Although most syrphid flies overwinter as larvae in leaf litter, S. pyrastri overwinters as an adult. There are usually three generations per year in Washington, although in very warm years or areas there may be more. Adults need pollen from wild flowers or weeds in order to produce eggs. They move into the orchard relatively late in the season, usually about mid-May. Adults often hover around flowers where they feed on nectar and honeydew from aphids and scale insects. Females lay eggs on leaves near or in aphid colonies, where the young maggots will locate prey easily. Despite being blind and legless, the larvae move about and locate prey very efficiently. Often a larva will lift an aphid off the leaf surface while sucking out its body fluids. When mature, larvae go to the ground to transform into pupae and eventually to adult flies. The life cycle takes 2 to 4 weeks to complete.
Adult syrphid flies are easy to see in flight but are often mistaken for wasps or bees. Syrphid flies have just one pair of wings, whereas wasps and bees have two, but it may be hard to distinguish them when the insect is at rest with the wings folded over the back. Black oily smears, the excrement of syrphid fly larvae, on plant foliage are typical signs of active syrphid fly populations. Larval densities can be determined in conjunction with visual counts of aphids.
Syrphid flies are highly susceptible to insecticides. Avoid insecticidal control of aphid infestations if you see syrphid fly larvae in them.