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 (L’riophyes pini Nalepa) is a very minute 
yellow blister or gall mite which has been found 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. 
Red spiders (Z'etranychus 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. Many 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. 
Oligonychus americanus 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 broadleaved 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 will be 
mentioned, 
