52 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 



type of damage, while important on shade trees, is seldom so on forest 

 trees under natural conditions. 



The pine needle mite {Eriophyes pini Nalepa) is a very minute 

 yellow blister or gall mite which has been founcl causing considerable 

 injury to the needles of Monterey pine in Golden Gate Park, San 

 Francisco, and to Torrey pine and Jeffrey pine in other parts of 

 California. It feeds within the basal sheath of the needle cluster and 

 causes a premature shedding of the needles and a weakening of the 

 tree. A 10-percent miscible-oil spray has given fairly satisfactory 

 control, but the removal of badly infested pine may at times be 

 necessary. 



E,ed spiders {Tetranychus spp.) are frequently the worst enemies 

 of shade trees in the central valleys of California, especially during 

 long, dry, hot seasons, when they attack the leaves and cause them 

 to fade and die. One species in particular is found on incense cedar 

 and another on ^Monterey pine. Alany species are found on the broad- 

 leaved trees. Both sulphur dusts and the combination oil and nico- 

 tine sprays have given good control on shade trees. More than one 

 application during a year is frequently needed. 



Oligonychiis americanns Riley has appeared as a rather common 

 pest of Douglas fir along the Madison River of Yellowstone National 

 Park. It webs the needles and turns them a dirty brown. 



GALL MAKERS 



A very large group of insects and mites have the unique ability to 

 irritate various plants so as to produce a gall, swelling, or peculiar 

 malformation. The common oak apple is a familiar example. Some 

 galls take the form of large, globular protuberances, others take 

 the appearance of buds or flowers, while some are simply an enlarge- 

 ment of the leaf or stem. These galls seldom are seriously harmful, 

 however, and control measures are called for only where ornamental 

 trees are made unsightly by such growths. On forest trees their 

 presence can usually be ignored. 



Galls may be formed by several groups of insects. The cynipids, 

 sawflies, gall midges, and gall aphids include most of the gall-form- 

 ing insect species. Gall mites of the family Eriophyidae are also 

 responsible for a large number of peculiarly shaped galls on broad- 

 leaved and other trees. Other important plant galls are formed by 

 fungi and various parasitic plants. 



On ornamental trees some of the gall-forming insects can be con- 

 trolled by spraying at the proper season of the year, but for forest 

 trees such treatment is impractical and seldom would be justified 

 by the importance of gall damage. 



There are innumerable types of galls on the various species of 

 western forest trees {32), particularly on the broaclleaved trees, such 

 as poplar and willow. Space in this publication would not permit 

 even the listing of the various species. However, a few of the more 

 important gall insects on commercially important forest trees ^^ill be 

 mentioned. 



