﻿104 BULLETIN 1369, V. S. DEPAETMENT OF AGRICULTURE 



of cattle by the application of insecticides. The difficulty of re- 

 moving the grubs from certain breeds, however, led them to give 

 very serious consideration to this method of treatment. As a re- 

 sult of rather extended experiments and practical demonstrations 

 of this method, it became evident that no ill effects were visible 

 in the host. It is true that certain materials may be used in the 

 destruction of the grubs which are detrimental to the host, as, for 

 instance, copper sulfate. In the writers' tests this material gave 

 decidedly bad local effects. Abscesses were developed in a con- 

 siderable percentage of the cysts treated. On the other hand, with 

 many of the most effective larvicides tried, pus formation in the 

 cysts was checked and the size of the lump caused by the grub was 

 rapidly reduced. 



With some treatments, notably those with carbon tetrachloride, 

 and with derris in various forms, the exit holes of the grubs were 

 observed to close rather rapidly and there was little tendency for 

 the host to expel the insect. In most instances the grubs were found 

 to disintegrate rather rapidly under such conditions, especially if 

 they were not in the late stages of development. At times some of 

 the grubs, especially in the late fifth stage, were found to rise in 

 the aperture in the skin and the posterior segments to harden, thus 

 retarding the closing of the hole in the skin. 



When the grubs are not further developed than the early part of 

 the fifth stage, they are promptly absorbed after being killed by 

 substances applied to the back of the host. After the skin and 

 spines have become thoroughly chitinized their elimination by the 

 host is more difficult. Some collapse and work out or are licked 

 out of the skin. Most of them are reduced in bulk by absorption 

 and the skin heals up smoothly over them. A considerable per- 

 centage of these mature grubs may remain in the skin for several 

 months. This shows the advisability of applying treatments while 

 the grubs are young; furthermore, at this time the holes in the 

 hides are much smaller and heal up more quickly. 



When ointments such as iodoform, derris, or pyrethrum and 

 petrolatum are used, the writers have found that a considerable 

 percentage of the grubs are expelled from the cysts after their 

 death. Some of them are found completely emerged from the skin 

 of the host two days after treatment and they continue to be thrown 

 off for about two weeks. Some project slightly from the skin and 

 may remain so for weeks if not mechanically removed. The per- 

 centage of grubs which are expelled after treatment varies widely, 

 apparently differing with the character of the skin of the host. 



In order to determine the effect on the host and the rapidity 

 of healing of the grub holes, some special observations were made 

 on herds (1) where grubs left the hosts normally, (2) where the 

 larvae were punctured and pulled out, (3) where they were ex- 

 tracted by hand, and (4) where various materials were applied 

 to the backs of the cattle. The rapidity of healing of the grub 

 holes was found to vary much in different animals, both when the 

 grubs emerged normally and when they were extracted or destroyed 

 in situ. It was found that the time required for healing of holes 

 where the grubs emerged normally ranged from about 13 to 76 

 days. The scars from the skin injury often persist for several 

 months. The presence of foreign material in the form of a plug in 



