Applying for general release or release with controls
Knowledge about the geographic and altitudinal distribution of a control agent in its region of origin may define the climatic limits within which the organism can live and reproduce. Successful biological control requires the introduced control agent to establish, and then to thrive sufficiently to control the target pest. The likelihood of both is advanced by selecting the founding population in the home range of the pest from a climate closely matching the climate (temperature and rainfall patterns) into which the agent will be released. Photoperiod may provide important cues that synchronise the life histories of agent and target, so the latitude of the source may also be important.
Selecting the correct source may be straight forward if the target pest occupies a limited geographical range in New Zealand. However, if the target is widespread then (given New Zealand's wide latitudinal range and varied climates) selection of the correct source may be difficult. Does a single population have a sufficient range of traits to perform at its best in a range of climates? Is the agent sufficiently physiologically plastic to quickly adapt to a wide range of climates? Are populations in different regions adapted to local climates? If this is the case, then founding populations would have to be sourced from more than one climate to succeed across the range of environments present in New Zealand. This approach is complicated by the possibility of differing host range between agent populations in the native range (see above).
There is little generalised theory to assist biological control practitioners in making these decisions, and judgments about how many populations should be sourced and from where must be made on a case by case basis, using information sources such as those outlined above.
While temperature is probably the dominant driver in determining the size that populations can attain, rainfall can be a determinant of initial establishment. Gorse spider mites sourced from southern England were introduced to New Zealand to attack gorse in 1988 (Hill et al. 1989). Following release it was noted that mites could not establish in western and northern areas of New Zealand, and this was attributed to poor adaptation to heavy rainfall. Further strains of mites were introduced from the wet, northwest coast of Spain and Portugal, and gorse spider mite subsequently established throughout New Zealand (Hill et al. 1993).
Hill R.L., Gourlay, A.H. and Wigley P. (1989). The introduction of gorse spider mite Tetranychus lintearius for biological control of gorse. Proceedings of the 42nd New Zealand Weed and Pest Control Conference: 137–139.
Hill R.L., Gourlay, A.H. and Winks, C.J. (1993). Choosing gorse spider mite strains to improve establishment in different climates. In: Proceedings of the 6th Australasian Grasslands Invertebrate Ecology Conference, R.A Prestidge (Ed.). AgResearch, Hamilton, New Zealand.
Previous use as a biological control agent, and efficacy in that use