THE THYSANOPTERA 



155 



longer in the cooler temperatures of the northern states, and several genera- 

 tions are produced in a season. 



Control. In general, spraying with nicotine sulfate 40 per cent, 

 standard formula, or with kerosene emulsion, 1 part in 4 parts of water, 

 is a good treatment. Success with these materials, however, depends 

 largely upon the thoroughness of the application and the number which 

 are killed. A favorite formula in California consists of 1J4 gal. of com- 

 mercial lime-sulfur, and 3^ fl. oz. of nicotine sulfate 40 per cent in 50 

 gal. of water, applied as a spray. Where the adults are wintering in grass 

 fields and it is practicable, burning these over will 

 destroy many. 



The Onion Thrips (Thnps tabaci Linde.;. This 

 pest is present practically everywhere in Europe 

 and the United States, having first been noticed 

 here about 1872 (Fig. 149). The adult is about a 

 twenty-fifth of an inch long, rather light yellow, 

 but turning brown as it becomes older. It feeds 

 on a great variety of plants but being the species 

 which is particularly injurious to growing onions, 

 is generally known as the Onion Thrips. The onion 

 leaves are whitened by the removal of their juices, 

 and soon begin to bend sharply downward, and 

 later they may curl or twist and even die, an area 

 much affected in a field being noticeably pale 

 colored and the plants stunted, while the bulbs 

 make little growth. 



Winter in the North is spent as the adult in 

 protected situations such as in dead grass close to 

 the ground or in rubbish left on the field. In 

 spring the young onion plants are attacked soon 

 after they come up, first in the bud, later on the leaves, in which the eggs 

 are laid. The life cycle from egg to adult is influenced by the tempera- 

 ture, varying from a little less than 3 weeks to over a month, and in 

 the most southerly states the generations overlap so that practically all 

 stages may be found at the same time. Sometimes in the North this 

 insect becomes a greenhouse pest on roses, carnations, cucumbers and 

 tomatoes, though the Green-house Thrips (Heliothrips hcemorrhoidalis 

 Douche") is most often responsible for this injury. 



Control. Any methods of farming which will reduce the oppor- 

 tunities for this insect to pass the winter successfully, are of value. The 

 destruction of all refuse on the field after the crop has been gathered: 

 fall plowing of such fields, and burning over grass lands adjacent to them, 

 at the proper time in the spring, are all beneficial. Cultivation and fertili- 

 zation to pus.h the crop ahead early to "keep it ahead of the thrips" is 



FIG. 149. Nymph 

 of the Onion Thrips 

 (Thrips tabaci Linde.), 

 greatly enlarged. 

 (From Britton: Third 

 Rept. Conn. State 

 Entomologist.) 



