Illinois sweat bees are 1/4-inch-long, metallic green, brown, or black bees that are also be attracted to perspiration. The only yellow and black stinging insects commonly attracted to perspiration are yellowjackets. These wasps are about 1/2 inch long and easily recognized by most people. Although commonly referred to as sweat bees, syrphid flies cannot sting. While they are feeding on perspiration, their mouthparts can usually be felt; and on sensitive areas, a person may feel a slight pinch. These insects have one pair of wings instead of the two pairs characteristic of bees and wasps. Like many people in the Midwest, I grew up being told that these insects were sweat bees. Only when I went to college did I learn the difference.
Syrphid flies are a diverse group, with the larvae being primarily feeders on dead organic matter and predators of aphids. The white to gray, legless larvae may have become abundant, feeding on aphids and other small, soft-bodied, slow-moving insects. A possibility is that the larvae have been feeding on the aphids that are very common on corn in Illinois at this time of year.
In any case, the adult insects are common around flowers and in shady areas. Control efforts are not recommended because these flies are, at the least, pollinators or non-pests and, at the most beneficial, useful in controlling aphids. Insecticide application is problematic because hovering flies easily avoid sprays aimed at them; large-surface spraying of insecticides is not recommended, and the effects are very short-lived. Put up with the flies. They will probably decline naturally within a week or two. (Phil Nixon)