Releasing biological control agents
Timing of releases
A new control agent can only establish if the target life stage of the host is present when the agent is released. Accurate timing of releases is therefore critical to establishment success. Even though univoltine or bivoltine pest species may be present year round, the life stages that are susceptible to the control agent may be present for a relatively short period. Releases must coincide with this window. In weed control, the susceptible plant structure at the right stage of development must be present when the agent is released. For example, Hill et al. (2000) found that the eurytomid seed-feeding wasp only laid eggs into the seeds of Acacia species for 3 weeks in late spring. If adult wasps were released outside this period, no suitable seeds would be available for oviposition. Similarly, Dymock (1987) found that adult ragwort seed fly, Botanophila jacobeae began emerging in late October and early November, 4-6 weeks before ragwort began to flower. During this time they fed in order to mature eggs. Release of new adults in early December, when flowers suitable for oviposition first appear, might not allow sufficient time for reproductive development before the resource became too mature for larval development.
Synchrony can be manipulated by seeding release sites with target pests of the correct stage, by introducing plants to the release site that are susceptible to the target pest out of season, or by judicious pruning to alter host plant phenology.
Seasonality and synchronicity are closely linked concepts. The period of the year during which agents can be released may be limited by the need to expose the control agents to climatic requirements that condition insects for successful diapause or hibernation. For weed control agents, it may be necessary to release agents during a narrow window when host-plant quality is adequate for successful development.
Hill (1982) found that the protein content of gorse foliage was generally low, peaking for only a short period in the spring. The life history of most foliage herbivores was keyed to this resource. Strategies for releasing foliage-feeding biological control agents in New Zealand were designed with this in mind (e.g. Hill et al. 1995).
Where an exotic agent is already established, or where native species are active, newly-introduced agents might face competition for hosts. Where possible, release multivoltine agents at a time of year when competition is at a minimum. Similarly, predators and host disease can interfere by removing susceptible stages of the pest, or even destroying parasitised individuals. Where such interactions are known, time releases to minimise such interactions.
Dymock J.J. (1987). Population changes of the seedfly, Pegohylemyia jacobaeae (Diptera: Anthomyiidae) introduced for biological control of ragwort. New Zealand Journal of Zoology 14:337-342.
Hill R.L. (1982). Seasonal patterns of phytophage activity on gorse (Ulex europaeus) and host plant quality. Pp. 237-242. In: Proceedings of the 5th International Symposium on Insect-Plant Relationships, J.H. Visser and A.K. Minks (Ed.) Wageningen, The Netherlands 1-4 March 1982.
Hill R.L., Gordon A.J. and Neser S. (2000). The potential role of Bruchophagus acaciae (Cameron) (Hymenoptera: Eurytomidae) in the integrated control of Acacia species in South Africa. Pp. 919-929 In: Proceedings of the X International Symposium on Biological Control of Weeds, N.R. Spencer (Ed.) Montana State University, Bozeman, Montana, USA.
Hill R.L., O'Donnell D.J., Gourlay A.H. and Speed C.B. (1995). The suitability of Agonopterix ulicetella (Lepidoptera: Oecophoridae) as a biological control agent for Ulex europaeus Fabaceae: Genisteae) in New Zealand. Biocontrol Science and Technology 5: 3-10.
Selecting release sites