TOXIC VALUES AND KILLING EFFICIENCY OF THE 



ARSENATES 



By A. L. LovETT, Entomologist, and R. H. Robinson, Assistant Chemist, 

 Oregon Agricultural Experiment Station 



INTRODUCTION 



Investigations covering a period of years have been under way at the 

 Oregon Agricultural Experiment Station to determine the killing effi- 

 ciency of various poison sprays for insects. More recently the problem 

 has resolved itself into a consideration of certain definite arsenical 

 sprays, their relative values as insecticides, and a determination of the 

 probable effective dilution for practical horticultural spraying. 



The present paper deals with the results obtained in a study of the 

 relative toxic value of pure samples of lead hydrogen arsenate (acid), 

 basic lead arsenate (neutral), and calcium arsenate. A preliminary 

 study of the relative toxicity of the arsenates of lead was made by 

 Tartar and Wilson,* of this Station. The present report on the lead 

 arsenates is the continuation of that work on an enlarged scale, which, 

 in addition to affording further verification of their results, gives material 

 data on the following points: (i) The comparative time required to kill 

 small caterpillars and nearly mature caterpillars; (2) the approximate 

 amount of lead hydrogen arsenate and basic lead arsenate required to 

 kill small caterpillars and nearly mature caterpillars; (3) the proportion 

 of these arsenates devoured by the small and mature caterpillars that 

 passes through the alimentary canal of the larvae. 



In addition to similar comparative tests of the calcium arsenate, it 

 was desired to determine the burning tendency on apple foliage {Malus 

 sylvestris) in the field. 



EXPERIMENTAL PROCEDURE 



Our common tent caterpillar (Malacosoma pluvialis Stretch) was used 

 throughout the whole series of experiments. The caterpillars were col- 

 lected in the field almost wholly from the wild rose {Rosa nukatana Presl.). 

 A few tents were obtained from the apple and some from Crataegus sp. 

 In collecting the caterpillars the limb to which the tent was secured 

 was severed and brought in, the caterpillars being disturbed as little as 

 possible. All the larvae used in one set or series of tests were collected 



> Tartar. H. V., and WasoN, H. F. the toxic values of the arsenates of lead. In Jour. Econ. 



Ent., V. 8, no. 5, p. 481-486. 1915. 



Journal of Agricultural Research, Vol. X, No. 4 



Washington, D. C. July 23, 1917 



jh Key No. Greg.— i 



(199) 



