10 BULLETIN" 609, U. S. DEPARTMENT OF AGRICULTURE. 



second application was made. The results of these treatments are re- 

 corded in this report. 



SPRAYING EXPERIMENTS. 



On October 19 some sweet potatoes at Brownsville were sprayed for 

 the sweet-potato leaf-folder with arsenate of lead at the rate of 1 

 pound (powder) to 50 gallons of water. At the time the spray was 

 applied the infestation was localized in spots, mainly on one side of 

 the plat. On the following night a shower washed a considerable 

 amount of the poison from the vines. An examination October 22 

 showed that somewhat more than one-half of the larvae were still 

 feeding actively and did not appear to be poisoned. By October 25 

 the living larvae had decreased to about 25 per cent of the original 

 number, though it appeared that birds were partially responsible for 

 the decrease. 



Afterwards few larvae were found up to the first few days of No- 

 vember, when they reappeared in large numbers and dozens could be 

 collected in a very small space. 



November 9 the sweet-potato patch was divided into three plats 

 and sprayed with lead arsenate and zinc arsenite at different 

 strengths. On account of the matting of the vines it was impossible 

 to ascertain the number of larvae by count on any one plant, so that 

 the number was estimated over a given space both before and after 

 spraying. 



Plat No. 1 was sprayed with lead arsenate: 2 pounds (powder) to 

 50 gallons of water. The powder was first converted to paste and 

 the remainder of the water was added. The spray was applied early 

 in the afternoon, when the foliage was perfectly dry and a fairly 

 uniform coating was secured. Particular care was taken to coat at 

 least one side of the foliage thoroughly. 



Plat No. 2 was sprayed immediately with lead arsenate at the rate 

 of 1 pound (powder) to 50 gallons of water. This was applied in the 

 same manner as in plat No. 1, but the spray seemed to spread more 

 uniformly over the leaves than did the heavier dosage. 



Plat No. 3 was sprayed with zinc arsenite: 1 pound (powder) to 

 40 gallons of water in which 12 pounds of cactus, to increase adhe- 

 siveness, had been placed 20 hours before. The solid cactus detritus 

 was thrown out and the zinc arsenite added. This spray adhered 

 better to the foliage than did either of the other sprays, though 

 the whitening of the foliage was less definite than in plat No. 1, and 

 hardly as much so as in plat No. 2. 



On November 11 an examination showed that in plat No. 1 the 

 leaves of the potato were scorched slightly, although not enough to 

 cause serious damage. It was estimated that about 94 per cent of 

 the larvae had been destroyed or were past feeding. In plat No. 2 



