THE CITRUS THRIPS. 23 
growth and the appearance of thrips larvae 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 larvae 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 
considerably 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 
