April, '17] KELLY: GREEN-BUG OUTBREAK 235 



fall of 1909. However, during the fall and winter of 1909-1910 an ex- 

 periment was conducted at Wellington, Kansas, laboratory, for the 

 purpose of obtaining information on this point. In November a num- 

 ber of Toxoptera were placed on a breeding plat near the laboratory at 

 Wellington for careful study of their resistance to freezes and to see 

 what potent factors determined the sexual forms. On December 31 an 

 examination was made, and it was observed that both adults and young 

 were still alive on the wheat plants; though we had had several days of 

 zero weather, no sexual forms occurred. January 27, 1910, many were 

 still alive, and had survived some very severe weather, and there were 

 still no sexual forms. On March 25, 1910, a few adults were found with 

 no young accompanying them. These had evidently passed the winter, 

 having found a protected place beneath some of the thicker growth of 

 the plants. Special searches were made on this date, for oviparous 

 forms and eggs, but none were found. On two occasions, one in De- 

 cember, 1909, and one in January-, 1910, full-grown aphids and young 

 which were frozen to bits of straw or ice, were brought into the 

 laboratory, were allowed to gradually thaw out, and were later placed 

 in breeding cages, where they grew, reproduced normally, acting as 

 if they had not been in the cold weather at all; yet they produced no 

 sexual forms. 



In the fall of 1915 the insect was carefully studied in reference to 

 sexual forms in the vicinity of W^ellington, Kansas. Wherever the 

 species was very abundant in wheat fields the plants, dry grass, and soil 

 were examined carefully for eggs and a large number of the aphids were 

 brought to the laboratorj^, where they were kept in breeding cages, both 

 indoors and out, but at no time did the sexual forms appear. The 

 question arises, then, what causes these forms to appear. Evidently it 

 is not cold weather, because we had extremely cold weather during the 

 winter of 1915-1916. We did not have such extreme cold weather 

 during the fall of 1907 and winter of 1908. However, the notes indicate 

 in the fall of 1908 that egg-laying forms were found on blue grass at 

 Leavenworth and on wheat at Wellington. 



Causes of Outbreaks 



There have been four disastrous outbreaks of this insect, one in 1890, 

 one in 1901, one in 1907, and the one in 1916. Aside from these, there 

 have been a few minor outbreaks, but it appears from information 

 gained from the weather records that the causes for the minor out- 

 breaks in restricted localities have been similar to the causes of the 

 larger outbreaks; at least, they are comparable to the seasons preced- 

 ing the 1907 outbreak and the season preceding the 1916 outbreak, 

 which were quite similar. 



