16 OUR SHADE TREES AND THEIR INSECT DEFOLIATORS. 



species or variety are differently impaired. As a rule, those which suit 

 the insect best are injured most by the poison, and those which resist 

 the insect most withstand the poison best. The latter have coarser 

 foliage with a darker green color and more vigorous general growth ; the 

 former have more delicate foliage, lighter in color and weight, appar- 

 ently less succulent. 



Certain elms of the species U. campestris and other species which were 

 overpoisoned, and shed most of their leaves in consequence, in the last 

 of June, 1883, sent out a profuse new growth of leaves and twigs. The 

 foliage fell gradually for three weeks, and this was somewhat promoted 

 by the succeeding rains. 



The larvse move from place to place so seldom that if the leaves are 

 imperfectly poisoned from the mixture being weakly diluted, or from its 

 application only in large, scattered drops, which are much avoided by 

 the larva?, they are not killed off thoroughly for several days, and in all 

 cases it requires considerable time to attain the full effect of the poison. 

 This result appears on the plant and on the insect. After each rain 

 the poison takes a new effect upon the plant and the pest, which indi- 

 cates that the poison is absorbed more or is more active when wet, and 

 that it acts by dehydrating thereafter. Where the tree is too strongly 

 poisoned, each rain causes a new lot of leaves to become discolored by 

 the poison or to fall. On some of the trees the discoloration appears in 

 brown, dead blotches on the foliage, chiefly about the gnawed places and 

 margins, while in other instances many of the leaves turn yellow, and 

 others fall without change of color. The latter may not all drop from 

 the effects of poison, but the coloration referred to is without doubt 

 generally from the caustic action. The poison not only produces the 

 local effects from contact action on the parts touched by it, but follow- 

 ing this there appears a more general effect, manifest in that all the fo- 

 liage appears to lose, to some extent, its freshness and vitality. This 

 secondary influence is probably from poisoning of the sap in a moderate 

 degree. TV hen this is once observable, no leaf-eater thrives upon the 

 foliage. Slight overpoisoning seems to have a tonic or invigorating 

 effect on the tree. 



Preventive Effects of the Poison. — In this grove the elms that were 

 poisoned in 1882 were attacked in the spring of 1883 less severely than 

 were those which were not poisoned the previous year. This would 

 seem to imply that the insects deposit mostly on the trees nearest to 

 where they develop, and are only partially migratory before ovipositing. 

 The attack afterward became increased, probably by immigration and 

 the new generation, so that later in the season the trees were mostly 

 infested to the usual extent. 



In the region of ^Yashington a preventive application of poison should 

 be made before the last of May or first of June, when the eggs are being 

 deposited and before they hatch. This will prevent the worms from 

 ever getting a start. By the preventive method the tree escapes two 



