32 



and showed no injury from poisons, except in the ease of one or two 

 trees, and even here very slight. Two or three months afterwards, 

 however, or toward the end of July, many of the trees treated showed 

 a varying percentage, not large, but yet noticeable, of the leaves which 

 were turning yellow and falling to the ground, apparently going through 

 the same stages of ripening and falling which is characteristic of the 

 autumnal foliage. The effect of arsenic on the plant, therefore, seems 

 to hasten maturity of foliage, and perhaps also of fruit. That this is 

 prejudicial in some degree to the plant can hardly be questioned, and 

 must be weighed against the destruction of foliage or fruit by insects 

 to determine the advisability of poisoning. 



Relative value of poisons against insects. — A large series of feeding 

 tests were made with the arsenicals referred to above, half-grown fall 

 webworms being used in the experiments throughout. From 20 to 50 

 larvre were caged for each test and fed on leaves wetted with freshly 

 prepared poisons. In all a total of 1,057 larva? were used in the exper- 

 iments. Fresh food was poisoned and supplied to the larva? every day 

 or two during the time of the experiments. The series of experiments 

 were three times repeated in the case of Paris green, arsenite of copper, 

 and London purple, and only once or twice performed with the other 

 arsenicals. Against these larva? the poisons were used at the strength 

 of 1 pound to 160 gallons, 1 pound to 100 gallons, and, with the excep- 

 tion of London purple, 1 pound to 80 gallons. One pound of poison to 

 160 gallons was rather unsatisfactory, although of considerable destruc- 

 tive effect against the larva?, causing the death within nine days of an 

 average of 71 per cent of the larva? with Paris green and arsenite of 

 copper and 20 per cent with London purple. The rate of 1 pound to 

 100 gallons was eminently successful with all the poisons except London 

 purple, which seemed to be inferior to the others and to vary greatly 

 in the different samples. Very little difference was shown between the 

 arsenicals with and without lime, and as between ordinary Paris green, 

 pulverized Paris green, and arsenite of copper there is little to choose 

 in the matter of effectiveness. Arsenite of lead, which I believe has 

 not hitherto been used as an insecticide, gave an excellent record and 

 seemed to be, if anything, more rapid in its action than the other 

 arsenicals. 



In the tests at the rate of 1 pound to 80 gallons of water all the 

 larva? were killed within seven days, no essential variation being shown 

 between the four poisons used, viz, Paris green (ordinary and pulver- 

 ized) and arsenite of copper and arsenite of lead, both with and with- 

 out lime. 



The following table includes the later tests at the strength of 1 

 pound to 100 gallons of water, and is introduced to illustrate the details 

 of the result. Except with London purple, the larva? were all killed 

 within nine days, and for the most part had succumbed within six days. 

 The slower and inferior action of the London purple is clearly shown. 



