Abstract: Aphids are important pests of crop plants, and are a particular problem on the 26,000 ha of vegetable Brassica crops grown annually in the UK. They cause direct feeding damage and are vectors of plant pathogenic viruses. At present, aphid control is heavily reliant on chemical insecticides but growers are under pressure to reduce insecticide usage. Aphid populations increase in the spring and autumn months interspersed with a period during midsummer when they decline rapidly. At present, the timing of this mid-season population ‘crash’ cannot be predicted accurately. If a forecast was developed growers could be alerted of when a crash was likely, which would give them the option to withhold insecticide applications. Natural enemies, and in particular entomopathogenic fungi, have been implicated in the crash, but little is known of their biology. The aim of this project is to quantify the effects of the associated guild of natural enemies of aphid populations in field brassicas.