32 FIFTH REPORT OF THE ENTOMOLOGICAL COMMISSION 



will need to be modified to correspond with the varying conditions in 

 habits and life-history of any particular species, as found detailed in the 

 following pages of Dr. Packard's report: 



Effects of Arsenical Poisons on Insect and Plant. — Species of elms are somewhat 

 differently affected by the poison. When treated alike there is always manifest some 

 differeuee in the susceptibility of different elms to the corrosive effects of the poison. 

 Even individuals of the same 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 iioison 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, apparently less succulent. 



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

 poisoned, and shed most of their leaves in consequence in the last of Jkme, 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 larvas 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 arc not killed off thor- 

 oughly 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 indicates 

 that the poison is absorbed more or is more active when wet, and that it acts by de- 

 hydrating 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 following this there appears a more general effect, manifest 

 in that all the foliage appears to lose, to some extent, its freshness and vitality. 

 This secondary influence is jjrobably from poisouing of the sap in a moderate degree. 

 When this is once observable, no leaf-eater thrives upon the foliage. Slight over- 

 poisoning 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 Washington a preventive application of poison should be made before the 

 last of May or tirst 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 kinds of injury : first, that directly from the eating by the in- 

 sect; second, that which follows indirectly from the deleterious effects of the poison 

 on the plant, for its caustic effect is much greater where the leaves have been so 

 gnawed that the poison comes in contact with tlie sap. 



Treatment with London Purple. — Already early in June the insect appears plentiful. 

 On June 7, 1882, it was at work on all the trees, and its clusters of eggs were numer- 

 ous beneath the leaves. Some of the trees had half of the leaves considerably gnawed 

 and perforated by larva? of all sizes, and by the adults. At this date fifteen trees, 

 constituting the south part of the grove, were treated. 



Preparation of the Poison. — London purple (one-half pound), flour (3 quarts), and 

 water (barrel, 40 gallons) were mixed as follows: A large galvanized iron funnel of 



