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



some new growth on the trees throughout the summer, but ordinarily 

 there are two periods in which the growths are most attractive to 

 thrips, one occurring in May and June, the other in August and Sep- 

 tember. During the August-September period the tissues of most 

 of the deciduous food plants and the rind of the oranges are tough 

 and distasteful, and there is a steady, often sudden, influx of thrips 

 from scattered locations to the new foliage of the orange and some 

 other citrus. The insects apparently become more numerous than 

 ever, being more closely concentrated than at any other time of year. 

 In this period of the season, therefore, the following conditions 

 favorable to the thrips progeny of the ensuing spring usually occur : 

 The food supply is abundant and concentrated. The proportion of 

 male thrips is increased, and mating and oviposition occur to a much 

 greater extent than at any other time. This most favorable circum- 

 stance of abundant food in comparatively small space at just the time 

 when the insect must produce the eggs for the next summer's gen- 

 eration is a principal factor in the rise of the species as a pest. It 

 has been brought about by the occurrence in recent years of large 

 numbers of vigorous young orange trees which the thrips was able 

 to substitute for the miscellaneous weeds previously constituting its 

 food supply. 



GENERATIONS. 



It is impossible to distinguish between generations of the citrus 

 thrips, except perhaps the first and second summer generations, 

 when growth is slow. Throughout the middle of the summer the 

 life cycle occupies from 15 to 16 days, while oviposition covers "from 

 25 to 30 days. Thus the complete life cycle may be passed by any 

 given lot of individuals and eggs deposited by them to begin another 

 generation while their progenitors are still ovipositing. This causes 

 an overlapping of broods, such that it is impossible to distinguish 

 between them. The number of generations will depend, of course, 

 upon the character of the season. An early, warm spring followed 

 by a prolonged, hot summer may result in the production of eight 

 or more generations. In seasons such as 1911, six full generations 

 may be expected between the middle of April and the first of 

 November. 



NATURAL CHECKS. 



FREEZES. 



Practically all sections of California where oranges are grown are 

 subject to occasional freezes of varying severity, which always occur 

 between November 15 and March 1, when the citrus thrips are prac- 

 tically all in the egg stage in the leaves and stems. The shoots most 

 severely injured by freezes are those in which a majority of the eggs 



