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 treatment 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 recovery was monitored using, in separate experimental designs, pitfall traps 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 the centre of large treated plots. 3. Invertebrate population recovery was shown to be dependent on the size of the plot treated. Two contrasting patterns of recovery were identified: (A) recovery progressing from 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 associated with the predatory groups Carabidae, Staphylinidae and Linyphiidae. It is consistent with that expected from the reinvasion of the pesticide-depleted plots from the undepleted surroundings. The speed of a group'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 Collembola. Predation pressure appears to be the dominant process in their post-treatment population recovery, with recovery most rapid in areas with the fewest predators. 6. The results suggest that small-scale within-field pesticide evaluations would in many instances fail to predict the impact of treatment on a commercial scale.