Grass thrips are orange or yellow, slender insects about 1/16 inch long. Adults have two pair of feather-like wings and are darker in color. Immature thrips are wingless, yellowish in color, and are most commonly found in whorls, tassels, ears, or on the underside of leaves. Both winged adults and immatures feed by penetrating leaf cells and sucking out their contents. Minute, longitudinal, whitish scars become so numerous that entire leaves may look grayish and desiccated. Most feeding takes place behind leaf sheaths in the whorl or on the underside of lower leaves.
Thrips are most noticeable and of greatest concern at two periods during the 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.
Damage is usually noticed only under extended periods of hot, dry weather conditions when plants are moisture-stressed. Gusty winds during these periods will further desiccate plant tissue and compound thrips injury. Usually the plants will outgrow the problem, just as they outgrow injury resulting from wind damage.
At ear formation, thrips and thrips injury to developing kernels provides entry for infection by Fusarium spp. and subsequent ear rot diseases from Fusarium and other organisms that can result in losses.
Survival And Spread
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 five days during the summer months; the immature stages take about five to seven days to complete development.
Treatment is usually not necessary on seedlings because vigorously growing plants with adequate soil moisture will recover from thrips injury. Thrips also might be beneficial at this time because of their role as mite predators. If dry conditions persist, plants are stressed, and thrips are present, treatment may be justified. Foliar insecticides are effective for suppression of thrips. 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.
Minute pirate bugs (Orius tristicolor) play a major role in controlling thrips populations. Indiscriminate use of broad spectrum insecticides can wipe out the bugs and flare thrips populations.
Good field sanitation and the preservation of beneficial insects will help to manage thrips in an organically grown crop. Thrips populations often build up on weeds. Cultivating nearby weedy areas before corn emerges will reduce potential thrips problems when the weeds begin to dry out. Cultivating weedy areas after corn emergence may increase problems.
If control is necessary, consult UF/IFAS recommendations for currently labeled insecticides for thrips control in sweet corn.