THE CONTROL OF INJURIOUS INSECTS 383 



3. Sulphur, 40 per cent.; arsenate of lead, 10 percent.; hydrated 

 lime, 50 per cent. 



In every case the sulphur was finely ground so that it could pass 

 through a 200-mesh screen. The results were very staisfactory, 

 comparing very favorably with those obtained from the liquid spray. 

 The cost of the materials in dust spraying, however, was high, but 

 the cost of labor was lower than in liquid spraying. The only objec- 

 tion, at present, to the complete substitution of the dust method for 

 spraying is that there is no known dust preparation that will kill scale 

 insects, or that is effective against aphis, pear psylla or red bugs. 



The dust method makes it possible to protect the orchard at 

 critical times, as a large orchard can be treated in about one-fifth of 

 the time required by liquid spraying (Fig. 249). 



POISONS FOR BITING INSECTS 



(Consult Farmers' Bull. 908, U. S. Dep. Ag.) 



The most important poisons used for the destruction of biting insects 

 are (a) Paris green; (b) arsenite of lime; (c) arsenate of lead; (d) arsen- 

 ate of lime; (e) hellebore; (/) sodium fluoride. When leaves or other 

 portions of plants, dusted or sprayed with suitable quantities of these 

 substances, are eaten by insects the latter are poisoned. As a rule, 

 better results are secured by spraying the plants than by dusting 

 them; sometimes, however, it is impracticable to spray and resort 

 must be had to dusting. 



(a) Paris Green.^ — Pure Paris green, 3Cu(As02)2-Cu(C2H302)2, 

 composed of 58.65 per cent, arsenious oxide, 31.29 per cent, copper 

 oxide, and 10.06 per cent, acetic acid, is soluble in ammonia but only 

 slightly soluble in water. In commercial forms there is usually a 

 small amount of free arsenious acid, so that it is necessary to add lime 

 to prevent injury from the burning of the foliage. The amount of 

 lime used should be at least equal to that of the Paris green, but when 

 it is added to Bordeaux mixture the excess of lime renders the addition 

 of lime unnecessary. One pound of Paris green is sufiicient for an acre 

 of potatoes. Paris green particles are heavy and sink readily in 



1 Paris Green first came into use as an insecticide about 1870 in the early effort 

 to control the Colorado potato beetle, and lead arsenate was first used in 

 Massachusetts about 1892 as a safe chemical against the gypsy and brown tail 

 caterpillars. 



