Larvae of the western flower thrips can become infected with tomato spotted wilt virus or impatiens necrotic spot virus by feeding on an infected plant for only 30 minutes. After a latent period of 3 to 18 days, these thrips can then infect new plants after feeding only 5 to 15 minutes. Only larvae can become infected by these viruses, but adults do most transmission of the disease. Western flower thrips are attracted to flowers of ornamental plants, although they breed readily on the foliage of chrysanthemum, gloxinia, tomato, vegetables, and grasses. The western flower thrips has acquired resistance to many insecticides and breeds successfully in many greenhouses all year long, so that flower growers often have an extraordinarily difficult time getting good control.
Thrips wedge down deep into the blossoms and buds and avoid the pesticide.
Because systemic insecticides do not readily translocate into the buds and flowers of ornamental plants, blossoms cannot be protected by applying systemic pesticides to the potting mix. Some flower growers report adding sugar at 1 pound per 100 gallons of spray mixture enhances thrips control. Our demonstrations have not corroborated the effectiveness of sugar in drawing the thrips out of deep crevices to areas of higher pesticide concentration. If sugar is used, the highly refined table sugar will not damage plants at 1 pound per 100 gallons whereas brown sugar may cause some damage. Treatments should be scheduled 3 to 5 days apart.