33(5 Report of the Department ok Entomology of the 



zinc arsenite with lime-sulphur and the combinations with glucose 

 lost their effectiveness. The precipitation had washed off the 

 evidence of spray from the foliage in all except the bordeaux treat- 

 ments, which was correlated with more extensive feeding by the 

 insects. This feeding was most abundant where glucose, lime- 

 sulphur and lead arsenate were used, while the minimum feeding 

 occurred with the bordeaux combinations and with the glue and 

 lead arsenate and the soap and zinc arsenite. 



FIELD FEEDING TESTS. 



Some tests were made in the field to compare the effectiveness 

 of zinc arsenite and arsenate of lead for the spiny elm caterpiller 

 (Vanessa antiopa) and the larvae of the willow beetle (Lina scripta). 



Results. — The shape and size of the trees upon which the beetle 

 larvae were feeding prevented actual counts of the insects, but as 

 near as could be determined the time required for effectiveness 

 was practically the same for the two poisons, there being a lapse 

 of four days before all the larvae died. Opposed to this the results 

 with the Vanessa larvae were quite striking. The spiny elm 

 caterpillars on the trees sprayed with zinc arsenite were all dead 

 within twenty-six hours, at which time the insects on the tree sprayed 

 with arsenate of lead showed no outward effects of the poison, 

 though all succumbed in a period of four days. The experiments 

 with the Vanessa larvae were repeated in the field and in the 

 laboratory with similar results. 



Two explanations are suggested to account for this difference in 

 the action of the poisons: First, that the zinc arsenite may be more 

 soluble than the lead arsenate in the digestive secretions of these 

 particular insects and that more time is required for the lead than 

 the zinc poison to exert its toxic properties in the body. Second, 

 that in drinking from the globules of spray as larvae were observed 

 to do, more of the zinc on account of its lower specific gravity 1 is 

 actually conveyed into the digestive tract of the insects. 



DISCUSSION OF RESULTS. 



EFFECTS OF THE POISONS ON THE INSECTS. 



In the experiments in which the treated twigs were fed to the 

 insects as soon as the spray on the foliage had thoroughly dried, 

 the effects of the two poisons were generally similar, although there 

 was a slight difference in favor of the combinations containing zinc 

 arsenite, which were quicker in action. The only marked difference 

 between the two arsenicals as regards speed of poisoning was noted 



1 Mont. Exp. Sta. Bui. No. 86. 



