Preventive insecticide use affects arthropod decomposers and decomposition in field crops

Insecticides use in field crops has grown increasingly prophylactic, which poses a potentially unnecessary risk to arthropod decomposers. These decomposers — including millipedes (Diplopoda), soil mites (Acari, mostly Oribatida), and Collembola — act as alternative prey for generalist predators, facilitate crop-residue breakdown, and can influence nutrient cycling. However, the abundance and activity of arthropod decomposers may be reduced by extensive prophylactic pesticide use. We conducted a three-year field experiment in no-till corn and soybean fields to assess how neonicotinoid seed coatings or broadcast applications of a pyrethroid insecticide affect arthropod decomposers and litter decomposition. Both insecticides reduced densities of arthropod decomposers and reduced decomposition of plant litter by over 10%. Neonicotinoid seed coatings reduced collembolan densities by 34% and millipede densities by 52%, while the pyrethroid significantly reduced oribatid mite densities by 24% and millipede densities by 82%. Lower mite and millipede densities correlated with slower litter loss, and decomposition was slower under neonicotinoid seed coatings and broadcast pyrethroid applications. These results indicate that perennial, widespread use of prophylactic insecticides is likely to have concerning negative effects on the availability of alternative prey, plant residue breakdown, and nutrient cycling in field crops.