Biological control against invasive species in simplified ecosystems: its triumphs and emerging threats

Although records show that the loss of susceptibility of pests to biological control agents is an exceedingly rare event, there are certain behavioural and ecological settings that may well predispose to it. In general, these circumstances rarely converge. Such a critical combination of factors could possibly occur in agroecosystems based on incomplete transplants imported from elsewhere. It can be argued that such ecosystems lack the biodiversity required to confer biotic resistance to invasive species and this can result in spectacularly high and damaging pest densities. Through exactly the same mechanism, introduced control agents such as parasitoid wasps, similarly can prove to be very successful in producing persistently very high levels of parasitism of pests, leading to triumph. However, this feeling may be short-lived. When success is based on very high selection pressure on the host pest species this could have the potential to lead to the evolution of resistance to the control agent. This is particularly so should it coincide with factors such as a lack of pest host refugia, parasitoid parthenogenetic reproduction, versus pest sexual reproduction, as well as suppression based on a narrow range of natural enemies. In effect, the very thing that can lead to spectacular success can eventually become the basis for failure. For the purposes of illustration, these considerations are illustrated via what seems to be a developing cause for concern about biological control in New Zealand's pastures.