Adult midges are delicate, gnatlike flies less than 1/8" long, with proportionately long legs and antennae. They lay their minute eggs on foliage. The emerging tiny, white, yellowish, reddish or orange maggots bore into plant tissue, sometimes forming galls within which they feed. Two types of midges will concern us here: those species that cause galls on chrysanthemums and various trees, particularly the honey locust; and the species which primarily affects roses.
The chrysanthemum gall midge produces pimple-like galls on the upper side of chrysanthemum leaves and generally distorts the foliage. It also affects greenhouse grown plants and is sometimes found in the garden as well. This pest is best controlled by picking off and destroying damaged leaves. You can also spray infested plants in their entirety with insecticidal soap; apply in the evening at 5 day intervals as soon as galls are discovered for up to a total of 3 applications. To prevent future problems, thoroughly clean up and discard garden debris each fall to destroy overwintering midges. Be sure not to compost any soil or plant material that may be infested.
A number of species of gall midges affect trees, but are generally very host-specific. Some trees affected by gall midges include coyote brush, dogwood, fir, oak, pine and willow. A gall-forming species that merits specific mention is the honey locust pod gall midge. This pest causes honey locust leaflets to form galls containing one to several small pinkish white maggots. Heavy infestations cause foliage to turn brown and to drop prematurely. Unfortunately, since the maggots are protected within their galls, there is little you can do to treat infested trees. On the other hand, established trees are rarely killed, so damage, while unsightly, can be tolerated. Consider planting alternative species or varieties in landscapes where the aesthetic value is high such as the 'Shademaster' variety of honey locust, or plant black locust instead. Avoid the 'Sunburst' variety of honey locust.
Rose midges are microscopic insects that can blacken and kill rosebuds and leaves. The destructive, whitish maggots usually hatch after the first bloom cycle and rasp tender plant tissue as they feed, causing leaves and blossoms to blacken and shrivel. An unchecked, heavy infestation can eliminate bloom from late spring to early fall. After feeding, the larvae drop to the soil, pupate, and emerge as reddish or yellowish brown flies within a week. To control this pest, remove and destroy affected flower buds and leaves as soon as you spot midge damage.