes, such processes as reproduction, competition, consumption, not on population size.
Hopper K.R. and Wajnberg E. (2006). The risks of interbreeding and methods for determination. In press In: Environmental Impact of Arthropod Biological Control: Methods and Risk Assessment, U. Kuhlmann, F. Bigler and D. Babendreier (Ed.) CABI Bioscience, Delemont, Switzerland.
Lockwood J.A. (2000). Nontarget effects of biological control: what are we trying to miss? Pp. 15-30 In: Nontarget effects of biological control introductions, P.A. Follett and J.J. Duan (Ed.) Kluwer Academic Publishers, Norwell, Massachusetts, USA.
Munro V.M.W. and Henderson I.F. (2002). Nontarget effect of entomophagous biocontrol: shared parasitism between native lepidopteran parasitoids and the biocontrol agent Trigonospila brevifacies (Diptera: Tachinidae) in forest habitats. Environmental Entomology 32: 388-396.
Vet L.E.M. and Godfray H.C.J. (2008). Multitrophic interactions and parasitoids behavioural ecology. Pp. 231-253 In: Behavioural ecology of insect parasitoids: from theoretical approaches to field application, E. Wajnberg, C. Bernstein and J.J.M. Van Alphen (Ed.) Blackwell Publishing, Oxford, UK
Willis A.J. and Memmott J. (2005). The potential for indirect effects between a weed, one of its biocontrol agents and native herbivores: A food web approach. Biological Control 35: 299-306.
van Veen F.J.F., Morris R.J. and Godfray H.C.J. (2006). Apparent competition, quantitative food webs, and the structure of phytophagous insect communities. Annual Review of Entomology 51: 187-208
What we don't know
Manipulation and surrogate systems