Influence of five pyrethroid insecticides on microbial populations and activities in soil

Laboratory tests were conducted to determine the effects of five pyrethroid insecticides-permethrin (FMC 33297) [3-phenoxybenzyl (±)-3-(2,2-dichlorovinyl)-2,2-dimethylcyclopropanecarboxylate]; FMC 45498 [(S)-α-cyano-3-phenoxybenzyl-(R)-cis-2-(2,2-dibromovinyl)-3,3-dimethylcyclopropanecarboxylate]; Shell WL 41706 [(±)-α-cyano-3-phenoxybenzyl 2,2,3,3-tetramethylcyclopropane-carboxylate]; Shell WL 43467 [(±)-α-cyano-3-phenoxy benzyl (±)-cis,trans-2-(2,2-dichlorovinyl)-3,3-dimethylcyclopropanecarboxylate]; and Shell WL 43775 [(±)-α-cyano-3-phenoxybenzyl (±)-2-(4-chlorophenyl)-3-methylbutyrate]-at 0.5 and 5μg/g on microbial populations and activities in a sandy loam. The insecticides had antimicrobial activity in early stages of incubation. The populations recovered after 2 to 4 weeks and stimulatory effects on populations were also observed in later stages. No inhibition of acetylene (C2H2) reduction was evident with any of the insecticides. However, WL 43467 at both concentrations and permethrin and WL 41706 at 5 μg/g increased nitrification after 4 weeks. Soil microbial respiration, as indicated by oxygen consumption, increased with increasing concentration of insecticides, suggesting the possibility of microbial degradation of the insecticides. Dehydrogenase activity showed that none of the insecticides inhibited formazan (2,3,5-triphenyltetrazolium formazan) formation, whereas urease activity was stimulated in most cases. The studies indicated that some of the pyrethroid insecticides may exert transient effects on populations and activities of the microflora in a sandy loam, but these were short-lived and minor in nature.