Description of the Pest
Thrips are small insects, about 0.04 inch long. Adult thrips have two pairs of narrow wings which are fringed with hairs. Immature thrips are wingless, whitish to yellowish in color, and are most commonly found in whorls, tassels, ears, or on the underside of leaves. Adults emerge continuously throughout the warm months. Adults and immatures may be found in corn at any time during the growing season. Eggs are deposited in plant tissue and hatching occurs in about 5 days during the summer months; the immature stages take about 5 to 7 days to complete development.
Damage
Thrips are most noticeable and of greatest concern at two periods during the corn growing season: on young seedling plants and at ear formation. On young seedlings their feeding makes the plants look stunted. A common sign of a heavy thrips infestation is distorted leaves that turn brownish around the edges and cup upward. Usually the plants will grow away from the problem, just as they outgrow severe ragging resulting from wind damage. At ear formation, thrips and thrips injury to developing kernels provides entry for infection by Fusarium spp. and subsequent Fusarium ear rot diseases. The actual thrips injury does little damage; however, the ear rot diseases can be devastating.
Foliage-feeding thrips are effective predators on early-season spider mite infestations. Both adult and immature thrips may be found in spider mite colonies feeding on spider mite eggs.
Management
Treatment is usually not necessary on seedlings because plants recover from thrips injury. Thrips are also beneficial at this time because of their role as mite predators. No threshold has been established for damage from thrips at ear formation. Treating for thrips will probably not prevent spread of Fusarium ear rot diseases.
Biological Control
Minute pirate bugs (Orius tristicolor) play a major role in controlling thrips populations.
Cultural Control
Thrips populations tend to build up on weeds. Cultivating nearby weedy areas before corn emerges will reduce the potential of a thrips problem when the weeds begin to dry out. Cultivating weedy areas after corn emergence will increase thrips problems.
Organically Acceptable Methods
Good field sanitation and the preservation of beneficial insects will help to manage thrips in an organically grown crop.