June, '15] 



BISHOPP; FLIES WHICH CAUSE MYIASIS 



323 



The damage chargeable to this species is of many classes. In certain 

 sections of the southwest stockmen have found it unprofitable to keep 

 breeding cattle on account of the heavy loss due to this pest at calving 

 time. In these sections the stockmen depend mainly for replenishment 

 of their herds upon purchase of yearlings. In nearly all of the cattle 

 country of the southwest, the screw w^orm has had the effect of causing 

 stockmen to reduce the size of their herds, especially during the fly 

 season, in order that they may watch more closely the animals and 

 promptly care for infestations. Under these conditions the actual 

 death loss is not great although throughout the entire region the num- 

 ber of cattle actually killed by the screw worm must number well into 

 the hundreds. Where animals become heavily infested with screw 

 worms they have a tendency to penetrate thickets and are thus not 

 found by the stockmen, and death usually results very shortly due to 

 the extensive destruction of tissue by the larvae or septicemia as a 

 result of their presence. 



Another important source of loss is the expense incurred by watch- 

 ing the herds and treating infested animals. Most of the pastures in 

 the southwestern country are of great extent, many of them containing 

 several thousand acres, and the promptness of treatment demanded 

 necessitates the employment of a number of men to continually ride 

 the pastures and treat the infested animals. The summer of 1913 

 was a year of especially great screw worm abundance and a large per- 

 centage of the stockmen found it necessary to employ extra help in 

 order to prevent heavy death losses from infestations. During this 

 season the percentage of infested animals in a herd ranged from ten to 

 one hundred. In a number of instances from three to ten men were 

 kept continuously busy on this line of work from May to November. 

 To this we must add the expense of materials used in treating the 

 infested animals. Chloroform is the most generally used, and I am 

 informed by dealers that their sales of this material alone often reach 

 five hundred dollars during one season. Individual owners of cattle 

 sometimes spend several hundred dollars for chloroform, creosote 

 products, pine tar, and other screw worm remedies. 



Source of Infestation. — The screw worm, C. macellaria, has de- 

 veloped a very marked tendency to attack living animals although it 

 breeds in great numbers in carcasses. This frequently gives rise to 

 infestation of an animal on the least provocation. Years when the flies 

 are very numerous, practically every scratch becomes a site for screw 

 worm entrance. These maybe made by brush, barbed wire, or the horns 

 of other animals. On account of the frequency with which wounds are 

 attacked the stockmen usually do not mark or brand live stock after 

 May first or before November first . In this way they avoid much trouble. 



