A metapopulation approach to modelling the long-term impact of pesticides on invertebrates.
Two simulation models are presented which predict the impact of local and episodic applications of pesticide on the dynamics of invertebrate metapopulations. Both models assume that invertebrates disperse at particular rates over a matrix of fields and that each field experiences a specific pesticide regime. Model 1 investigates the population dynamics of a polyphagous predator which experiences direct mortality from pesticide exposure but is unaffected by the availability of the targeted pest. Model 2 investigates a similar system but also considers the dynamics of the pest, which is influenced both by predation and by pesticide exposure. Using stability diagrams and isocline portraits, the likely long-term effects of factors such as the frequency and extent of spraying are characterized, factors which have hitherto not been widely considered in general pesticide models. To provide a realistic context, the first model was parameterized using data on the phenology, dispersal rate and susceptibility to pesticide of carabid beetles in winter wheat sprayed with the insecticide dimethoate. It is argued that the predator/prey metapopulation perspective adopted in the second model is an appropriate framework with which to view the dynamics of a number of pests, for instance Delia radicum preyed upon by epigeal invertebrates in brassica fields. The first (stochastic) model suggests that the chances of a polyphagous predator population persisting in a sprayed field are improved if few other fields are sprayed, the rate of pesticide application is low or if the pesticide is of low toxicity. Less intuitively, there appears to be an optimal dispersal rate of the predator population which maximizes the range of conditions under which it is able to persist in sprayed fields. The second (deterministic) model predicts that the regular application of pesticides can, under certain circumstances, eventually cause prey metapopulations to fluctuate at densities higher than they would otherwise fluctuate in the absence of pesticide. It was found that the dispersal rates of both the predator and the prey are highly important factors influencing the eventual population size of the prey. The importance is discussed of adopting the metapopulation approach if the long-term side-effects of pesticides are to be properly evaluated.