ECONOMIC ZOOLOGY ENTOMOLOGY. 355 



the cleauiug of fruit or reiuloriug it of a lower grade, and iu extreme cases 

 entirely Aalueless. 



The black scale, although a much larger insect than cither the red scale or 

 purple scale, appears to have generally little effect on the vitality of the tree. 

 Its commercial importance arises largely from its habit of secreting honeydew, 

 which spreads over the leaves, fruit, and branches, furnishing a growing 

 medium for a black or sooty-mold fungus, resulting in a black coating through- 

 out the tree. This coating is removed from the fruit by washing, or in light 

 attack^ by brushing, although the investigations of Powell et al. (E. S. R., 

 20, p. 43), have shown that the decay is greater in washed than in unwashed 

 fruit. To avoid the washing of fruit it is necessary to destroy the scale iii the 

 orchards. The more directly injurious effect to the tree resulting from the 

 attacks of the red, puri)le, and yellow scales appears to the author to be due to 

 the ability to produce some toxic effect in the host plant in addition to the 

 injury caused by the removal of sap. 



The equipment and methods of fumigation, including dosage and dosage 

 tables, methods of computing volume, measurements of trees, etc., are considered 

 at some length. An imported cyanid designated as 98 to 99 per cent pure is 

 reported as used almost exclusively for fumigation purposes in southern Cali- 

 fornia under the popular belief that it is superior to American cyanids for this 

 purpose. There appears, howevex", to be no real basis for this belief. In procur- 

 ing sulphuric acid for fumigation purposes only that should be purchased 

 which is entirely free of nitric acid and which is guarantied 6G° (Baume), or 93 

 per cent pure. The proportions of materials to be used are considered in detail. 

 The use of one part (fluid measure) of acid to each part of cyanid is recom- 

 mended. 



Water should always be used. Practice is said to have demonstrated that 

 with a greater proportion of water the injurious effect of the resulting gas on 

 the leaves and fruit is materially lessened. The lessening of the injury has been 

 attributed to the fact that the escaping gas was less heated when large propor- 

 tions of water were used. In order to determine the actual action an experi- 

 ment was conducted in which 1 part of cyanid, 1 part of acid, and from 1 to 8 

 parts of w'ater were used. It was found that the maximum temperature of the 

 escaping gas is always realized within the lirst minute, usually 30 to 40 

 seconds after the generation commences, and an examination of the maximum 

 temperatures of the gas in this experiment indicates that the temperature of the 

 gas is reduced when large proportions of water are used. When using from 1 

 to 4 parts of water the temperature is nearly uniform, but with .5 parts of water 

 the decrease becomes marked. The violence of the reaction and the temperature 

 of the gas are affected more or less by the size of tlie pieces of cyanid. A very 

 violent reaction results from the use of cyanid in powdered form. As the pro- 

 portion of water to sulphuric acid becomes larger the resulting temperature of 

 the mixture is lessened. 



Experiments were made to determine the temperature of gas resulting from 

 large and small doses, in wbich the proportions 1 part cyanid, 1 part acid, and 

 3 parts water were used. An examination of the results shows that the tempera- 

 ture of the escaping gas increases somewhat as the dosages become larger. The 

 highest temperature of the acid-water mixture occurs about i minute after the 

 mixing takes ])lace. The highest temperature of the hydrocyanic-acid gas 

 occurs about i minute after the generation conunences, and theu the temperature 

 of the gas rapidly decreases during 2 to 3i minutes, at the end of which time 

 most of the gas has b(>en evolved. At the expiration of 3 to 5 minutes the gen- 

 eration of gas has practically ceased. 

 5117— No. 4—00 



