276 CONTROLLING INSECTS AND DISEASES 



The calcium arsenate should be fresh and of a brand which 

 is stated by the manufacturer to be suitable for orchard spray- 

 ing. 



a : 3. Zinc arsenate is sometimes used as a substitute for 

 lead arsenate. It is somewhat safer than calcium arsenate as 

 long as 1 pound of lime is used for every pound of zinc 

 arsenate. 



a : 4. Less Common Stomach Poisons. At the present time 

 experimental work is in process with a group of new stomach 

 poison^ for orchard use. Hellebore, cuprous thiocyanate, 

 dinitro-o-cresol, and thiodiphenylamine (phenothiazine) are 

 some of the most promising. As yet none of these materials is 

 replacing the common arsenic compounds. 



It is certain that investigations will continue until ma- 

 terials both effective and economical, lacking the burning 

 properties of arsenic compounds and the undesirable accu- 

 mulation of poisons (spray residues) on the fruit, are devel- 

 oped. Progress in this direction should be watched carefully. 

 In the meantime follow the recommendations of your local 

 experiment station. 



Contact Insecticides 



a : 5, Lime-sulfur is a standard insecticide for sucking in- 

 sects which must be fought with a contact application. It is a 

 solution looking much like a red-brown syrup, obtained by 

 boiling together fixed amounts of burned or stone lime, sulfur, 

 and water. It is much used for control of San Jose scale. 

 It is one of the safest and best sprays to use in the dormant 

 season when the foliage is off, and has no bad cumulative 

 effects from years of use. It is not in common use as a 

 summer spray for insects, since on m^ost trees it would injure 

 the foliage, or even cause defoliation, at dilutions strong 

 enough to be effective. It is used extensively for the summer 

 control of fungus diseases. 



The manufacture of lime-sulfur, or concentrated lime- 



