Feeding by larvae of this gall midge (Cecidomyiidae) causes distorted, swollen growth of leaflets on honey locust, Gleditsia triacanthos. Established trees are rarely, if ever, killed by the galling, so damage can be tolerated.
Identification
Galls are most apparent during spring. Each distorted leaflet contains one to several pinkish white maggots up to 1/10 inch long. The oblong pupae are orangish to white and 1/10 inch long. Adult gall midges are delicate flies about 1/10 inch long with long, slender antennae and legs and a gray thorax with two black, lengthwise stripes.
Life cycle
Adults emerge from overwintering pupae in soil and most egg laying occurs during spring. By midsummer egg laying ceases, and plants often continue to produce new leaves that do not develop galls.
Larvae require succulent new terminals in order to feed and cause galls. Pupation of the spring and summer generations occurs within the galls. In the fall, mature larvae drop from galls or crawl to pupate under the tree canopy, within 1 inch of the soil surface. There are about six overlapping generations per year in California.
Damage
Infested leaflets form brown, green, or reddish galls. Heavily infested foliage turns brown and drops prematurely, leaving parts of branches leafless.
Solutions
Avoid the Sunburst honey locust, which has bright yellow spring foliage. Sunburst readily defoliates in response to drought or temperature changes, as well as gall midge damage. Consider planting the Shademaster cultivar of honey locust, which appears to be less susceptible. Black locust, Robinia spp., are not infested by this midge.
Several species of parasitic and predaceous wasps feed on this pest. Because the gall midge has many generations and the larvae and pupae occur protected in galls or soil, this insect is not easily controlled with insecticides.
On small plants, narrow-range oil, or horticultural oil, sprayed to thoroughly cover terminals at intervals during about March and April kills gall midge eggs and can substantially reduce damage.
To increase the effectiveness of oil, properly time its application. Beginning in early March, monitor for the presence of adults by hanging at least two yellow sticky traps in the lower outer branches of honeylocust. Inspect each trap at least twice a week using a 10X hand lens or a dissecting microscope to view and identify the adults, described above.
