REPORT ON THE DISEASES OF SILKWORMS IN INDIA 29 



less disease than the second generation. So that evidently the 

 climate at rearing time is not of so much importance as the history 

 of diseased generations behind them. ) 



Very interesting results were got in Cambodia by breeding from 

 diseased moths' progeny for several generations in succession 

 (Bull. Econ. $e VIndo-Chme, No. 108, 1914). It was found that 

 the disease gradually diminished, and that after six generations bred 

 always from diseased moths only l£ per cent, were diseased and these 

 only slightly. It is certain that layings of diseased moths can be 

 reared successfully if the rearing is good, but if they were subjected 

 to the treatment meted out by the average rearer to his worms they 

 would probably not be so successful. In Pusa I reared diseased 

 moths' progeny for three generations without the worms dying out. 

 Then I selected healthy layings from those that had three generations 

 of disease behind them, and I found that they gave quite good 

 results. Breeding from diseased stock for several generations does 

 not seem to lead to any real weakening or degeneracy. 



It is in this way, I believe, that epidemics run their course. 

 Something induces rearers to over-production, or in some way to 

 go in for bad rearing. Diseased moths are bred from and pebrine 

 becomes rife, resulting gradually in almost the extinction of the 

 worms in a district or country. Consequently production drops 

 and the industry almost dies out. But probably the best rearers, 

 who will be the most interested in the work, go on. There are 

 always a few healthy worms somewhere among the diseased ones as 

 we have seen. The. diseased ones will gradually die out, and by 

 careful rearing the few healthy ones will re-establish the race. 

 Thus we can understand why despite pebrine the race of silkworms 

 has persisted. 



The above results show how high a percentage of disease is to be 

 attributed to hereditary infection, and they also show what a 

 serious source of infection to healthy worms even one or two 

 layings of highly diseased worms might be. 



WHAT PERCENTAGE OF DISEASE IS^PICKEJJIPIN HXGHXYJOTECTED 

 SURROUNrfflSOS^Y JDATORPD^ SEED ? We 



have seen that the percentage of disease usually found in worms 

 reared from diseased seed in healthy surroundings is very high ; it is 

 next necessary to see what amount of infection worms reared from 

 disease- free seed pick up in highly infective surroundings. In the 

 first instance an extreme series was taken. A small hutcha 

 house was infected by sprinkling thickly over the floor, walls and 

 roof, dust made by crushing the bodies of highly pebrinised moths. 

 The resulting powder showed about 60 spores in the field of a mic- 

 roscope fitted with a Leitz ^objective and a No. 4 eye-piece. Worms 



c2 



