20 



CIRCULAR 270, U.S. DEPARTMENT OF AGRICULTURE 



Figure 21. 



-Apple leaf infested with pear leaf blister 

 mite. 



mites attacking apple and pear are distinct species, since very often 

 one fruit shows evidence of injury, while on adjacent trees the other 



fruit does not. The mite that 

 attacks the apple apparently 

 comes from the wild snowberry 

 (Symphoricarpos racemosus 

 Michx.), since a mite that seems 

 to be identical is found on that 

 plant, and infested apple or- 

 chards are most often in canyons 

 and along foothills adjacent to 

 wild growths of the snowberry. 

 The form found on the pear tree 

 is evidently the same as the spe- 

 cies occurring in Europe, and un- 

 doubtedly came from there origi- 

 nally. 



LIFE HISTORY 



The adult mites pass the winter 

 under the scales of fruit and leaf 

 buds. They are exceedingly 

 small, less than one one-hun- 

 dredth of an inch in length, and 

 are therefore practically invisible 



to the naked eye. Under a lens or a microscope they appear (fig. 23) 



as elongated, whitish, wormlike creatures, with four legs near the head 



end. Often hundreds 



of them can be found 



in a single bud. When 



the buds begin to 



swell under the in- 

 fluence of warm 



spring weather, 1 the 



mites lay eggs in 



them, and the young 



hatching from the 



eggs burrow into the 



unfolding leaves and 



form the character- 

 istic blisters. The 



overwintering mites 



may also attack the 



young leaves. The 



mites feed entirely 



within the blisters, 



and a succession of 



generations de- 



velops, practically 



out of the reach of sprays. With the approach of cold weather the 



mites migrate to the buds, beneath the scales of which they hibernate. 



CONTROL 



The pear leaf blister mite can be controlled very easily with lime- 

 sulphur solution used as a dormant spray at the same strength as for 



Figure 22.— Apple deformed and russeted by pear leaf blister mite. 



