Excerpt from the IOBC-APRS Newsletter, No. 2, June 2015
Dr Stephen Goldson
AgResearch and Lincoln University, NZ
As is obvious to all in the IOBC, when it works, classical biological control is of immense value; it requires minimum ongoing input and is often long-term in its benefits (but see below). The literature indicates that acquired pest resistance to classical control agents is very rare, particularly when compared to what happens with pesticides. Possible reasons for this are that biological control agents can co-evolve with a pest and thereby counteract any resistance developing in the latter (i.e. an evolutionary arms race). Temporal and/or physical refugia allow the persistence of susceptible genes in the pest population. There is often an adaptive cost to a species acquiring resistance, which can mean the reappearance of the fitter genotypes from the refugia, if and when, selection pressure ceases. Finally diverse ecosystems may stabilise biological control; they often comprise a wide range of natural-enemy guilds that collectively contribute to pest suppression. Perversely, it is turning out that some economically productive ecosystems, especially those in geographical isolated areas like New Zealand can lack these sort of factors that promote biological control longevity. Paradoxically, in the short term at least this may lead to impressive biological control successes. New Zealand’s extensive but intensively managed improved pastoral ecosystem is effectively an incomplete transplant (mainly ryegrass and clover) of the complex native grasslands of Europe. The species paucity and consequent lack of biotic resistance has resulted in severe damage being caused by catastrophically high populations of invasive species such as the Argentine stem weevil (Listronotus bonariensis
and the clover root weevil (Sitona obsoletus). However, in the last 25 years the impact of these pests have been significantly reduced by the parthenogenetic braconid endoparasitoids agents M. hyperodae and M. aethiopoides (Irish strain) both of which established very rapidly and reached parasitism rates of over 80%. Is seems that the very feature that allowed the pests to build up to such serious levels also applied to the parasitoid. They too encountered little or no biotic resistance and flourished. Now after a period of triumph there are signs that after about 45 generations M. hyperodae is losing is efficacy possibly though resistance. The auguries are not particularly good. The wasp has had no ability to coevolve with its weevil host, there are few if any refugia and the pastoral ecosystem is very species poor. Resistance and its mechanism have yet to be proven conclusively but the point remains that biological control cannot be taken for granted particularly in ‘created’ habitats like New Zealand’s improved grasslands. Vigilance must be maintained and understanding improved; there is indeed work to be done.