46 BULLETIN 1453, TJ. S. DEPARTMENT OF AGRICULTURE 



acid gas, using at least 2 ounces of sodium cyanide per 100 cubic 

 feet. 



Since hydrocyanic-acid gas is very poisonous to human beings 

 when it is inhaled, its use in occupied buildings or in locations where 

 the gas may leak through into adjoining occupied buildings should 

 not be attempted. Only careful persons thoroughly informed as to 

 the proper methods of procedure should undertake to generate this 

 gas. After fumigation, rooms must be thoroughly ventilated before 

 being entered. 



On account of the uncertainty which attends attempts to kill all 

 the maggots that are deep in the tissues of infested meats, it is sug- 

 gested that infested stocks be removed from storage spaces from 

 which it is desired to eradicate the insect. 



NATURAL ENEMIES 



Indefinite reference to a parasite of Piophila casei was made by 

 Swammerclam (73, p. 69), Taylor {75, p. 609), and Fletcher (&£, 

 p 9Ii). In the summer of 1921 the writer reared from pupa? of the 

 cheese skipper numbers of a pteromalid parasite (Hymenoptera), 

 identified by A. B. Gahan of the Bureau of Entomology as Pachy- 

 crepoideus clubius Ashmead. Records of the distribution of this 

 parasite include Illinois, the Philippine Islands, Hawaii (intro- 

 duced), Canada, and eastern Australia. It is an enemy of a num- 

 ber of Diptera, notably the house fly (Musca domestica L.). In 

 Hawaii, where it is also a parasite of the Mediterranean fruit fly 

 (Ceratitis capitata Wied.), it has been introduced from the Philip- 

 pines, propagated, and liberated as a parasite of the horn fly (Hae- 

 matohia irritans L.), and in 1910 it was reared in Canada from a 

 breeding jar containing the puparia of the cabbage root maggot 

 (Hylemyicb brassicae Bouche). In Australia it parasitizes various 

 sheep-maggot flies. The records do not indicate ability on the part 

 of P. dubius to make an important contribution to the control of 

 any of its hosts, and there is small probability that it will ever be of 

 much assistance in the control of the cheese skipper. 



In the laboratory in the summer of 1921 a minimum period of 16 

 days elapsed between the exposure of skipper puparia to oviposition 

 by P. dubius and the emergence of adult parasites. As a rule only 

 one parasite developed in each puparium, but three puparia pro- 

 duced two parasites each. Three females, which emerged September 

 9 and reproduced, and which were not fed, lived respectively 9, 9, 

 and 13 days. 



Several insect associates of P. casei prey upon it. Chief among 

 these is the clerid beetle Necrobia rufipes DeG. (the ham beetle), 

 which is predacious both as adult and larva. Young larvae of N. 

 ruflpes are unable to kill migrant skippers, but vigorous migrant 

 skipper maggots are readily killed by larger larvae of this beetle, and 

 in rearing experiments with the latter skippers have been found to be 

 the best food for larva? and adults. Table 19 shows the predacity 

 of this beetle, which is also cannibalistic. In the laboratory, colonies 

 of skippers to which these beetles had access were observed to be 

 rapidly depleted. Adult ham beetles also have eaten skipper eggs, 

 dead adults, and puparia. 



