Insect guide > Flies > Gall Gnats

Gall Gnats

Gall Gnats(Family Cecidomyiidae)
The minute flies of this family are small delicate flies somewhat resembling mosquitoes, but do not bite. The antennae are many-jointed, and are furnished with whorls of hair. The wings have only a few longitudinal veins and but a single cross vein in some genera. The legs are not swollen, and the body and wings are clothed with hairs which are easily rubbed off.

There are many species, and some of them are known only by the work of their larvae. About one hundred species have been described in the United States.

The larvae differ considerably in their habits, but most of them form galls on the twigs or leaves of different plants. Some live under bark and others (of the genus Diplosis) prey upon plant-lice and bark-lice. Others still produce plant deformities of different kinds which cannot strictly be called galls.

The larvae are small, somewhat flattened maggots, tapering at each end, and frequently brightly colored with some shade of red or yellow. The pupa is either naked or is enclosed in a delicate silken cocoon.

The most famous member of this group is the so-called Hessian fly (Cecidomyia destructor Say.) This species lives, in the larval state, in stems of wheat, and annually damages the wheat crop of the
United States to the extent of many millions of dollars. It is supposed to have been introduced into the United States in the straw brought over for bedding by the Hessian troops during the War of the Revolution. Hence the popular name.

Other American species form curious galls on willow, one of them for example resembling a pine cone. Another species lives on the surface of the maple leaves. Still another forms a gall in the stem of Chrysopsis.

There are also several Cecidomyiid galls on golden rod, sunflowers and Aster.

The species of two genera have been found in Europe to give birth to young while in the larval state. This phenomenon is known as paedogenesis, but has not been observed in any American forms.

Life History of a Gail-Gnat
(Cecidomyia leguminicola Lint.)
This insect, commonly known as the clover-seed midge, occurs throughout a large part of the United States and breeds in the flower heads of the common red clover. It was first noticed in New York State in 1879, but has since been found in most of the clover-growing regions of the country.

The very minute, long, oval, pale yellowish eggs are pushed down by the female between the hairs which surround the seed capsule of the yet undeveloped florets. They are generally deposited singly, but are sometimes found in clusters of from two to five. After young larvae hatch they work their way through the mouth of the flower to the seed.

They feed upon the seed, usually destroying it, and when full grown work their way out of the closed florets, wriggling violently until they fall to the ground where each forms an oval, compressed, rather tough cocoon of fine silk with particles of the surrounding earth adhering to the outside and rendering its detection extremely difficult. The pale orange pupa remains within the cocoon about ten days, after which period the adult fly emerges.

There are two generations annually in New York, and three in the District of Columbia. The species also breeds in white clover, and is frequently so numerous as to destroy the clover-seed crop over a large section of the country.

The remedy is a simple one, and affords an excellent example of the value of accurate knowledge of the life history of injurious insects. It is the custom in the northern states to cut clover twice in the season, once when the clover is in full bloom, for hay alone, and again in the autumn for seed. lf the hay crop be cut from two to three weeks earlier than usual the first generation of the insect will be destroyed and the seed crop in the autumn will not be affected, or at least only to a comparatively slight extent.