4 BULLETIN 416, U. S. DEPARTMENT OF AGRICULTURE. 



FOOD PLANTS. 



As the result of tlie investigations at Batesburg, S. C, supple- 

 mented by observations throughout the cotton belt, this mite has been 

 taken from 183 species of plants, including weeds, ornamentals, and 

 garden and field crops. Upon most of these the pest has been seen 

 only occasionally, but it is found commonly throughout the active 

 season upon the following plants : Cotton (Gossypium spp.) , cultivated 

 violet (Viola spp.), (PI. IV, fig. 4), Jerusalem oak (Chenopodium 

 hotrys), wild blackberry {Rubus spp.), wild geranium {Geraniums,^^.), 

 ironweed {Sida rJiombifolia) , garden bean (PJiaseolus spp.), pokeweed 

 (Phytolacca americana), tomato (Lycopersicon ly coper sicon), dahha 

 (DaMia spp.), (PI. IV, fig. 3), sweet pea (Lathyrus odoratus), (PL IV, 

 fig. 5), and hollyhock (Althaea rosea). 



Of these 183 host plants, 100 (or 55 per cent) are cultivated species 

 and 83 (or 45 per cent) are native wild species. It should be stated, 

 however, that in the preparation of this host list more time was 

 devoted to house yards than to rural localities. It seems reasonable 

 to suppose, therefore, that the common red spider occurs on fully as 

 many wild plants as on cultivated species. The fact that Harvey's 

 (1892) 37 host plants reported from New England, and Swing's 

 (1914) 30 hosts from the Northwest are practically all cultivated 

 species may be accounted for by presummg that these investigators 

 did not extend their research to the wild plants. 



Throughout the past five years of the red-spider investigation it has 

 been brought to our attention repeatedly that certain plants possess 

 a peculiar importance due to their restriction to certain seasons; 

 hence they may thus form a series of links in the cycle of infestation. 



The cultivated violet, which has come to be recognized as perhaps 

 the most important wintering host, and as a source of dispersion to 

 neighboring weeds and near-by cotton in the spring, is probably the 

 most commonly infested plant in the South. 



The pokeweed (Phytolacca americana) occupies an important posi- 

 tion as a host, but its exact status has never been clearly determined. 

 Among farmers in various parts of the cotton belt there is a strong 

 belief that red-spider infestation, called by them ''rust," has its 

 origin in pokeweed. The result of much careful study during the 

 winter and early spring months seems to refute the idea that poke- 

 weed normally supports mites during these periods. It does, how- 

 ever, function as a very desirable secondary host during the early 

 season migratory movements of the mites by intercepting a few indi- 

 viduals. These intercepted mites multiply rapidly, until the poke- 

 weed no longer furnishes sufficient nourishment, and at such times 

 the infestation spreads to cotton if it is available. (PI. V, fig. 6.) 



Native blackberry vines also constitute an important overwinter- 

 ing host, since many of the leaves remain attached throughout the 



