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
