AJV ECONOMIC ENTOMOLOGY. 



own localities. The resin washes will not act well where frequent 

 rains and much dew dissolve the varnish formed by them over 

 the scales. 



Caustic potash and caustic soda, dissolved in water, are very- 

 useful winter washes for fruit-trees. They can be used at the rate 

 of one pound in one or two gallons of water, and will destroy all 

 fungus growth and most of the common scale insects, leaving 

 the bark pure, fresh, and glossy for the following season. Inci- 

 dentally, all insects hiding under loose bark, or bark scales, are 

 also destroyed, as are many insect eggs. The application should 

 be confined to the trunk and larger branches, as there is danger 

 to green bark in so strong a wash, and no foliage or active plant- 

 tissue should ever be so treated. 



Water alone, either very hot or very cold, is sometimes a use- 

 ful insecticide. Boiling water is quickly fatal to most insects 

 and to vegetation as well. At 125° it is effective against the rose- 

 chafer, killing it instantly, and at 140° it is still safe on grape 

 foliage. At 160° it kills all vegetation on which I tried it. It 

 has been applied with some satisfaction against cabbage-worms 

 without injuring the plants, and perhaps we may say that water 

 at a temperature of 125° is fatal to most insects and harmless to 

 most plants. But while this seems a very convenient and cheap 

 material, there are really grave practical difficulties in the way of its 

 general use. Even boiling water applied in a spray becomes too 

 cold twelve inches from the nozzle to be of any effect ; and so 

 rapidly do the small particles lose heat, that at a distance of three 

 feet it is reduced to atmospheric temperature. Its usefulness is 

 limited, therefore, to cases where it can be applied at short range 

 by a sprinkler, — e.g , on cabbage-plants ; and here the arsenites 

 are cheaper and more certainly effective. 



Very^ cold or ice- water is effective against plant-lice under 

 some circumstances, and I have known of trees and bushes com- 

 pletely cleared by one appHcation. But our knowledge of the 

 effects of cold on insects and plants is yet too small to make 

 practical recommendations possible. 



Under some circumstances vapors can be made useful. Bi- 

 sulphide of carbon volatilizes readily at ordinary temperatures ; 

 the vapor is deadly to insects of all kinds, and is especially useful 

 against species infesting stored products, like grain and other 



