218 Acarology 



they migrate to other hosts, such as cotton. There appears to be a 

 predominance of the females during the summer, but as cold weather 

 approaches the numbers of the sexes become, more nearly equal. The 

 female lives from seventeen days in midsummer to several months in 

 winter, whereas the male is shorter lived. The female is not active, 

 but the male, when not mating, moves rapidly about. The male is 

 smaller and narrower than the female and the abdomen can be tele- 

 scoped considerably, as during mating. Parthenogenesis, the giving 

 birth to males by unfertilized females, has been reported. 



Little work has been done on the transmittal of plant diseases by 

 the spider mites. Moskovetz 1940 reports a virus disease of cotton 

 being transmitted by Tetranychus telarius (L.). 



Of the plant-feeding mites, the Tetranychidae are probably the most 

 important economically. Hardly a plant is free from at least one spe- 

 cies, and practically all species of the family are or appear to be poten- 

 tial pests. The mites damage the plants by piercing the leaves with 

 their stylet-like chelicerae and draining out the cellular material near 

 the puncture. When the mite population is large the leaves may be 

 killed and drop from the tree. Populations of red spiders may build 

 up to fantastic numbers if left uncontrolled. In Porterville, California, 

 Tetranychus pacificus McGregor has reproduced so rapidly in the 

 summer on chinaberry (umbrella) trees that they were defoliated 

 within a few days, and the dropping mites completely covered the 

 entire surface of the ground as well as fence posts. The use of DDT 

 for agricultural pests apparently has upset a spider mite-predator com- 

 plex, causing the tetranychids to become serious problems in areas 

 where they had been of no or of secondary importance. Little known 

 or undescribed species are not only becoming pests, but some are build- 

 ing up resistance to the newer acaricides. In many cases, however, no 

 reason can yet be given for increased populations of these mites. 



There are many species of importance in the United States, but only 

 a few are listed here. Metatetranychus iilmi (Koch) is distributed 

 over continental Europe from Italy to Sweden and Norway, and from 

 France to Russia. In the United States it is distributed generally along 

 the Atlantic coast from Nova Scotia to North Carolina and west to 

 Ontario, Michigan, Indiana, Tennessee, Kentucky, Wisconsin, Illinois, 

 and Missouri. In the West it is found from British Columbia to cen- 

 tral California and east to Idaho and northern Utah. The favorite food 

 plants are the deciduous fruits, especially the plum, prune, apple, and 

 pear, but this mite has been found on many other plants. M. citri 



