THE CITRUS THRIPS. 23 



growth and the appearance of thrips larva? occur about 10 days 

 earlier on the foothill slopes than on the valley level. Adult thrips 

 begin to appear rapidly about the time the Washington navel trees 

 are in full bloom, and the growth from 1 to 3 feet long, usually about 

 the middle of April, and the resultant oviposition and feeding on 

 the spring growth soon bring their injury into prominence. About 

 the time one-third or more of the petals have fallen the first growth 

 of foliage begins to harden and the thrips transfer to the little fruits. 

 This period of transfer usually will fall between April 15 and May 

 30, depending upon conditions of growth and bloom. 



The first injury to the fruit is caused largely by a comparatively 

 few of the first larva? which issue from it, feeding deeply in a circle 

 about the stem, the injury so made eventually developing into the 

 ring scars typical of citrus-thrips injury. Severe injury to the fruit 

 begins as soon as the petals drop, increasing from that time until the 

 oranges average about the size of a tennis ball. The more serious 

 damage usually is done between the middle of April and the early 

 part of July. Adult thrips then begin to leave the oranges, which 

 become tough and distasteful to them, and transfer to the second 

 growth of foliage. A few thrips feed and oviposit on the fruit prac- 

 tically throughout the season, however, and slight injury may often 

 occur until the last of September. In 1911, the leaf growth was 

 sparse and the amount of late injury to the fruit was unusual, being 

 considerabty greater than had been expected from the relative 

 scarcity of thrips. 



MIGRATION OF THRIPS TO SECONDARY FOOD PLANTS DUE TO SCARCITY OF 



ORANGE GROWTH. 



The transfer of thrips from oranges to the secondary food plants 

 takes place every season in greater or less degree, depending upon 

 food conditions. The scarcity of suitable citrus food in 1911, ap- 

 proximately from June 30 to August 30, led to an unusual increase 

 of the insects on certain other plants during that period. Grapes 

 are especially subject to infestation at such times, and one case was 

 reported to the writer in which both leaves and berries of a small 

 vineyard were severely injured. During this period in which the 

 thrips are scattered widely over their various food plants — roughly, 

 during June, July, and August — reproductive activity is at its min- 

 imum. 



CONGREGATION OF THRIPS ON LATE SUMMER GROWTH OF CITRUS. 



Washington navel and Valencia orange trees in the San Joaquin 

 Valley make from three to six successive new growths during the 

 summer, depending on weather conditions, care of the grove, and age 

 and health of the trees. In exceptionally favorable seasons there is 



