Insect outbreaks, especially of migratory pests, are often associated with particular weather patterns, e.g. outbreaks of the desert locust and Spodoptera spp. The weather can also directly affect pop ulation development, if temperatures are favourable for population growth at appropriate period during the insect's life cycle then outbreaks can occur, e.g. mild winters in the UK are associated with out breaks of cereal aphids. Weather can produce a differential development of pests and their natural enemies causing a de- coupling of their association and thereby permitting an unregulated pest population increase.