CACTUS-FEEDING INSECTS AND MITES 151 



of Opuntia as quimilo, ficus-indica, sulphurea, glomerata, and aoracantha. 



It has been established in Australia for many years (recorded by H. 

 Tryon in 191 1) on 0. inermis, 0. stricta, and 0. tomentosa, but evinces a de- 

 cided preference for the last-named. Occasionally it becomes abundant 

 on a few plants in local areas, but in the main it is held in check by a 

 chalcid parasite and by predacious Lepidopterous larvae. 



This scale rarely attains importance in America. In Mexico and 

 Argentina, heavy infections have been observed to cause damage to a 

 limited number of plants. Often one prickly pear plant is severely 

 infested, with a light attack on adjacent plants, and little sign of scale 

 elsewhere in the district. 



ACARINA 



Tetranychidae 



TETRANYCHUS Dufour 

 Tetranycbus opuntiae Banks 



This spider mite is found on the shrub pear 0. lindheimeri in Texas as 

 far north as Waco, west to Uvalde, and south to Laredo and Brownsville 

 and in the Mexican States of Coahuila, Nuevo Le6n, and Tamaulipas, 

 as far as Tampico. Around Uvalde at times plants are so heavily in- 

 fested that many segments fall off and are killed. Red spider is rather 

 general and is probably the most destructive agent on Opuntia lindheimeri 

 in the Brownsville district. In Tamaulipas work of the mites has been 

 observed everywhere on the same host plant. 



The species occurs over a wide area on the Mexican plateau and is 

 often prevalent around San Luis Potosi and Aguascalientes, the re- 

 corded host plants being the shrub pear 0. cantabrigiensis, the semitree 

 0. robusta and 0. durangensis, and the Cylindropuntias 0. leptocaulis 

 and 0. imbricata. The typical corky areas caused by these mites have 

 been observed on 0. tomentosa at Cuernavaca, State of Morelos. 



In Haiti a few segments of 0. dillenii were injured by red spider. 



In Australia, 7". opuntiae freely attacks 0. inermis and 0. stricta but 

 prefers the former plant. 



Both in Australia and in America this species increases rapidly during 

 dry periods. Thunderstorms and other heavy rains reduce or even 

 stop its activity. 



The feeding of the mites gives rise to circles of whitish cork, usually 

 around the areoles, and when feeding is severe these areas extend and 

 coalesce. When the affected segments are completely or wholly covered 

 by the corky growth, they fall from the plant and shrivel up. 



