ONION INSECTS 



247 



enter the ground a short distance, where, by twisting and turning 

 their bodies, they construct small earthen cells. In these cavities 

 the insects molt and thus enter the third nymphal, or so-called 

 prepupal, stage. These third-stage nymphs are yellowish 

 white in color and the wing-pads extend to the middle of the 

 second abdominal segment. They do not feed in this stage 

 but remain quietly resting in their earthen cells. 

 They are able to walk, however, and wdien 

 disturbed soon, crawl into hiding again. In 

 two or three days the insect molts and enters 

 the fourth and last nymphal stage (sometimes 

 known as the popular stage) . The fourth stage 

 is whitish in color and the wing-pads extend 

 to the eighth abdominal segment ; the body 

 is more spiny than in the preceding stages. In 

 this stage, which lasts from one to six days, the 

 insect remains in a quiescent condition and 

 takes no food. At the next molt the thrips 

 becomes adult, returns to the plant and re- 

 sumes feeding. The life cycle is completed 

 in about twenty-six days. There are five or 

 six generations annually and breeding con- 

 tinues until stopped by cold weather. 



A careful examination of infested onion plants at any time 

 during the summer will disclose the presence of four stages of 

 the onion thrips, viz., eggs, first- and second-stage nymphs, 

 and winged adult females. The first- and second-stage nymphs 

 may be distinguished by the difference in size and by the more 

 distinctly yellow coloration of the latter. Both the nymphs 

 and adults feed on all parts of the leaves but are to be found in 

 greatest numbers hidden under the sheath at the base of the 

 leaves or between the young leaves at the center of the plant. 

 In feeding, the thrips punctures the epidermis of the leaf with 

 its sharp needle-like mouth-parts and then sucks out the juices 



Fig. 150. — The 

 onion thrips, 

 second stage 

 nymph ( X45)'. 



