248 MANUAL OF VEGETABLE-GARDEN INSECTS 



of the plant, killing the cells and causing them to turn whitish. 

 When the infestation is severe and the punctures consequently 

 very numerous and close together, the leaves lose their dark 

 green color and assume a dirty white or bleached appearance. 

 The tender leaves at the center of the plant do not turn white 

 when punctured by the thrips, but instead curl inward and 

 downward and present a thickened deformed appearance. 



Badly injured plants wilt and the leaves fall over on the 

 ground ; in such cases, the bulbs do not make the proper 

 growth and the crop is of small size and inferior in quality. 

 The plants are most susceptible to injury by thrips during 

 June and July when they should be making their most rapid 

 growth. Thrips are most destructive in years of drought 

 because, under unfavorable conditions, the plants are less able 

 to outgrow the injury. Onions grown on poor soil or when 

 poorly cultivated are more likely to be injured by thrips than 

 when planted on good ground and well cared for. When 

 onions are grown for seed the thrips often infest the flower- 

 heads in sufficient numbers greatly to reduce the crop. 



Infested cabbage and cauliflower leaves turn brownish or 

 rusty instead of dirty white, as in the case of onions. As a 

 rule the injury is confined to the outer leaves, and is usually 

 outgrown by the plants, provided other conditions are favorable. 



Control. 



Onion thrips can be killed by application of such contact 

 insecticides as whale-oil soap, kerosene emulsion and tobacco 

 extracts. The best results have been obtained by spraying 

 early, before the leaves turn down, with "Black Leaf 40" 

 tobacco extract, 1 pint in 100 gallons of water in which there 

 have been dissolved 5 pounds of soap. There are great difficul- 

 ties in applying the spray to large fields ; heavy traction or 

 power sprayers cannot be employed on the soft muck soil on 

 which onions are usually grown. The knapsack sprayer can 



