Terrestrial non-target invertebrates and the autumn application of synthetic pyrethroids: experimental methodology and the trade-off between replication and plot size.

Abstract

The effects of the synthetic pyrethroid, deltamethrin ((S)-a-cyano-3-phenoxybenzyl (I R)-cis-3-(2,2-dibromovinyl)- 2,2-dimethylcyclopropanecarboxylate) on the non-target invertebrate fauna of winter-wheat was monitored over two cropping cycles. The product was applied at decimal growth stage 12-13 in the first week of November in 1986 and 1987. In 1986, demeton-S-methyl (S-2-ethylthioethyl 0,0-dimethyl phosphorothioate) was applied as a toxic standard. Short-term depletions were detected in the autumn-active, field-overwintering Carabidae, especially Bembidion obtusum, Nebria brevicollis and Pterostichus melanarius. Staphylinidae were largely unaffected during the winter months however, significant increases in summer-active species were detected in both years. The most profound effects were on the Linyphiidae with 168-221 day depletions in the 4 ha plots in 1986/7 and 89-145 day reductions in the I ha plots used in 1987/8. Meioneta rurestris was the most affected species. In general, pyrethroid effects were greater than organophosphate effects, especially in the case of spiders. Summer aphid infestations tended to be lower in the pyrethroid-treated plots, possibly as a result of overwintering aphid control. No evidence for sub-lethal effects (number of eggs per female and proportion with empty crops) were found in the carabid, Trechus quadristriatus however, the proportion of female beetles was higher in the pyrethroid-treated plots. B. obtusum and P. melanarius numbers were significantly reduced within small, barriered areas 160 days after treatment within the pyrethroid-treated plots in 1987/8. This suggests that recolonisation of treated areas may reduce or dilute the duration of effects that would be detected in the whole-field treatments characteristic of the real world. Support for this was also obtained from the direct relationship between the duration of effects and plot size in the two years of the study. The potential for using models incorporating spatial dynamics to assist interpretation of experimental results is discussed .