Gall Midge

Gall midges belong to the family Cecidomyiidae, one of the largest fly families with thousands of described species and likely many more undescribed. Adults are fragile, short-lived flies rarely noticed by people, but their larvae are famous for inducing galls — abnormal plant growths — on leaves, stems, buds, and flowers of a huge range of trees and herbaceous plants.

Each gall midge species is typically highly host-specific, targeting a single plant species or a narrow group of related plants, and the galls they produce are often distinctive enough in shape and color to identify the midge species without ever seeing the insect itself. Familiar examples include the Hessian fly, a historically significant pest of wheat, and various oak, willow, and goldenrod gall midges that produce conspicuous swellings on their host plants.

Because the larvae feed and develop entirely within the protective gall tissue, gall midges are rarely seen as adults; most people only ever encounter the plant galls they leave behind.

• Adults are tiny and mosquito-like, 1–5 mm, with long, beaded antennae
• Slender legs and a single pair of delicate, often hairy wings
• Soft, weakly sclerotized body, usually tan, gray, or reddish
• Larvae are small, legless maggots, often orange, yellow, or white, found inside plant galls
• Best identified indirectly by the distinctive gall shape on a known host plant rather than by examining the adult fly

Gall midges occur wherever their host plants grow, from garden shrubs and ornamental trees to agricultural fields, forests, and wetlands. Because most species are tied to specific plants, their distribution mirrors that of their hosts and spans temperate and tropical regions worldwide.

Adult gall midges are short-lived, often surviving only a day or two, during which their main task is mating and locating an appropriate host plant on which to lay eggs. Larvae feed within the gall tissue the plant forms around them, drawing nutrients from the surrounding cells; the plant reaction that creates the gall is thought to be triggered chemically by the feeding larva. Ecologically, gall midges are an important food source for parasitoid wasps and other insects that specialize in attacking gall-dwelling larvae, and heavy infestations of certain species are treated as agricultural or garden pests on crops such as wheat, sorghum, and various ornamentals.

Females lay eggs on or near host plant tissue, often on young leaves, buds, or shoots. After hatching, larvae burrow into the plant and their feeding stimulates the surrounding tissue to swell into a gall that encases and protects them as they feed and grow. Larvae typically pass through three instars before pupating, either within the gall or after dropping to the soil, and many species overwinter as larvae or pupae before emerging as adults the following season, with some species producing multiple generations per year.