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 larve 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 larvee, 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. When 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 Hffects 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 Washington 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 
