88 JOURNAL OF ECONOMIC ENTOMOLOGY [Vol. 3 



Conclusions 



1. It has been shown above that a cyanide containing a high per- 

 centage of cyanogen is not the only consideration in determining its 

 use for fumigation purposes. Of equal importance is that it be prac- 

 tically free from sodium chloride. A cyanide containing in excess of 

 one per cent of sodium chloride should be condemned. 



2. In field work, as well as in the laboratory, high grade sodium 

 cyanide has produced exactly as satisfactory results as high grade 

 potassium cyanide. In fact, the results from the field tests thus far 

 examined indicate that the former, under the crude conditions prev- 

 alent in orchard fumigation, is slightly superior to the latter. A 

 sodium cyanide of less than 123 per cent purity should never be used. 

 Preferably the purity should be from 126 to 130 per cent. Such a 

 high grade chemical seldom contains more than a trace of sodium 

 chloride. 



The price of the 126-130 per cent sodium product is almost the same 

 as that of the 98-99 per cent potassium salt. The former produces 

 at least one fourth more gas. After deducting the cost of the greater 

 amount of acid required to liberate the gas from the sodium cyanide 

 we still have an economy of between 10 and 20 per cent in favor of 

 the sodium salt. 



From these considerations, as well as the fact that the rapid exhaus- 

 tion of the deposits of potassium carbonate, which is the source of 

 potassium cyanide, has been causing the price of this cyanide gradu- 

 ally to increase, leads the writer to believe that in a few years the use 

 of sodium cyanide for fumigation purposes will become general. 



SUPERPARASITISM: AN IMPORTANT FACTOR IN THE 

 NATURAL CONTROL OF INSECTS^ 



By W. F. FiSKE 



In his study of the parasites of the white marked tussock moth, the 

 results of which were pulilished as Bulletin No. 5 of the Technical 

 Series, Bureau of Entomology, Dr. L. 0. Howard says of the oviposi- 

 tion of the common parasite, Pimpla inquisitor: 



"It happened on several occasions that the adult Pimpla was 

 observed to oviposit in tussock-moth caterpillars which were already 

 infested with Tachinid larvfB. Several such caterpillars were isolated 



lOccasional contributions from tlie Gypsy Moth Parasite Laboratory, II. 



