152 U.S. NATIONAL AIUSEUM BUIvLETIN 25 6 



Acarina: Eriophyidae 



ERIOPHYES Von Siebold, sp. 



Since 1931 this tiny acarid has infested joints of Opuntia inermis through- 

 out the prickly pear territory of Queensland and New South Wales. 

 The effect on the plant is very similar to that of T. opuntiae. A. M. 

 Massee of the Imperial Institute of Entomology has expressed his 

 opinion that the species is an undescribed form. Hence, its country of 

 origin and host relations other than that of 0. inermis, are unknown. 



SCAVENGERS 



Decaying cactus, whether prickly pear or other types, offers a very 

 suitable breeding ground for insects that live on rotting vegetation. 

 Hence, scavenging insects are well abundant, both in numbers and in 

 species, and include many syrphid, muscoid, stratiomyid, and other 

 Diptera, and histerid and staphylinid Coleoptera. Most of these 

 scavengers are associated with decaying vegetation generally, and can 

 in no way be regarded as true cactus insects. The rotting of the plants 

 from the attack of Melitara and other phycitid larvae, or the attack of 

 Moneilema, Cactophagus, and other coleopterous primary enemies is 

 followed by the advent of the various scavengers. 



However, many syrphid Diptera belonging to the genera Volucella 

 and Copestylum are mainly cactus insects, although they do breed in 

 other rotting vegetation. It is possible, also, that certain species in 

 these genera are restricted to prickly pear and other cacti. They are 

 very prevalent in both North and South America. 



The flies are attracted to cactus joints that are infested with lepi- 

 dopterous and coleopterous borers, have been injured by mechanical 

 means, or are attacked by bacterial and fungus diseases. 



The general scavenging insects infest cactus pulp that has reached 

 a considerable degree of decomposition. The syrphid group, however, 

 will attack the green joints, provided that insects, disease, or injuries 

 have set up a certain amount of decay, which attracts the adult flies. 



Thus, these insects are of considerable importance as cactus 

 destroyers, inasmuch as they extend the scope of the injury. Wounds 

 that might otherwise dry up and cause little material damage are 

 enlarged by the action of the syrphid larvae to such an extent as to 

 bring about complete rotting of the affected joints and, in extreme 

 cases, of whole plants. The most common species are: 

 Volucella pusilla, Florida, Texas. 

 Volucella fasciata, Florida, Texas. 

 Volucella esuriens, Texas, Florida, Arizona, Mexico. 



