Thrips, Vol. 4, No. 8

Various species

Order: Thysanoptera
Family: Thripidae

With a mature body length of about 1/20^th of an inch, thrips are some of the smallest insect pests.  Despite their small size, thrips can cause big problems for roses, cotton, tomatoes and many other plants.  In certain row crops and vegetable crops, thrips damage emerging seedlings, causing distorted leaves and terminals and slow growth.  Some species, such as western flower thrips, spread serious virus diseases, such as tomato spotted wilt virus.  Other species feed on leaves of older plants, causing galls or “bleached,” inefficient leaves.  Flower thrips are an especially vexing problem for rose growers.  They feed on blooms causing the petals to be discolored and distorted. Light-colored blooms, whites and yellows, are especially attractive and usually suffer greatest damage. 
 
Thrips populations are often highest during warm, dry springs.  Large numbers of adults develop on spring weeds and crops such as wheat.  As these spring plants mature and begin to dry out, they become unsuitable hosts and emerging adult thrips take wing, allowing prevailing winds carry them to new places.  During heavy migration periods, sticky cards placed around field borders or in rose gardens are quickly peppered with thrips and susceptible crops are heavily infested.  
 
Control:  Thrips can be difficult to control for several reasons.  One is their preference for feeding on the undersides of leaves and inside blooms and buds, where they are difficult to reach with contact insecticides. Another major challenge is controlling migrating adults during the spring, because even when you get good control, more are coming!  Systemic insecticides, such as imidacloprid, are some of the most effective treatments for controlling thrips on seedlings of susceptible row crops like cotton, and insecticides containing spinosad, acetamiprid, imidacloprid, or acephate are some of the most effective treatments for controlling thrips on ornamental plants such as roses.
 
But insecticide treatments are not very effective for preventing thrips-born virus diseases.  Even if you have an effective insecticide treatment on, or in, the plant, the thrips usually feeds a little before being controlled and this is enough to infect the plant.  Disease resistant varieties and reflective row covers are the most effective tools for protecting tomatoes and other susceptible vegetable crops from being infected with tomato spotted wilt virus.  Physical exclusion is yet another method of controlling thrips.  Commercial greenhouses use thrips-proof screening to keep thrips from getting inside.  It takes a mesh size of 1/75000^th of an inch to do this.
 
For more information on thrips in roses and ornamental plants see, Publication 2472, Insect Pests of Roses, and Page 10 of Publication 2369, Insect Pests of Ornamental Plants in the Home Landscape.
 
Thanks to Dr. Jack Reed, retired Entomology Professor, Mississippi State University, for providing this highly magnified photo.  From top to bottom these are tobacco thrips, western flower thrips and eastern flower thrips.