October, '15] 



TARTAR-WILSON: ARSENATES OF LEAD 



481 



Mr. F. C. Bishopp: Was there a record of the previous treatment 

 that had been appUed to the orchards mentioned? 



Mr. a. L. Melander: We have a record for ten years in the or- 

 chard at Charleston, where 5 per cent to 10 per cent survived treat- 

 ment. In some cases we got as good results with a weak as with a 

 heavy spray. In some cases from 20 per cent to 50 per cent of the 

 insects survived treatment. 



Vice-President Cooley: The next paper will be read by Mr. H. 

 F. Wilson. 



THE TOXIC VALUES OF THE ARSENATES OF LEAD 



By H. V. Tartar, Chemist, and H. F. Wilson, Entomologist, Oregon Agricultural 



College Experiment Station 



For several years investigations have been carried on at the Oregon 

 Agricultural Experiment Station to determine the relative insecticidal 

 values of the different insecticides in common use. The problems 

 taken up have involved both chemical and entomological work and 

 the^ data obtained show some very interesting results. Among other 

 things the work done has 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. We designate these compounds 

 as lead hydrogen arsenate (acid) and basic lead arsenate (neutral) and 

 experiments made, using both substances in a pure state, show that 

 the first is quicker acting than the second and apparently more efficient 

 in strengths containing equal amounts of arsenic. The data pre- 

 sented in this paper deal with experiments conducted to determine the 

 comparative toxic value of these compounds. 



Considerable difficulty was experienced in securing a satisfactory 

 agent for the determination of these toxic values. Very few insects 

 were found to occur in desirable numbers and all of them are present 

 only for a more or less limited period. Of these the common tent 

 caterpillar, Malacosoma pluvialis Stretch, was found to offer the 

 best means. This species occurs gregariously in colonies of a hundred 

 or more individuals and during the months of April, May and June 

 large numbers of the larvse are available. They also feed on all kinds 

 of foliage and eat greedily so that the poison is easily fed to them. 



The general plan of the work was to cover the leaves of the twigs 

 with the arsenates in different strengths; then, by the use of some 

 insect which would feed on the foliage, to determine the comparative 

 killing properties and finally to determine by chemical analysis the 

 exact quantity of arsenic consumed. 



