Treatment for warbles.—During the winter and spring examine the cattle for the presence of warbles. By passing the hand over the backs of the animals the swellings marking the location of the grubs may be readily found. Pressure properly applied to the swellings will cause the grubs to "pop out" if they have reached a late stage of development. They may be more easily removed by means of slender forceps inserted into the opening of the warbles, and a still more certain method of removing them, particularly if the lumps are still very small, is to cut into the swellings with a sharp knife or bistoury, after which they may be pressed out. Care should be taken to crush all grubs removed, so as to prevent the possibility of their further development and transformation into flies. In order that none may escape it is advisable to examine the cattle every two weeks during the late winter and spring, at each examination removing the grubs which have developed sufficiently to cause perceptible swellings.
Another method of treatment is to force grease or oil into the openings of the warbles, which kills the grubs. This method is less certain than that of removing the grubs, and has the further objection that the dead grubs remain beneath the skin.
Cattle may be treated during the summer with fly repellents ([p. 502]) to keep off the warble flies. The efficacy of repellents against these flies is probably, however, not very great.
In localities where the character of the cattle industry is such as to render practicable the systematic examination of cattle and the removal of the grubs—that is, where the herds are comparatively small and subject to the close supervision of the owners—it is possible, by the exercise of a little care and with very little effort on the part of the cattle owners, provided they work together, each doing his share by seeing to the removal of grubs from his own cattle, so that as few as possible survive to transform into flies, to reduce the number of grubs within one or two seasons almost, if not entirely, to the point of extinction.
Investigations not yet completed indicate that grub eradication may sometimes be accomplished by the use of arsenical dips, which are extensively used at the present time for destroying cattle ticks. (See [p. 497].) It is possible that the destructive action of arsenical dips upon warbles is more or less dependent upon the fact that arsenic is stored up in small quantities in and upon the skin of cattle that are repeatedly dipped in arsenical dips. The arsenical dip appears to act, not upon the well-developed grub beneath the skin, but upon the eggs or the newly hatched larvæ, probably the latter. Accordingly the dipping of cattle to destroy grubs should be carried out during the fly season and repeated treatments should be given every two or three weeks, as in dipping cattle to eradicate ticks.
LICE.[16]
Cattle in the United States are commonly infested with three species of lice, two of them sucking lice (Hæmatopinus eurysternus, the short-nosed cattle louse, and Linognathus vituli, the long-nosed cattle louse), commonly known as blue lice, and one biting louse (Trichodectes scalaris), commonly known as the red louse.
Fig. 7.—Short-nosed blue louse (Hæmatopinus eurysternus) of cattle. Enlarged. (From Bureau of Entomology.)