Eriophyes rudis Canestrini. 



These mites cause one of the most remarkable and interesting 

 forms of British vegetable gall-growths. They cause deformities 

 which are of prodigious proportions when compared with the 

 extreme minuteness of a single mite. 



The deformities, or galls, are known as " witch-knots," 

 " witches' brooms," " rooks' nests," and other local appellations, 

 and may be seen, while walking or driving through the country 

 during the winter or spring, in birch-trees in almost any part 

 of England. They are less noticeable at other seasons because 

 of the foliage of the trees. 



They occur in varying numbers,, some small trees having 

 many of the galls upon them, while large trees may have but 

 two or three. The variation appears to depend entirely upon 

 the length of time the mites have been established upon the 

 tree and the multiplicity of their numbers, rather than upon any 

 climatic conditions or environment of the tree. Apparently 

 they do little or no harm to its general growth and development. 



The origin of these galls is in buds in which the mites have 

 lived during the winter. When the buds begin to expand in 

 the spring, they are retarded from successfully sending out 

 leaves, owing to the mites absorbing for their nourishment a 

 great amount of the leaf-producing sap. A continuation of the 

 twig is also prevented. The bud increases a little in size and 

 assumes a somewhat globular form, which, as summer proceeds, 

 is altered into a rosaceous shape and is composed of a number of 

 irregular-sized, imbricated, leaf-like scales. The outer scales in 

 course of time fall off, the inner ones also fall with the growth of 

 new buds. These new buds appear at the base of the old one, 

 and they are quickly subjected to the same treatment by the 

 mites. The mites increase in numbers very rapidly, and are con- 

 tinually moving from one bud to another, rendering them abortive. 



This is repeated again and again, until, after a period of 

 several years, the end of the twig is covered by buds which 

 under normal conditions would have been distributed along 

 several inches of new wood. If, owing to the concentration of 

 attack at one spot, the twig has not the power to lengthen or 

 continue forward growth, a mass of numerous short twigs is 

 formed, which proceed from a hard and woody core. But where 

 the mites are distributed along the twigs a bushy tangle of long 

 and slender twigs is the result. See also pp. 62, 64, 66, 68, 

 and 70, and plates 16, 17, 18, 19, and 20. 

 56 



