No, 4.] SHADE-TREE INSECT PROBLEM. 97 



discussed are the ones most commonly troublesome to the 

 shade trees of this State ; and, with their treatment well un- 

 derstood, it will be easy to apply similar methods in the 

 cases of allied pests. 



It will be noticed that, of the insects in the previous list, 

 all but one are amenable to treatment by spraying, although 

 in the case of the brown-tail moth by far the cheapest method 

 is to destroy the winter webs by hand. This leads us to 

 consider the best insecticides for spraying, and the means 

 of applying them. There are certain simple tests by which 

 the merits of an insecticide for use against leaf-eating insects 

 may be determined : it must kill the insects ; it must not 

 injure the foliage ; it must adhere to the leaves for a reason- 

 able length of time ; and its price must not be prohibitive . 



Three compounds of arsenic fall within these specifica- 

 tions, viz., London purple, Paris green and arsenate of lead. 

 Of these, London purple, while effective and cheap, is open 

 to the objection of being a by-product, and therefore of ir- 

 regular chemical composition. Some samples contain a large 

 percentage of soluble arsenic, and therefore cause serious 

 injury to the leaves. It settles rapidly in the spraying 

 tank, and requires considerable stirring. It does not ad- 

 here well to the foliage, and where its application is followed 

 by repeated rains, a notable burning of the leaves usually 

 occurs. In the market the price of London purple ranges 

 from 15 to 20 cents per pound ; and, as but 1 or 1J- pounds 

 are used to 100 gallons of water, it is by far the cheapest 

 arsenical insecticide. While its use is not advised, should 

 it be adopted, it is well to add 2 or 3 pounds of fresh-slaked 

 lime for every pound of the insecticide. This will convert 

 the soluble arsenic into arsenite of lime, and thereby lessen 

 the burning effect. 



Paris green for years has been the mainstay in work 

 against leaf-eating insects. It is probably the quickest in 

 its action of the three insecticides mentioned. Where all 

 the insects it is desired to destroy have hatched and are feed- 

 ing, and good weather prevails, there is no better insecticide 

 for use than Paris green. But these conditions are almost 

 impossible of attainment in the damp, changeable climate 



