Selecting biological control agents
Predicting direct non-target impacts
Selecting agents for safety
Taxonomic status and synonyms
Clarification of the current taxonomic status is vital to ensure that the correct natural enemy species or entity is introduced. An application to introduce a biological control agent to New Zealand must contain a definitive identification to species level, supported by expert opinion. If this is not possible, evidence must be provided to confirm that the proposed agent is a biologically and reproductively stable entity. For example, Tortrix sensu lato "chrysanthemoides" was approved for release against the weed boneseed in 2004 even though this moth has not yet been formally described (See NOR03001, click on Documents and Decision [http://www.epa.govt.nz/search-databases/Pages/applications-details.aspx?appID=NOR03001#]).
The first step in selecting control agents is to compile information on the identity and taxonomic history of the proposed control agent from literature sources (Sands and Van Driesche 2004), collection data and the expertise of taxonomic specialists. The synonymy may also indicate whether close relatives of the agent occur in the receiving range, providing better definition of likely food web relationships. The current taxonomic status and any past synonyms of the agent must be established before any effort is made to compile the literature of the candidate natural enemy. Unrecognised synonyms can cause important information to be overlooked, and use of an incorrect taxon may lead to the inclusion of false host records (Sands and Van Driesche 2004). The synonymy may provide information on the hosts of related species that might be important in the design of host-range tests. The taxonomic stability of related species may also be important in selecting non-target species for testing. The synonymy of the gorse spider mite, Tetranychus lintearius Dufour included a known pest, which led to experiments to check whether significant hybridisation was likely between gorse spider mite and two pest species (Hill and O'Donnell 1991).
It is equally important to identify the target species. Inconsistent identification of hosts in the native and introduced ranges can lead to the introduction of agents that are either incapable of establishing because the host is absent, or are ill-suited to damage the pest form of the target. Correct matching is most important for highly specific agents such as plant disease pathovars (Morin et al. 2006). The utility of molecular techniques is increasing rapidly, and resolution of these issues will become routine (Sheppard 2003). Better characterisation of population and phylogenetic relationships at the sub-specific level, will result in better physiological and behavioural matches between the agent and host populations, increasing the efficiency of agent selection.
Gassmann and Schroeder (1995) reviewed the major biological control programme against Euphorbia esula L. (leafy spurge) in the U.S.A. After 30 years the taxonomy of the host plant remained difficult, and the taxon was more variable in the native range than in the exotic range. In the native range it grew across a range of biomes and ecoclimatic areas, with various forms, and defining the centre of origin in Eurasia to match US populations was controversial. Although there was a large complex of natural enemies in Europe, they considered that morphological and biochemical differences between the target weed in the native and exotic ranges may have led to host-plant incompatibility, handicapping both the establishment and the performance of the agents released.
Gassmann A. and Schroeder D. (1995). The search for effective biological control agents in Europe: History and lessons from leafy spurge (Euphorbia esula L.) and cypress spurge (E. cyperissias L.). Biological Control 5: 466-472
Hill R.L. and O'Donnell D.J. (1991). Reproductive isolation between Tetranychus lintearius and two related mites, T. urticae and T. turkestani (Acarina: Tetranychidae). Experimental and Applied Acarology 11: 241-251
Morin L., Evans K.J. and Sheppard A.W. (2006). Selection of pathogen agents in weed biological control: critical issues and peculiarities in relation to arthropod agents. Australian Journal of Entomology 45: 349-365
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.
Sheppard A.W. (2003). Prioritising agents based on predicted efficacy: beyond the lottery approach. Pp. 11-21 In: Improving the selection, testing and evaluation of weed biocontrol agents, H. Spafford-Jacobs and D. T. Briese (Ed.) Cooperative Research Centre for Australian Weed Management, Adelaide, Australia
Conflicts of interest