Spatial Dynamics of Invertebrate Population Recovery in Cereals Following Treatment with the Insecticide Dimethoate.

Abstract

Data are presented from a three-year study into the spatial dynamics of recovery of invertebrate populations following treatment with the broad-spectrum aphicide dimethoate. Such a study was required because of the difference between the area treated in current field trials designed to evaluate the side-effects of pesticides on non-target invertebrate populations and the area treated under commercial practice.
Two distinct spatial patterns of invertebrate population recovery were identified following dimethoate use. Firstly, there was a pattern in which populations recovered most rapidly in small treated areas and at the edge of larger treated areas. This pattern of recovery was shown by the invertebrate predators: Carabidae, Staphylinidae, Linyphiidae and Coccinellidae and was interpreted in terms of the invasion of individuals from the untreated surroundings. The second spatial pattern of population recovery was shown by Aphididae and Collembola. Recovery of these groups was most rapid in the centre of the large treated areas. This pattern of recovery was interpreted in terms of a response to the prolonged reductions of the populations of their predators in these positions.
These interpretations were supported by experiments which demonstrated the importance of re-invasion in the recovery of carabid populations and by the fact that a prolonged reduction in predation pressure occurred at the centre of large treated areas.
The results from this study are discussed in terms of potential ecological and economic consequences of treatment with dimethoate. The results also have important implications for the evaluation of pesticide-side effects and criteria are given to modify current evaluation methodology accordingly. It is concluded that current evaluation methodology fails to predict the impact of commercial practice and suggestions are made for future work to address this problem.