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Selecting biological control agents

Selecting effective agents

Selecting more effective agents

Characteristics of successful biological control agents

Perhaps the most successful predictor of how successful an agent will be is its successful use elsewhere. Julien et al. (1999) recorded many examples of the transfer of successful control agents from place to place. The 'Silwood Project' analysed the success or failure of all weed control projects and attempted to draw conclusions about which attributes generally led to control. Conclusions proved difficult, but Crawley (1989) stated that weevils (Coleoptera: Curculionidae) and chrysomelids (Coleoptera: Chrysomelidae) had greater average success rate than other orders. The project also found that characteristics that predicted ability to establish also broadly predicted success, including small size, high voltinism, high fecundity, and long-lived adults. Plant characters that appeared to make plants more susceptible to biological control included genetic uniformity, lack of perennating organs, and susceptibility to secondary infections. Plant characteristics that militated against success included possession of rhizomes, high powers of regrowth and low food quality. Morin et al. (1997) evaluated the relative performance of three agents for mistflower (Ageratina riparia) in Hawaii before selecting a biological control strategy for New Zealand. Froud and Stevens (1998) and Froud and Stevens (2004) selected Thripobius semiluteus as a control agent for glasshouse thrips because it was known to have contributed to control of the thrips in closed habitats in Australia and elsewhere.

The success of related natural enemies and species in the same guild may also help predict success. Conversely, failure elsewhere should contraindicate use in New Zealand, but only if the reason for failure is known. However, this has not stopped the transmission of gorse spider mite worldwide, despite its susceptibility to generalist predators in New Zealand, and wherever it has been released.

Price (2000) contends that bottom-up regulation of insect herbivore populations through plant quality is more common than top-down regulation by natural enemies. Eruptive species tend to be those that feed on a range of plants. This creates the paradox that selecting agents for a high degree of host-specificity also reduces the chance of selecting agents that can outbreak. Stability of the parasitoid-host relationship sometimes reduces control potential.

Mills and Gutierrez (1999) stated that understanding what leads to lasting depression in pest population requires consideration of three processes: spatial heterogeneity, parasitoid coexistence, and tritrophic interactions. Other papers in Hawkins and Cornell (1999) examine the theory surrounding the roles played by ecological factors in the development and implementation of biological control of pests such as spatial heterogeneity and refuges, tritrophic relationships and genetics. Thomas and Waage (1996) demonstrated convincingly how both bottom-up and top-down effects may lead to better pest control than either alone.

McClay and Balciunas (2005) suggested that a pre-release efficacy assessment (PREA) should be conducted as an additional filter in the agent selection process. Although based on weed control agents, their principles of PREA apply equally to arthropod agents. They suggested that predicted impact of an agent could be estimated as:

Impact = Range Abundance x Per-Capita Impact

Range is estimated by such factors as climatic limits, survival and dispersal, geographic range, diapause and aestivation requirements, and climate matching. Abundance is governed by such factors as voltinism, fecundity, host suitability and survivorship. Per-capita effect can be estimated from native field range studies, experimental manipulation, and from constructs like 'damage curves' - change in fitness with agent load. Many of these contributing factors are discussed in more detail below. In fact, the requirements of the HSNO Act mean that an application to release a control agent could be seen as such an assessment. The selection of effective control agents cannot be concluded in isolation from the effects of land and grazing management (Hatcher and Melander 2003); synergies with other control methods (Buckley et al. 2004); plant competition (for example, Fowler and Griffin 1995, Davies et al. 2005); and more complex tritrophic relationships (Hatcher 1995).


Buckley Y.M., Rees M. and Paynter Q. (2004). Modelling integrated weed management of an invasive shrub in tropical Australia. Journal of Applied Ecology 41: 547-560

Crawley M.J. (1989). Plant life history and the success of weed biological control projects. Pp. 17-26 In: Proceedings of the VII International Symposium on Biological Control of Weeds, E. S. Delfosse (Ed.) Rome, Italy. Istituto Sperimentale per la Patologia Vegetale (MAF).

Davies J.T., Ireson, J.E. and Allen G.R., (2005). The impact of gorse thrips, ryegrass competition, and simulated grazing on gorse seedling performance in a controlled environment. Biological Control 32: 280-286

Fowler S.V. and Griffin D. (1995). The effect of multi-species herbivory on shoot growth in gorse Ulex europaeus. Pp. 579-584 In: Proceedings of the VIIIth International Symposium on Biological Control of Weeds, E.S. Delfosse (Ed.) Christchurch, New Zealand

Froud K.J. and Stevens P.S. (2004). Importation biological control of Heliothrips haemorrhoidalis (Thysanoptera: Thripidae) by Thripobius semiluteus (Hymenoptera: Eulophidae) in New Zealand - a case study of non-target host and environmental risk assessment. Pp. 90-102 In: Assessing host ranges for parasitoids and predators used for classical biological control: a guide to best practice, R. Van Driesche and R. Reardon (Ed.) USDA Forest Service, Morganstown, West Virginia, USA

Froud K.J. and Stevens P.S., (1998). Parasitism of Heliothrips haemorrhoidalis and two non-target thrips by Thripobius semiluteus (Hymenoptera; Eulophidae) in quarantine. Pp. 526-529 In: Pest Management - Future Challenges. Proceedings of the 6th Australasian Applied Entomological Research Conference, M.P. Zalucki, R.A.I. Drew and G.G. White (Ed.) Brisbane Australia.

Hatcher P.E. (1995). Three-way interactions between plant pathogenic fungi, herbivorous insects and their host plants. Biological Reviews 70: 639-694

Hatcher P.E. and Melander B. (2003). Combining physical, cultural and biological methods: prospects for integrated non-chemical weed management strategies. Weed Research 43: 303-322

Hawkins B.A. and Cornell H.V. (1999). Theoretical approaches to biological control. Cambridge University Press, Cambridge, UK. 412 pp.

Julien, M. and Griffiths M.W. (1999). Biological Control of Weeds. A World Catalogue of Agents and their Target Weeds. CABI Publishing, Wallingford, UK. 223 p.

McClay A.S. and Balciunas J.K. (2005). The role of pre-release efficacy assessment in selecting classical biological control agents for weeds - applying the Anna Karenina principle. Biological Control 35: 197-207

Mills N.J. and Gutierrez A.P. (1999). Biological contro of insects: a tritrophic perspsective. Pp. 89-102 In: Theoretical approaches to biological control, B. A. Hawkins and H. V. Cornell (Ed.) Cambridge University Press, Cambridge, UK

Morin L., Hill R.L. and Matayoshi S. (1997). Hawaii's successful biological control strategy for mist flower (Ageratina riparia) - can it be transferred to New Zealand? Biocontrol News and Information 18: 77-88

Price P.W. (2000). Host plant resource quality, insect herbivores and biocontrol. Pp. 583-590 In: Proceedings of the Xth International Symposium on Biological Control of Weeds, N.R. Spencer (Ed.) Montana State University, Bozeman Montana, USA

Thomas M. and Waage J.K. (1996). Integration of biological control and host plant resistance breeding. A scientific and literature review. Technical Centre for Agricultural and Rural Cooperation (CTA), 99 pp.