THE RAVAGES OF LEAF-EATING CATERPILLARS. 245 



mixed. A similar experiment was conducted at the same time, substituting a 

 diseased silkworm instead of the cotton worm. In the latter experiment the 

 silkworms died in rapid succession after the fourth day and none reached 

 maturity. In the experiment where the cotton worm remains had heen used 

 as a means of infection only a few worms died on the fourth day, and the 

 incidence of mortality fluctuated considerably for a period of fifteen days or 

 more, but no larva reached the spinning stage. 



In the early part of May 1913, cotton worms were unprocurable and by the 

 middle of the month, at which period they are usually numerous, about 80 three- 

 quarter-grown larvae were obtained. These were fed upon leaves sprayed with 

 water containing the macerated body of one diseased silkworm, with the result 

 that on the third day the disease appeared and on the fourth day more than 

 50 per cent, had succumbed to it. Only about half a dozen survived to pupate 

 and of these none is likely to emerge. 



A demonstration in the field of the proper development from these experiments 

 has been hindered by a scanty appearance of the cotton worms themselves ; a 

 result which was anticipated from the severity of the disease in the autumn 

 brood in 1912. 



In order to avoid confusion it should be mentioned here that the official 

 method employed in Egypt of computing the virulence of an attack is, although 

 unintentionally so, not only unreliable but often misleading. At the commence- 

 ment of the season the discovery of two or three leaves bearing egg-masses in a 

 field of 100 acres is sufficient to cause a native official to report that the whole 

 area is infected. It is realised, however, that if more stringent accuracy in 

 estimation were insisted on, the tendency would be to go to the opposite 

 extreme. 



One word more in connexion with the application to other countries of the 

 remedy which we are employing in Egypt. Our investigations are as yet 

 insufficiently complete to make it possible to say what influence climatic 

 conditions may have upon the infective power of the disease, but there is no 

 doubt that the artificial dissemination of it by spraying is facilitated in rainless 

 Egypt to a greater extent than would be the case elsewhere. Nevertheless the 

 cheapness and simplicity of the treatment fully merits trials being made upon a 

 much wider scale, the results of Avhich would be of great interest to economic 

 agriculturists throughout the world. 



