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



affected trees or render them susceptible to attack by other tree- 

 killing insects. 



Burke (25) rates them as the most important pests of shade, 

 park, and ornamental trees in the Pacific Coast States. They are 

 less important on forest trees under natural conditions. A few 

 species have been reported as seriously injuring or killing native 

 conifers, but their work is much more common on broadleaved 

 trees. 



Spider mites belonging to the genera Tetranychiis and Para- 

 tetranychtis are frequently the worst enemies of shade trees, 

 especially in the warmer parts of the region and during long, dry, 

 hot seasons. They suck the juices from leaves and cause spotting, 

 fading, yellowing, or heavy leaf cast. Sometimes leaves are 

 covered with silvery webs, or turn brown and rusty, as though 

 scorched. Many species are found on broadleaved trees, and 

 several are also found on conifers. Both sulfur dusts and wettable 

 sulfur have given good control on shade trees when applied in 

 the spring and summer when the temperature is high; or a dor- 

 mant miscible oil spray may be used during the winter to kill the 

 egg stages. 



GALL MAKERS 



A very large group of insects and mites have the strange ability 

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

 culiar malformation (51). The common oak apple is a familiar 

 example. Some galls take the form of large, globular protuber- 

 ances, others take the appearance of buds or flowers, while some 

 are simply an enlargement of the leaf or stem. These galls seldom 

 are seriously harmful, however, and control measures are called 

 7 or only where ornamental trees are made unsightly by such 

 growths. On forest trees their presence can usually be ignored. 



The cynipids, sawflies, gall midges, and gall aphids include most 

 of the gall-forming insect species. Gall mites of the family Erio- 

 phyidae are also responsible for a large number of peculiarly 

 shaped galls on broadleaved 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 

 controlled 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 (50), particularly on the broadleaved trees 

 such as poplar and willow. Only a few of the more important gall 

 insects on commercially important forest trees will be mentioned. 



