Selecting biological control agents

Selecting effective agents

What is success in biological control?

What the HSNO Act requires

Like the Resource Management Act 1991 [http://www.legislation.govt.nz/act/public/1991/0069/latest/DLM230265.html] before it, the HSNO Act [http://www.legislation.govt.nz/act/public/1996/0030/latest/DLM381222.html] is 'effects-based', enabling legislation. It requires applicants wishing to introduce a new biological control agent to comprehensively assess the probability and magnitude of both the adverse and beneficial effects that might flow from the introduction. The Act is not concerned with changes in the population dynamics, abundance or vigour of a weed or pest resulting from control per se but how those changes affect the pest status of the target. Applicants must focus on the effects of damage to the target rather than the intensity of that damage.

Defining success

Successful control occurs when the abundance or the vigour of the damaging stage falls below a damage threshold (Greathead and Greathead 1992). Rather than the effects of control, percent damage or parasitism of the damaging stage has often been used to describe the impact of biological control agents even though these measures have little relevance to damage threshold, and can sometimes be misleading. For example, the braconid parasitoid Apanteles ruficrus Haliday often kills over 90% of Mythimna separata (Walker) (Noctuidae) larvae in maize crops (Hill 1977, but this level of larval parasitism does not appear to lower the average population level of the pest. In fact, successful control is based more on the reduction in average food consumption as a result of heavy rates of larval parasitism, doubling the tolerance of the plant (Hill 1988). Similarly, very high levels of seed predation by a biological control agent may have little effect on the density of the target weed if the target species is not seed-limited (). Conversely, life-table studies indicated that only a small increase in generational mortality is required to reduce the population density of codling moth from year to year (Wearing 1979). Immigration of feral females from unsprayed trees play an important role in the population dynamics of the moth, such that even partial success of an efficient biocontrol agent may have significant economic benefits (Wearing and Charles 1989). This illustrates that in intensively managed, high value crops, biological control is but one of many complementary insect pest management tools. It may be difficult to quantify the impact of individual biological control agents, although the sometimes great increase in pests when natural enemies are accidentally killed can dramatically illustrate their collective value.

Planning for success

Understanding what is required of biological control should be an important starting point for planning a biological control project, but rarely is. The potential success of a biological control project can only be judged in the context of other management strategies for the pest (), and defining success at the outset may determine whether biological control is the most appropriate management option at all (). Standish et al. (2001) have established a threshold biomass of the forest weed Tradescantia fluminensis below which regeneration of native forest plants is possible. This has become a key performance target in assessing the potential benefits to forest health of a biological control programme against this weed (Hill 2008). Briese (2006) published a protocol used to develop the biological control programme against Onpordum spp. that could equally be applied to the introduction of parasitoids or predators.

In most cases where success thresholds for arthropod pests are identified, the research has been retrospective. For example, Barlow and Goldson (1993) showed that the economic threshold for damage to lucerne by the weevil reduction in the density of Sitona discoideus Gyllenhal was 60% of the pest's natural equilibrium population, a level easily achieved by the parasitoid Microctonus aethiopoides Loan. Setting better goals for biological control of both weeds and pests would assist the EPA in evaluating potential benefits.

References

Barlow N.D. and Goldson S.L. (1993). A modelling analysis of the successful biological control of Sitona discoideus (Coleoptera: Curculionidae) by Microctonus aethiopoides (Hymenoptera: Braconidae) in New Zealand. Journal of Applied Ecology 30: 165-179

Briese D.T. (2006). Can an a priori strategy be developed for biological control? The case for Onopordum spp. thistles in Australia. Australian Journal of Entomology 45: 317-323

Greathead D.J. and Greathead A.H. (1992). Biological control of insect pests by insect parasitoids and predators: the BIOCAT database. Biocontrol News and Information 13: 61N-68N

Hill M.G. (1988). Analysis of the biological control of Mythimna separata (Lepidoptera: Noctuidae) by Apanteles ruficrus (Braconidae: Hymenoptera) in New Zealand. Journal of Applied Ecology 25: 197-208

Hill R.L. (1977). Parasite helps control armyworm. New Zealand Journal of Agriculture 134: 21-23

Hill R.L. (2008). Application to release from containment a beetle, Lema obscura F. (Chrysomelidae), for the biological control of the weed tradescantia (Tradescantia fluminensis). http://www.ermanz.govt.nz/appfiles/execsumm/pdf/NOR07001-002.pdf

Standish R.J., Robertson A.W. and Williams P.A. (2001). The impact of an invasive weed Tradescantia fluminensis on native forest regeneration. Journal of Applied Ecology 38: 1253-1263

Wearing C.H. (1979). Integrated control of apple pests in New Zealand 10. Population dynamics of codling moth in Nelson. New Zealand Journal of Zoology 6: 165-199

Wearing C.H. and Charles J.G. (1989). Cydia pomonella (L.), codling moth (Lepidoptera: Tortricidae). Pp. 161-169 In: A review of biological control of insect pests and weeds in New Zealand 1874 to 1987, P.J. Cameron, R.L. Hill, J. Bain and W.P. Thomas (Ed.) CAB International Institute of Biological Control 10, CAB International, Wallingford, UK