Selecting biological control agents

Factors influencing host selection in the target range

Host suitability and physiology

The nature of the physiological relationship between the control agent and its host ultimately controls the degree of host specificity, and the most categorical measure is whether the host can support complete development of the agent or not. There is considerable evidence to back the hypothesis that herbivorous insects (and diseases) have evolved to exploit particular plant species share common features of chemistry, form, and/or phenology. Secondary plant chemistry usually closely tracks host taxonomy. Although the relationship is not nearly so clear cut, this is also true for parasitoids and predators (van Driesche 2004, Kuhlmann et al. 2005). Strict monophagy is not uncommon amongst parasitoids, and the ever growing body of observation shows that specialisations are often taxonomically conserved within groupings (Kuhlmann et al. 2005. There are many examples: for instance we know that aphidiine braconids only attack aphids and that Metaphycus species only attack soft scales in the family Coccidae (Sands and Van Driesche 2004). This is not surprising as closely related hosts will share similar physiology and defensive mechanisms, and will tend to occupy similar habitats. However, we also know that host ranges are often disjunct between genera, and the factors that mediate host range in parasitoids and predators do not appear to be as simple or straightforward as those for weed biocontrol.

Larval survival in the host may be the most discriminatory stage in determining the host range of a parasitoid (van Driesche and Murray 2004). Those that cause their hosts to cease development once parasitised are called idiobionts, and are usually external or egg parasitoids. Koinobionts live within the host and manipulate and overcome the immune response to maintain a favourable environment in which to grow. The complex mechanisms by which koinobiont larvae influence host defences include venoms and toxins, and even symbiosis with polydnaviruses (). Parasitoids that contend with physiological host defences in this way probably require very specific adaptations to the physiology of the host and will tend to be host-specific. Idiobionts do not require such finely-tuned physiological adaptation and tend to be more polyphagous than koinobionts (Godfray 1994, Althoff 2003, van Driesche 2004). Pennacchio and Strand (2006) have reviewed developmental strategies in parasitic Hymenoptera.

Host suitability can be controlled by a range of other attributes. Grandgirard et al. (2006) determined that indigenous cicadellids of French Polynesia differed sufficiently from the exotic pest Homalodisca vitripennis (Germar) in size, ecology, and oviposition biology to be at no risk from an introduced mymarid egg parasitoid.

Fungal pathogens have been favoured over other plant pathogens for weed biocontrol, partly because they often have the required host-specificity. The most commonly used are biotrophic or obligate species that are generally host specific and debilitate vigorously growing plants by accessing nutrients directly from living tissue (Morin et al. 2006). These include the rusts and smuts and are analogous to koinobiont parasitoids. The recent control of mistflower (Ageratina riparia) using Entyloma ageratinae is an example (Barton et al. 2007). More facultative species that kill cells and then utilise dead plant material have also been used (Morin et al. 2006), and these are analogous to idiobionts. The biological control programme of old man's beard (Clematis vitalba) using Phoma clematidina is an example (Gourlay et al. 2000). Discussion about various aspects of the use of fungi as both classical and augmentative biological control agents for insects can be found in the book edited by Butt et al. (2002).

References

Althoff D.M. (2003). Does parasitoid attack strategy influence host specificity? A test with New World braconids. Ecological Entomology 28: 500-502.

Barton J.E., Fowler S.V., Gianotti A.F., Winks C.J., de Beurs M., Arnold G.C. and Forrester G. (2007). Successful biological control of mist flower (Ageratina riparia) in New Zealand: Agent establishment, impact and benefits to the native flora. Biological Control 40: 370-385

Butt T.M., Jackson C. and Magan N. (2002). Fungi as Biological Control Agents: Progress, Problems and Potential. CABI Publishing, Wallingford, U.K. 390 pp.

Godfray H.C.J. (1994). Parasitoids: Behavioural and Evolutionary Ecology. Princeton University Press, Princeton. 473 pp.

Gourlay A.H., Wittenberg R., Hill R.L., Spiers A.G. and Fowler S.V. (2000). The biological control programme against Clematis vitalba in New Zealand. Pp. 709-718 In: Proceedings of the X International Symposium on Biological Control of Weeds, N. R. Spencer (Ed.) Bozeman, Montana, USA Montana State University.

Grandgirard J., Hoddle M.S., Petit J.N., Percy D.M. and Roderick G.K. (2006). Pre-introductory risk assessment studies of Gonatocerus ashmeadi (Hymenoptera: Mymaridae) for use as a classical biological control agent against Homalodisca vitripennis (Hemiptera: Cicadellidae) in the Society Islands of French Polynesia. Biocontrol Science & Technology 17: 809-822

Kuhlmann U., Schaffner U. and Mason P.G. (2005). Selection of non-target species for host specificity testing of entomophagous biological control agents. Pp. 566-583 In: Second International Symposium on Biological Control of Arthropods, Davos, Switzerland, 12-16 September, 2005, M.S. Hoddle (Ed.) United States Department of Agriculture, Forest Service, Washington.

Morin L., Evans K.J. and Sheppard A.W. (2006). Selection of pathogen agents in weed biological control: critical issues and peculiarities in relation to arthropod agents. Australian Journal of Entomology 45: 349-365

Pennacchio F. and Strand M.R. (2006). Evolution of developmental strategies in parasitic Hymenoptera. Annual Review of Entomology 51: 233-258

Sands D.P.A. and Van Driesche R.G. (2004). Using the scientific literature to estimate the host range of a biological control agent. Pp. 15-23 In: Assessing host ranges for parasitoids and predators used for classical biological control: a guide to best practice, R.G. Van Driesche and R. Reardon (Ed.) USDA Forest Service, Morgantown, West Virginia.

van Driesche R. (2004). Predicting host ranges of parasitoids and predacious insects - what are the issues? Pp. 1-3 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, Morgantown, West Virginia.

van Driesche R.G. and Murray T.J. (2004). Overview of testing schemes and designs used to estimate host ranges. Pp. 68-89 In: Assessing host ranges for parasitoids and predators used for classical biological control: a guide to best practice, R.G. Van Driesche and R. Reardon (Ed.) USDA Forest Service, Morgantown, West Virginia.