A model was developed to estimate the effect of predation byEpisyrphus balteatuslarvae on aphid populations in fields of winter wheat. Feeding rate of syrphid larvae was estimated from experiments in field cages using a Holling type-III function. During larval development, syrphids consumed a maximum of 396 aphids, which is only half the potential feeding rate reported from laboratory experiments. The lower consumption rate in the field may be attributed to the additional time required by the predators to find prey. Isoclines derived from the model were used to estimate the abundance of syrphid larvae necessary to prevent further pest increases.