Crops, Insect Pests and Natural Enemies: Effects of Organic and Conventional Fertilisers

Abstract

Agricultural intensification can have negative impacts on the environment and there is increasing interest in the use of low intensity or organic agricultural methods to improve sustainability. Fertiliser is an important component of all agricultural systems and can affect the performance of crop pests and their natural enemies. This thesis presents the results from a quantitative review of the literature on both farming system and organic and conventional fertiliser effects on pests and natural enemies. Results from a series of laboratory and field experiments investigating the effects organic and conventional fertiliser on cereal aphids and their natural enemies are reported. The review demonstrates that crop pests and their natural enemies benefit from organic or low intensity methods and this is evident for natural enemies in farm scale experiments. The effect of organic and conventional fertilisers on arthropod pests is variable although the influence of manures is consistently negative while the effect of plant composts is positive. More studies investigating organic and conventional fertilisers and the response of natural enemies are needed. Field and laboratory experiments show that conventional fertilisers can benefit cereal aphids but the mechanism behind this response is species specific. Rhopalosiphum padi is sensitive to temporal nutrients availability and is influenced by the timing of fertiliser application, while Metopolophium dirhodum is responsive to plant morphology with aphids performing better on plants with a high proportion of vegetative matter. The implications of pest performance on fertiliser management strategies are discussed. Parasitoid abundance in the field was not found to be influenced by fertiliser treatment although in the laboratory, indirect effects of fertiliser, mediated through its aphid host, were found to affect parasitoid fitness with larger parasitoids emerging from larger aphids. A positive influence of conventional fertiliser on syrphid oviposition in the field was also apparent.