Airplane Dusting in Control of Malaria Mosquitoes 7 



determine the concentration of Paris green which had reached the 

 water, and on the morning following the dusting a corroborative ex- 

 amination for larvae was made. The stations were usually located 

 in the parts of the area most difficult for treating — that is, at the ends 

 and in the brush at the margins. 



Of the six applications given this lake the highest average mor- 

 tality for one set of 10 stations was 93.5 per cent. A set of sta- 

 tions in another portion of the lake, treated in the same operation, 

 showed a mortality of 87.5 per cent. In one other trial 93 per cent 

 was killed, but the remaining applications were variable in results 

 and in one instance a complete failure. 



The treatment of the open part of the lake was largely a matter of 

 skillful flying and estimation of the wind drift. (Fig. 4.) In the 

 operation the planes were brought down in a dive to below the line 



Fig. 4. — Dusting plane making a flight over the center of Ex Via Lake, flying below 

 the tops of the trees on each side 



of surrounding trees, then leveled out across the lake, and " zoomed " 

 over the timber at the other end, the upward direction of the plane in 

 " zooming " serving to blow the dust with great force down into the 

 pockets and thick brush at the ends. 



At the time at which the flights were made, usually between 8 

 and 11 a. m., there was always more or less of a breeze, and the 

 effect on the drift of the dust cloud had to be rather carefully esti- 

 mated to make the applications effective. This was especially im- 

 portant in treating the wooded areas, since the dust, when released 

 at the height of the tree tops, is easity carried away by the wind. 

 Dusting was usually discontinued when the wind velocity exceeded 

 from 10 to 15 miles per hour. On the other hand, when the dust 

 is released at the right place, a slight wind drift is of advantage in 

 distributing it uniformly, and especially in carrying it into the 

 woods at the margins. (Figs. 5 and 6.) 



