FARMERS BULLETIN 713. 



to maturity and deposit another brood of eggs, Gerlach has esti- 

 mated that the sixth generation from one pair of mites, which may 

 appear in 90 days, may consist of as many as a million and a half 

 individuals. Another 15 days may bring the nmnber up to 15,000,000 

 and 15 days more to 150,000,000. This calculation, though theo- 

 retical and only approximate, gives an idea of the rapidity with 

 Avhich these parasites multiply and shows that a few mites gaining 

 a foothold on one or two sheep may in a short time have descend- 

 ants enough to infest 

 heavily the enti re 

 flock. The i m p o r - 

 tance of prompt treat- 

 ment when scab is 

 discovered in a flock, 

 or if sheep have been 

 exposed to infection, 

 is evident. 



Some of the points 

 in the life historj'' of 

 the scab mite, as out- 

 lined aboAe, have a 

 certain bearing on the 

 question of the time 

 which should elapse 

 in the treatment of 

 sheep scab between 

 the first and second 

 dippings. It has been 

 found that a single 

 dipping usually fails 

 to free a flock from 

 scab, the apparent 

 reason being either 

 that all the females 

 in the course of de- 

 positing eggs at which time they appear especially tenacious of 

 life) or all the eggs already deposited have not been destroyed by 

 the dipping; consequently, after the eggs which may have escaped 

 the first dipping have had an opportunity to complete their incu- 

 bation, it is essential to dip the sheep again. It is also essential 

 that the second dipping take place before any mites which may 

 hatch out after the first dipping have had time to become ma- 

 ture and deposit more eggs. As the eggs on the body of the host 

 usually require 4 days' incubation, sometimes a week, and prol)ably 

 never more than 10 days, and as 10 to 12 days are necessary for 



FUJ. 2. — Sheep-scab mite (Psoroptfs oris). Female. Dorsal 

 view, greatly enlarged, (.\fter Salmon and Stiles, 1898.) 



