﻿90 BULLETIN' 1369, TJ, S. DEPAETMEiSTT OF AGElCtrLTtTEfi 



In 1923 two series of tests were made. In the first of these 192 

 eggs deposited April 10 on cow hair were divided into four lots, 

 each of which was kept in an incubator at about 90° F. The check 

 lot contained 99 eggs. These began hatching on April 13, and 

 45 per cent of them hatched. The other three lots were submerged 

 for one minute after they had been incubated for 1 hour, 25 hours, 

 and 49 hours, respectively. The respective percentages of larvae 

 which hatched in these three lots were 42, 45, and 57.5. All of 

 these lots began hatching on the same date as the check. 



The seconcl series contained 74 eggs deposited on April 11, 1923 ; 

 18 of these were submerged for three minutes in a 2 per cent solution 

 of a coal-tar creosote dip on April 13. These began hatching in 

 the incubator on April 14, and 44 per cent of the larvse emerged. 

 The other eggs were kept in an incubator except for the time re- 

 quired to submerge them for one minute in crank-case oil, drained 

 from an automobile. Of the 22 eggs dipped immediately after ovi- 

 position none hatched, of the 23 eggs dipped 24 hours after being 

 laid 26 per cent hatched, and of the 11 submerged 65 hours after 

 oviposition none hatched. 



ATTACK DIRECTED AGAINST HYPODERMA LARV^ IN THE BACKS OF CATTLE 



A number of facts point to the practicability of combating this 

 pest by destroying the larval stages while in the subcutaneous tis- 

 sues of the back. Control by this procedure is favored (1) by the 

 fact thac the species confine their attack almost entirely to cattle, 

 (2) by the fact that there is a rather definite and somewhat re- 

 stricted seasonal occurrence of the larvae in that situation, (3) by 

 the presence of an aperture through the hide immediately over the 

 grub during its entire existence in the subdermal tissue, and (4) 

 hj the fact that the larvae are limited in their distribution to the 

 back of the host between the withers and tail. In order to proceed 

 intelligently against the species in this situation it is necessary to 

 have very definite information regarding its seasonal occurrence in 

 the subdermal tissues in each section where control work is under- 

 taken, as also a knowledge of the developmental period in the sub- 

 cutaneous tissues, particularly as to the minimum time required. 



The possibilities of attacking the species in this situation appear to 

 have been recognized in a general way many years ago. Work along 

 this line, however, has been of a very superficial and sporadic na- 

 ture, with the exception, perhaps, of that carried on in certain dis- 

 tricts in Denmark (6), Hollancl, and Germany {67 and 77.) 



EFFECT OF MATERIALS ON LARV^ REMOVED FROM HOSTS 



In order to determine the toxicity of various insecticides and 

 other materials to the larvae of Hypoderma, a series of tests was 

 carried out in which larvae in different stages of development were 

 removed from the backs of cattle and submerged in these materials. 

 This method of testing the insecticidal value of various chemicals 

 is open to objection, owing to the fact that the conditions are ab- 

 normal. Another objection to the use of larvae extracted from hosts 

 is that practically none of them will produce adults even though 



