THE EFFECT OF SPATIAL SCALE OF TREATMENT WITH DIMETHOATE ON INVERTEBRATE POPULATION RECOVERY IN WINTER-WHEAT

1. The influence of plot size in assessing the impact of pesticide tre atment on invertebrate populations in cereals was investigated. Within -field experiments were performed in southern England in winter wheat using the broad-spectrum aphicide dimethoate. 2. Post-treatment recove ry was monitored using, in separate experimental designs, pitfall trap s in the centre of treated plots of different size from 4 x 4 m up to 288 x 288 m, and pitfall traps in transects running from the edge to t he centre of large treated plots. 3. Invertebrate population recovery was shown to be dependent on the size of the plot treated. Two contras ting patterns of recovery were identified: (A) recovery progressing fr om the edge to the centre of treated areas; (B) recovery most rapid in the centre of the large treated areas. 4. Type A recovery was associa ted with the predatory groups Carabidae, Staphylinidae and Linyphiidae . It is consistent with that expected from the reinvasion of the pesti cide-depleted plots from the undepleted surroundings. The speed of a g roup's recovery was shown to be a function of the invasion potential, which in turn was related to the mobility and phenology of the group. 5. Type B recovery was associated with the prey groups Aphididae and C ollembola. Predation pressure appears to be the dominant process in th eir post-treatment population recovery, with recovery most rapid in ar eas with the fewest predators. 6. The results suggest that small-scale within-field pesticide evaluations would in many instances fail to pr edict the impact of treatment on a commercial scale.