The thrip normally lays its eggs in slits cut in leaf or flower tissue and lives the first 

 stage of its life inside the tissue of the plant - which is why ordinary surface sprays 



do not kill the insects at these stages 



first infected may show brown or 

 yellowish ringspots, large circular 

 areas of brown dying tissue, and 

 browning of the midrib. 



In all host plants, symptom ex- 

 pression will vary depending upon 

 the age of the plant at the time of 

 infection, environmental conditions 

 (especially temperature), host 

 variety or cultivar, and strain of the 

 virus. 



Disease Cycle 



Tomato spotted wilt virus, in 

 virtually all cases, is u-ansmiited by 

 thrips. Thripsare tiny (l/16of an 

 inch) winged insects that feed on 

 plants by sucking sap from cells of 

 leaves and flowers. 



Immature thrips acquire the 

 virus when feeding on diseased 

 plants; the insects then spread the 

 virus as adults feeding on healthy 

 plants. The virus multiplies within 

 the plant and may exist throughout 

 it, even though some parts of the 

 plant may look perfectly healthy. 



TSWV survives the winter in 

 infected plants, including those held 

 over the winter in greenhouses, and 

 in many species of weeds. These 

 plants act as reservoirs of the virus 

 from which it can spread in the 

 spring by the feeding activities of 

 the thrips. 



Disease Control in 

 Greenhouses 



There are three primary 

 methods of control: (1) sanitation, 

 including weed control; (2) exclu- 

 sion of the vector and diseased 

 plants, and (3) insecticides to control 

 thrips. Sanitation practiced dili- 

 gently will prevent spread of the 

 virus. Keep the greenhouse area 

 free of weeds because they may 

 harbor both thrips and the virus. 

 Destroy plants known to be infected; 

 do not use these plants for propaga- 

 tion, even if some parts appear to be 

 normal and healthy. Inspect incom- 

 ing crops carefully for thrip infesta- 



FREEZE FRACTURE OF SOYBEAN LEAF ENLARGED 480 TIMES. 



tions or unusual symptoms. Accept 

 only healthy, insect-free plants. Fine 

 mesh screen (400 mesh) can be used 

 on doors, air intakes, and screens to 

 prevent thrips from entering houses 

 from the outside. 



Monitoring thrips in the green- 

 house using yellow sticky cards 

 placed near vents and above crop 

 level will allow detection of increas- 

 ing thrip populations. Thrips are at- 

 tracted to the yellow cards and be- 

 come trapped on the sticky surface. 

 Cards should be checked at least once 

 a week. When the number of trapped 

 thrips seems to be increasing, an in- 

 secticidal spray may be warranted. A 

 second application should be made 

 four days after the first. 



Thrips, especially the Western 

 Flower Thrips, have been found to de- 

 velop populations resistant to certain 

 insecticides that are used repeatedly, 

 so it is important to rotate the class of 

 insecticide used. For example, pyre- 

 thoids, carbamates, chlorinated hydro- 

 carbons, organophosphates, and soaps 



PHOTOGRAPH: JOHN KNOWLES 



should be used in rotation instead of 

 constantly applying just one chemical 

 class. 



Insecticides should be applied in 

 the early morning, when thrips are 

 most active and when the potential for 

 plant toxicity reactions are mini- 

 mized. 



For the complete brochure, 

 contact the Maryland Department of 

 Agriculture, 50 Harry S. Truman 

 Parkway, Annapolis, MD 21401. 

 (Telephone 301-841-5920) * 



(This is a synopsis of an 

 article written by Melody 

 Putnam, Maryland Depart- 

 ment of Agriculture, and 

 Ethel Dulky, University of 

 Maryland at College Park) 



