8 BULLETIN 616, U. S. DEPARTMENT OF AGRICULTURE. 



A few eggs also pass the winter in the ripening fruit and are thus 

 distributed to practically every part of the United States. Some 

 of these eggs may hatch and survive from fruit marketed in tropical 

 and subtropical climates, but there is very little danger of the insects 

 becoming established in any of the localities where oranges are 

 marketed generally. 



Thrips larva? are to some extent distributed on nursery stock 

 and, though not so difficult to detect as eggs, would be overlooked 

 very largely in commercial inspection because of their minute size 

 and habit of hiding in crevices in the bark. It is probable that 

 occasionally the larvae are shipped on fruit, especially on the Valencia 

 orange, but nearly mature fruit has little attraction for them, and 

 it is further probable that few survive the handling incidental to 

 picking and packing. 



Local spread is accomplished largely by flight. The citrus thrips 

 is a very ready flier when disturbed or when it desires better food. 

 An observer standing in a favorable position with regard to the 

 light may see numbers of these minute insects leave one tree and 

 make for another, usually disappearing at a distance of 4 or 5 feet 

 because of the light not striking just right. The insect will leave 

 either tree or orchard as soon as leaves, stems, and fruit become 

 somewhat tough. Thus in a grove of young Valencia trees where 

 thrips were present to the number of 80 to 150 per leaf at one exami- 

 nation, not a single adult thrips could be found two weeks later. 

 The explanation was that the leaves had hardened and were no 

 longer suitable as food, while on some older navel trees near by a 

 luxuriant growth of leaves had just reached prime condition for 

 food, and here the insects were found in as large numbers as they 

 had been found on the trees they had deserted. 



The citrus thrips usually takes a rapid spiral or zigzag course in 

 short-distance flights, but in extended flights the course is more direct. 

 The flight somewhat resembles that of katydids and grasshoppers. 

 By mounting in a strong wind the thrips undoubtedly would fly a 

 mile or possibly more. 



FOOD PLANTS. 



Although the citrus thrips thrives best and reaches its maximum 

 abundance on plants of the citrus group, it feeds continuously or 

 occasionally on plants widely separated botanically from the citrus 

 group. In Table I the food plants are arranged according to the 

 extent of infestation as nearly as possible in the order of their im- 

 portance as food for thrips. There is very little difference, however, 

 in the extent to which the different plants in Group III are infested, 

 and certain plants in Group II are infested almost as badly as any 

 of the citrus fruits of Group I. It will be noted in Table I that the 



