Does scale influence the recovery of dimethoate-depleted invertebrate populations in winter wheat?

Abstract

To evaluate the potential side-effects of agrochemicals on non-target invertebrates most field experiments are designed to cover a maximum area of a few hectares; for example the maximum area treated in recent trials by Cole et al. (1986), Purvis, Carter and Powell (1988), Vickerman et al. (1987) and Wratten et al. (1988) was 6.5, 5.0, 5.1 and 2.8 hectares, respectively. The results of these trials may be used for registration purposes and subsequently may lead to approval, and the commercial treatment of thousands of hectares. A recent survey of insecticide application in the UK (Wratten, Watt, Carter and Entwistle, 1990) showed that in 1984 the organophosphorous aphicide dimethoate was applied to 13 250 ha of the 115 000 ha surveyed.
Two distinct scales of pesticides use have been identified above: the small scale used in side-effects evaluation and the large scale of commercial usage. If the scale of pesticide treatment itself influences the extent and duration of subsequent side-effects the use of small scale 'screening' to predict of application, on the commercial scale, may be unrealistic.
This paper discusses the results of experiments designed to: identify the influence of the scale of treatment on the duration of side-effects following pesticide application, and determine the mechanism underlying such results. The trials were performed in southern England on winter wheat using the insecticide dimethoate, a broad spectrum aphicide known to cause significant reductions in populations of many 'non-target' invertebrates species found in winter cereals (Vickerman and Sunderland, 1977; Frampton, 1988). The use of dimethoate as an experimental compound was relevant to practice, because it is widely used as an aphicide in the U.K. (Wratten and Mann, 1988; et aI., 1990).