10 Department Circular 367, U. S. Dept. of Agriculture 



occurred in the two tests on Texas Field Lake, which was the third 

 lake in the series, and consisted of a central open area with a fairly 

 wide overflow out into the woods. The most abundant breeding 

 was found in the overflow water, and the dusting observations were 

 confined to this part of the area. 



In nearly all of the lake-dusting experiments one or more places 

 in the lake failed to receive more than a very small quantity of 

 poison; and, on one occasion, the lake under treatment was almost 

 completely missed because the dust drifted away before settling. 

 When the results of the various experiments were analyzed, how- 

 ever, it became apparent that in a number of cases a large quantity 

 of the Paris green had settled upon the water surface without de- 

 stroying the larvae. The uniform coating of dust on the glass 

 plates indicated that the distribution had been thorough; and, on 

 one or two occasions, it was observed by the examiners that living 

 Anopheles larvae and green arsenic pellets floating on the water were 

 to be picked up in the same dip. This was contrary to the writers' 

 experience with hand distribution, and suggested some connection 

 with the method of application. 



For various reasons it was not possible to conduct a large series 

 of tests in 1924; but, from the few which were made, it was indi- 

 cated that the difficulty in 1923 had been due to some defect in the 

 Paris green itself. Two lots had been used that year, and the 

 trouble had arisen with the second one, although it has not as yet 

 been determined wherein this differed from other samples. A new 

 stock was obtained for 1921; and, although an excessive quantity 

 was put in the mixtures because of the previous experience, a num- 

 ber of stations gave a high larval mortality where only a small 

 quantity of the arsenical was found to have reached the water. 



Owing to the long drought in 1924 the woods became almost com- 

 pletely dry, and mosquito breeding was limited to the water re- 

 maining in the more open parts of the swamp lakes. The experi- 

 ments could not therefore be repeated in timber, as had been de- 

 sired; but, in the two lakes where applications were made (fig. 7), 

 the growth of water chinquapins was extremely dense; and the 

 water surface was a mat of floating and partially submerged vege- 

 tation, so that the conditions for the tests were sufficiently difficult. 



DUSTING A FLOODED RICE FIELD 



Another test of special interest was made in a rice field. The plat 

 selected for this was a T^-acre cut, with the rice about 20 inches 

 high, and the field flooded with from 4 to 6 inches of water. 

 Anopheles breeding was well distributed throughout. The dusting 

 plane was flown from Tallulah, a distance of about 25 miles, and 

 was landed in a near-by pasture to take on the dust load, which for 

 this test consisted of 5 pounds of Paris green in 100 pounds of 

 Tripoli earth. The actual time required for dusting the field was 

 a matter of only a few minutes. 



During the flight the wind was rather strong, and as a result one 

 edge of the field was missed on account of the failure of the plane to 

 go far enough upwind to offset the effect of the drift. Except for 

 this, the distribution was rather thorough and the quantity of dust 



