759 



diluting the sprays, thoroughness of application, or apparently to any 

 combination of extrinsic factors. The prevalence of the scale at 

 Clarkston and its scarcity at Weenatchee, where effective spraying has 

 kept it in complete control, further bear out the supposition that there 

 is an inherent biological difference in the insects of the two places. 



Tartar (H. V.) & Wilson (H. F.). The Toxic Values of the Arsenates 

 of Lead. —J?. Econ. Entom., Concord, viii, no. 5, October 1915, 

 pp. 481-486. 



The investigations carried out for several years at the Oregon 

 Agricultural Experiment Station to determine the relative insecticidal 

 values of the different insecticides in common use, have shown that 

 there are two different arsenates of lead present in the commercial 

 material and that they are quite variable in their action and efficiency. 

 These compomids are lead hydrogen arsenate (acid) and basic lead 

 arsenate (neutral). The former contains approximately 33 per cent, 

 arsenic oxide, the latter 25 per cent. Malacosoma pluvialis, Dyar 

 (common tent caterpillar) proved a satisfactory subject for deter- 

 mining the comparative toxic value of these compounds. As soon as 

 the sprayed foliage became dry, the caterpillars were placed on the 

 twigs. Each morning the dead larvae were gathered, counted, and 

 placed in glass bottles. At the conclusion of the experiments they 

 W'Cre analysed for arsenic content. The data are tabulated in detail. 

 Lead hydrogen arsenate acts more quickly than the basic salt and 

 smaller amounts are required for efficiency. Although the caterpillars 

 sprayed with the basic form lived longer, they finally died, and where 

 not less than 2 lb. to 200 U.S. gals, was used, the damage done before 

 they died was not serious ; at certain strengths, the destnictive value 

 is lost because the caterpillars cause very serious damage before the 

 quantity of poison eaten is large enough to kill. The lead hydrogen 

 arsenate in strengths of 2 lb. to 50 gals, acted more quickly than the 

 basic salt, but the results obtained with the latter were satisfactory, 

 in that practically the same amount of foliage was eaten in both cases. 

 In strengths of 2 to 100, the difference in action was greatly in favour 

 of the lead hydrogen arsenate, but only a slight difference was noticed 

 in the quantity of foliage destroyed. In strengths of 2 to 200 similar 

 conditions were noticed, but with the basic lead arsenate the amount 

 of foliage destroyed was increased. In strengths of 2 to 400 and 2 to 

 800, both materials acted more slowly and a considerable part of the 

 foliage was eaten in the basic lead arsenate experiment. The lead 

 hydrogen still remained satisfactory. In strengths of 2 to 1,200 

 neither salt prevented serious damage, and with the basic salt, the first 

 twigs were completely defoliated and a second set partly destroyed. 

 Further comparisons showed that the lead hydrogen arsenate, at 

 2 to 200 and 2 to 400, was respectively more efficient than the basic at 

 2 to 100 and 2 to 200. The tables show that only a few caterpillars 

 died on the first day or two, the heaviest mortality occiu-ring within 

 two or three days. Some caterpillars ate only a small amount of 

 foliage or ate slowly, and although they became sick, they managed 

 to survive for a longer period in proportion to the amount eaten ; 

 others ate rapidly and devoured a considerable quantity of foliage 

 before the poison could act. To ascertain the amount of arsenic in 



