Predator mortality depends on whether its prey feeds on organic or conventionally fertilised plants

Natural enemy abundance and diversity can be increased under sustainable farming systems, but this has not been shown to consistently increase predation and parasitism rates or decrease herbivore abundance. ‘Top-down' regulation of herbivore populations may depend on ‘bottom-up’ factors such as plant quality, and not solely on predator diversity or abundance. Specialised herbivore species can sequester secondary chemicals from plants to use in a defensive system against predators which mimics that of their host plants, but this herbivore defence may vary with the concentration of plant defences. We investigated whether fertiliser type and concentration alter the mortality of coccinellids feeding on two aphid species from Brassica plants growing in fertilisers typical of organic and conventional farming systems, due to differences in concentrations of defensive glucosinolate compounds cascading up the food chain. Coccinellid larval mortality was 10% higher when feeding on aphids from synthetically fertilised plants compared with those in organic fertilisers, regardless of the aphid species. Concentrations of both constitutive foliar glucosinolates, and those induced by aphids, varied with fertiliser type but this did not affect the glucosinolate concentrations sequestered by the aphids. The efficacy of predators as biological control agents may thus differ between conventional and sustainable farming systems.