﻿ARSENICALS. 29 



less, this spraying was done more thoroughly than is possible in prac- 

 tical spraying;, (70 It was . often difficult to separate the dead in eel 

 from those apparently dead. This was accomplished in a fairly sati 

 factory manner by placing these insects almost against the globe pf an 

 electric light. If they exhibited no signs of lif'o after being subjected 

 for five minutes to the heat from this light they were considered 

 dead, (i) In these experiments it was impossible to feed definite 

 amounts of the arsenicals to individual insects. In a very limited 

 way it would be possible to feed insects singly, but it is almo-l impos- 

 sible to make them eat definite amounts of poisons. It would be 

 possible to feed definite amounts of arsenicals to individual bees. If 

 they remain isolated singly in cages, however, they live for only a 

 few hours, although when 50 or more are confined in one case they 

 freely feed one another and usually live for 9 or 10 days. 



Because of these uncontrollable factors, a large number of insects 

 were used for each individual experiment and the experiments were 

 repeated several times, if possible. The results thus obtained are 

 only comparative and are based on the average time required to kill 

 the insects tested rather than on the absolute single lethal doses 

 required to kill them. It was assumed that the insects ate equal 

 amounts of the poisons, although this may never have been true. 

 In the light of these probable errors it is easy to explain the delayed 

 deaths of many of the insects poisoned and the great variation in 

 their daily mortality. 



PRELIMINARY TESTS. 



During the summers of 1917 and 1918, many preliminary tests 

 were performed on silkworms, tent caterpillars, and fall webworms. 

 While no conclusive data were obtained, the following indications 

 may be given. 



The 14 commercial acid lead arsenates (samples 1, 2, 3, 4, 13, 14, 

 29, 38, 39, 40, 44, 47, 48, and 49) used showed no important differ- 

 ences in insecticidal properties. All proved efficient. The two basic 

 lead arsenates (samples 21 and 28) did not kill as quickly as did the 

 acid lead arsenates. Only two of the five commercial calcium 

 arsenates (samples 5, 6, 7, 24, and 34) tested proved efficient. The 

 insoluble calcium meta-arsenate (sample 45) prepared in the labor- 

 atory had no effect, while the laboratory sample of water-soluble 

 monocalcium arsenate (No. 46) killed quickly. The arsenious oxid 

 samples (Nos. 9, 19, 27, and 37), arsenic oxids (samples 10 and 16), 

 sodium arsenates (samples 25, 31, and 41), potassium arsenate 

 (sample 26), and zinc arsenites (samples 23, 30, and 33) were usually 

 efficient. One of these with a high percentage of water-soluble 

 arsenic, however, was not necessarily more toxic than another with 

 a lower percentage of water-soluble arsenic. The bases — lead oxid, 

 (samples 12 and 20), calcium oxid (sample 11), and zinc oxid (sample 

 22) — had little effect when used alone. From the inseetieidal 

 viewpoint, there seems to be no advantage in combining calcium 

 arsenate and lead arsenate (sample 8) . When lime was added to the 

 laboratory sample of calcium arsenate (No. 42) the toxicity seemed 

 to be decreased. 



