Success and safety in the biological control of environmental weeds in NewZealand

Biological control, using specialist insect herbivores and plant pathogens, can be a self-sustaining, cost-effective and low-risk tool for the managem ent of environmental weeds. Agents have been recorded attacking non-target plants in New Zealand and elsewhere, but the effects are usually minor and/ or transitory. It seems probable that only two cases, worldwide, will resul t in significant damage to non-target plants (representing 0.5% of the near ly 400 insect, mite, or fungal species used in classical weed biocontrol). Both of these cases were predictable from host range testing. Negative indi rect, or 'downstream', ecological effects from specific weed biocontrol age nts are difficult to predict and measure. They are probably insignificant c ompared to the impacts of the invasive plants that the agents are introduce d to control. However, it is necessary to balance the risks associated with any introduction against the environmental benefits from controlling a wee d to a predicted level. Recent analyses suggest that success rates are bett er than generally perceived. For New Zealand programmes, where enough time has lapsed to allow assessment, we calculate a full/partial success rate of 83%. Many of the costs associated with environmental weeds are difficult t o quantify. Detailed risk assessment will make biological control programme s more expensive and time-consuming, so that reliance on non-biological man agement methods for environmental weeds may actually increase. The costs of biocontrol programmes against some New Zealand weeds can be kept down by u sing research already carried out in Australia and other countries, and the process is reciprocal. Developing international consortia of sponsors is a lso a potential way to fund programmes against weeds shared by several coun tries.