Opposing effects of organic and conventional fertilizers on the performance of a generalist and a specialist aphid species

1 Sustainable and conventional farming systems use fertilizers that differ in the availability of nitrogen, which may affect plant quality to alter the abundance and performance of potential pest species. 2 We grew brassica plants in several types of fertilizer, including those commonly used in conventional and sustainable farming systems, and an unfertilized control. The effects of fertilizer type on the performance of two aphid species and foliar glucosinolate content were investigated. 3 Both aphid species performed poorly (with reduced fecundity) on the unfertilized treatment compared with those feeding on fertilized host plants. 4 Brevicoryne brassicae, the brassica specialist, performed best on Brassica oleracea plants fertilized with an organic animal manure, with a 72% increase in fecundity and an 18% increase in intrinsic rate of increase compared with plants fertilized with ammonium nitrate. 5 By contrast, the generalist Myzus persicae had an intrinsic rate of increase that was reduced by 15% on plants growing in the animal manure compared with those growing in ammonium nitrate. 6 These results may explain earlier findings on the effects of fertilizer type on aphid populations in the field, and are discussed in the context of pest species' responses to sustainable and conventional agricultural systems.