Releasing biological control agents

Selecting release sites

Climate matching

Agents are usually sourced from areas of the native range that broadly match the area in which the agent is to be established, and sometimes more than one biotype is introduced to manage pests in different climates (van den Bosch et al. 1982). Climate can influence establishment success in a variety of ways. At its simplest, weather can influence biological control agent survival and dispersal at the time of release. Once released, the chosen release site should provide adequate conditions for the control agent to develop through all stages, while temperature, humidity and rainfall extremes should not limit survival of any stage. Inappropriate weather conditions can also interfere with insect behaviours such as mating and oviposition. Where it is uncertain exactly what conditions the agent needs to survive or breed are uncertain, releases should be made over a range of climates to maximise the likelihood that an agent population will survive. Climate can also influence your choice of the best time of year to release agents.

Habitat

If a pest insect attacks a range of plants, it is advisable to release control agents onto a range of alternative host plant species. Release amongst diverse vegetation rather than a monoculture may assist the agent to use natural host-seeking behaviours that might increase the chance of establishment. Non-crop vegetation also provides a refuge for the agent that is not directly affected by agricultural practices such as pesticide application. Release in permanent vegetation also increases security that the new population won�t be destroyed by harvest, cultivation or grazing.

Habitats can be enhanced in anticipation of the release of control agents. Fertilisers have been applied to weeds to enhance the reproductive potential of the agents applied. Establishment success can be improved by planting additional plants infested with the host into the release site, or simply as an 'out of season' host plant for natural populations of hosts.

Parasitoid host-finding behaviour is often mediated by kairomones generated by damaged plants. Creating infestations within release sites could arrest emigration from the release site once agents are released.

Adult parasitoids sometimes need to feed on nectar as adults to maximise egg-production (see references in Araj et al. 2006). Planting appropriate flower species within the release site could increase the parasitism rate and fitness of the control agents following release, and increase the likelihood of agent establishment.

References

Araj S.A., Wratten S.D., Lister A.J. and Buckley H.L. (2006). Floral nectar affects longevity of the aphid parasitoid Aphidius ervi and its hyperparasitoid Dendrocerus aphidum. New Zealand Plant Protection 59: 178-183.

van den Bosch, R., Messenger, P.S. and Gutierrez, A.P. (1982). An introduction to biological control. Intext Educational Publishers, Plenum Press, New York and London. Pp 247