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Selecting biological control agents

Experimental confirmation of host range

Indicators of effects

Retrospective studies

Post-release studies tell us nothing about the risk profile of any particular natural enemy proposed for introduction, and are therefore not strictly relevant to the preparation of any application to the EPA to introduce a new organism. However, such studies are essential if we are to later verify the predictions made in applications, and so build confidence in methodologies for predicting non-target effects. Barratt et al. (2006) review the role of post-release studies in improving pre-release evaluation of the risks of biological control. They acknowledge that no accepted standards yet exist for doing this.

A high degree of host specificity limits the number of interactions that a control agent will have in its new environment (. The reliability of host- range testing is therefore an important issue in minimising non-target community effects. Comparisons of the field host range of natural enemies following release with the predictions of host range testing are becoming more common, and for the most part appear to be validating the predictive capacity of host-range testing. Paynter et al. (2004) described a large survey of the non-target plants of 20 control agents introduced to New Zealand for the biological control of weeds since 1926. Eighteen were either host-specific or the field host range was originally predicted by testing. For two species, Bruchidius villosus (F.) and Cydia succedana (D&S) minor attack on several exotic non-target hosts related to the true host was not predicted by testing regimes. However, the levels of attack are likely to be insignificant to community structure. These results complemented those of Fowler et al. (2003) who found that (with the exceptions mentioned) host range testing for weed control agents in New Zealand has reliably predicted post-release field host range.

Hill et al. (2001) predicted that Phytomyza vitalbae, a leaf mining control agent of Clematis vitalba, was likely to occasionally attack leaves of native Clematis species. Paynter et al. (2006) surveyed damage to the leaves of Clematis species throughout New Zealand and found that this prediction was largely correct. There are other examples of post-release field studies that confirm the host range predicted by testing, including Haye et al. (2005) and Coombs (2003). Barratt et al. (2006) reviewed their extensive research into the performance of the weevil parasitoid Microctonus aethiopoides in New Zealand following its release for the control of Sitona discoideus. They have examined how spatial distribution, phenology and habitat affect the propensity of the parasitoid to utilise non-target native weevils, and the consequences for weevil populations.

Barron et al. (2003) used a life table analysis to investigate the effect of the pupal parasitoid Pteromalus puparum (L.) on the population dynamics of the red admiral, Bassaris gonerilla (F.) in New Zealand, over fifty years after the parasitoid was introduced for white butterfly control. They found that although common and apparently damaging, parasitism by P. puparum was less important to the population dynamics of the host than egg parasitism by Telenomus sp. and mortality caused by a second pupal parasitoid, Echthromorpha intricatoria (F.). Such studies could help pinpoint key factors in the population dynamics of important non-target species, and avoid increasing mortality in critical life stages.

Barlow (1999) found no existing population models aimed at predicting non-target impacts but predicted that the demand for such models would grow. However, he pointed out that there is likely to be far less information about non-target species on which to base such models. Barlow et al. (2004) then went on to model the population dynamics of a non-target host in the presence and absence of the introduced braconid M. aethiopoides. The difference in population impact between habitats was explained by the difference in the intrinsic rate of increase of the non-target species in those habitats.

Non-target impacts can occur only if natural enemies and potential hosts share habitats, e.g. Barratt et al. (2006). Pre-release climate modelling based on the biological characteristics of the agent has the potential to predict such overlaps (see above). Louda et al. (2005) found that the environment influences and can alter host use and population growth, leading to higher than expected direct impacts on the less preferred native host species at several spatial scales. They concluded intensive retrospective ecological studies provide guidance for future assessment of candidate biological control agent dynamics and impacts.


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.

Barlow N.D., Barratt B.I.P., Ferguson C.M. and Barron M.C. (2004). Using models to estimate parasitoid impacts on non-target host abundance. Environmental Entomology 33: 941-948.

Barratt B.I.P., Blossey B. and Hokkanen H.M.T. (2006). Post-release evaluation of non-target effects of biological control agents. Pp. 166-186 In: Environmental Impact of Arthropod Biological Control: Methods and Risk Assessment, U. Kuhlmann, F. Bigler and D. Babendreier (Ed.) CABI Bioscience, Delemont, Switzerland.

Barron M.C., Barlow N.D. and Wratten S.D. (2003). Non-target parasitism of the endemic New Zealand red admiral butterfly (Bassaris gonerilla) by the introduced biological control agent Pteromalus puparum. Biological Control 27: 329-335.

Coombs M. (2003). Post-release evaluation of Trichopoda giacomellii (Diptera: Tachinidae) for efficacy and non-target effects. Pp. 399-406 In: Proceedings of the 1st International Symposium on Biological Control of Arthropods, Honolulu, Hawaii, 14-18 January 2002, R. G. Van Driesche (Ed.) United States Department of Agriculture, Forest Service, Washington

Fowler S.V., Gourlay A.H., Hill R.L. and Withers T. (2003). Safety in New Zealand weed biocontrol: a retrospective analysis of host-specificity testing and the predictability of impacts on non-target plants. Pp. 265�270 in Proceedings of the XI International Symposium on Biological Control of Weeds, Canberra, Australia, 2003, J.M. Cullen, D.T. Briese, D.J. Kriticos, W.M. Lonsdale, L. Morin and J.K. Scott (Ed.).

Haye T., Goulet H., Mason P.G. and Kuhlmann U. (2005). Does fundamental host range match ecological host range? A retrospective case study of a Lygus plant bug parasitoid. Biological Control 35: 55-67.

Hill R.L., Markin G.P., Gourlay A.H., Fowler S.V. and Yoshioka E. (2001). Host range, release, and establishment of Sericothrips staphylinus Haliday (Thysanoptera: Thripidae) as a biological control agent for gorse, Ulex europaeus L. (Fabaceae), in New Zealand and Hawaii. Biological Control 21: 63-74

Louda S.M., Rand T.A., Russell F.L. and Arnett A.E. (2005). Assessment of ecological risks in weed biocontrol: Input from retrospective ecological analyses. Biological Control 35: 253-264.

Paynter Q., Waipara N.W., Peterson P.G., Hona S.R., Fowler S.V., Gianotti A.F. and Wilkie P. (2006). The impact of two introduced biocontrol agents, Phytomyza vitalbae and Phoma clematidina, on Clematis vitalba in New Zealand. Biological Control 36: 350-357

Paynter Q.E., Fowler S.V., Gourlay A.H., Haines M.L., Harman H.M., Hona S.R., Peterson P.G., Smith L.A., Wilson-Davey, J.R.A., Winks, C.J. and Withers T.M. (2004). Safety in New Zealand Weed Biocontrol: A nationwide survey for impacts on non-target plants. New Zealand Plant Protection 57: 102-107