THE CITRUS THRIPS. 25 



are deposited, and thus it occasionally happens that a considerable 

 number of the eggs are frozen. In the winter of 1911-12 more or less 

 severe freezing occurred throughout the citrus-growing sections of the 

 State. The leaves and stems of the principal summer's growth were 

 browned and withered, and the thrips suffered a considerable reduc- 

 tion in numbers in many orchards. For example, an orchard which 

 had been severely infested by thrips during the three years from 1909 

 to 1911, inclusive, and particularly in 1911, when the entire crop was 

 rejected because of thrips scabbing, was frosted so badly during the 

 winter of 1911-12 that the tender leaves and stems were wilted and 

 blackened well down into the leaf expanse of the trees. In August, 

 1912, examination of the fruit from this orchard showed only IS 

 per cent of it to be marked by thrips, the degree of injury being noted 

 at the time as mostly very slight stem-end rings. Thrips were very 

 scarce and the leaves showed little injury except in scattered spots. 



RAINS. 



From the fact that rains are supposed to be a powerful agency in 

 the natural control of some species of thrips, it has been argued that 

 a season of unusually heavy rainfall would reduce greatly the number 

 of citrus thrips. Whatever merit rainfall may have in checking 

 thrips of other species, its effect on the citrus thrips under the con- 

 ditions prevailing in the San Joaquin Valley and in Arizona could 

 not be of the least importance, since the heavier rains do not begin 

 there until October at the earliest, and usually not before November 

 or December, by which time almost all the overwintering eggs have 

 been deposited. In spring the rains cease before the thrips emerge 

 in any considerable number. The season of 1909, following the 

 heaviest rainfall in the three-year period from 1909 to 1911, inclusive, 

 in the San Joaquin Valley, was the worst season for thrips injury 

 on record. 



NATURAL ENEMIES. 



INSECTS. 



The most important insect enemy of the citrus thrips is the larva 

 of the common lacewing fly of California {Chrysopa calif ornica 

 Coq.) - 1 In its early stages this larva feeds largely upon larvse of the 

 citrus thrips. In October, 1911, an examination of 10 one-year-old 

 orange trees on which thrips were numerous disclosed an average of 

 25 eggs of this Chrysopa per tree. A number of the sickle- jawed 

 larvae were also present in the trees, and several of the smaller ones 

 were engaged busily in feeding upon thrips. The thrips were indeed 

 almost the only food available to the lacewings in quantity on these 



1 Identified by Nathan Banks. 



