Selecting biological control agents

Selecting effective agents

Population matching for maximum performance

Matching agent-host populations

Biocontrol practitioners have generally worked on these two principles: firstly, that the more control agents that attack the pest, the more likely it is that there will be significant reduction in the effects of the pest, and secondly, that agent populations will be greatest when the natural enemy is well adapted to its host and to its new environment. Hufbauer and Roderick (2005) reviewed the importance of microevolution and local adaptation in the success of biological control in detail, and explored whether such generalities were valid. They found examples in the biological control literature illustrating that natural enemies often perform better on genotypes from their local population than genotypes from others, but this was not always true. The strength and importance of local adaptation is influenced by host range, relative generation time, migration rates and the metapopulation structure of host and natural enemy. For example, short-lived natural enemies (such as weed control agents) on long-lived hosts tend to be more locally adapted than on short generation hosts. This may explain the strong emphasis on habitat, biotype and climate matching in the weed biocontrol literature. Similarly, local adaptation is more likely if the migration rate of the host is much lower than that of the natural enemy.

Although not universal, it is clear that local adaptation exists, and genotypic variation of both biological control agents and hosts can influence the level of control achieved. Goolsby et al. (2006) point out examples of where good matching of biological control agent and target has resulted in more host specific and effective agents, and those where agents collected from different biotypes of the host have been ineffective. These authors have reviewed factors to consider during native range research for weed biological control agents. Better awareness of these and other issues reviewed by Hufbauer and Roderick (2005) would allow the selection of better control agents. However, it is clear that occurrence and strength of local adaptation is itself variable from system to system, and theory is not sufficiently well developed to provided day-to-day assistance to practitioners. Clearly these are important issues to address at some level when selecting agents, but the need for more information on local adaptation and its genetics has to be balanced by geographical and financial limitations of overseas research.

Matching agents with the receiving environment

The importance of matching biotypes so that agents can find and exploit the host well is discussed below. The natural enemy must also be physiologically able to survive and reproduce well in its new environment if populations are to build. The climatic conditions in the receiving range must not limit the intrinsic rate of increase of the natural enemy. Both pre-requisites are normally met by introducing a population of the natural enemy that is sourced from the centre of origin of the pest, and from a climate that is broadly similar to the target habitat.

At its simplest, climate-matching must ensure that the temperature and humidity thresholds for survival and development of the introduced agent are met in the new habitat or the agent will not establish. Temperature, humidity and rainfall extremes should not limit survival of any stage. Hill et al. (1993) describe the introduction of additional strains of gorse spider mite to overcome apparent susceptibility to rainfall.

Hufbauer and Roderick (2005) cover a wide range of other issues in their review including the risk that bringing individuals together from disparate locally adapted populations may produce out breeding depression in performance, the distinction and variation between infectivity and virulence (attack and damage) in assessing the value of agents, the role of post-release adaptation in achieving control, the influence of genetic bottlenecks, and the risk of evolution of adverse effects post-release. Increasing awareness of the issues posed by local adaptation is leading more and more to the permitting of only single provenances of agents.

Systems such as CLIMEX and other modelling approaches ( can be used to predict which area within the native range will be an appropriate source of control agents ( and for determining the climate range of both host and natural enemy within the target area. With limited biological information about developmental thresholds and rates, DYMEX modelling can make a first estimate of the life history, potential population increase an distribution of a control agent on release (Kriticos et al. 1999. Barlow (1999) describes several other examples of the use of models to estimate performance post-release.

References

Barlow N.D. (1999). Models in biological control: a field guide Pp. 43-70 In: Theoretical approaches to biological control, B.A. Hawkins and H.V. Cornell (Ed.) Cambridge University Press UK.

Goolsby J.A., Van Klinken R.D. and Palmer W.A. (2006). Maximising the contribution of native-range studies towards the identification and prioritisation of weed biocontrol agents. Australian Journal of Entomology 45: 276-286

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.

Hufbauer R.A. and Roderick G.K. (2005). Microevolution in biological control: Mechanisms, patterns, and processes. Biological Control 35: 227-239.

Kriticos D.J., Brown J.R., Radford I.D. and Nicholas M. (1999). Plant population ecology and biological control: Acacia nilotica as a case study. Biological Control 16