4i8 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



the recent government and experiment station bulletins 

 should be consulted. (See p. 419). 



Pyrethrum, Persian insect powder, or buhach, is a powder 

 made by pulverizing the flowers of pyrethrum. It gives off 

 a volatile oil which is poisonous to insects but does not affect the 

 higher animals. It is used principally as a household remedy, 

 where it can be dusted on or near the insects. 



Gases. As the citrus trees and some others have a very 

 dense foliage and retain their leaves the year round it is difficult 

 to spray them with the ordinary insecticides. When it is 

 necessary to treat them for insect pests they are usually fumi- 

 gated with hydrocyanic acid gas, which is generated under a tent 

 that is placed over the tree. Nine ounces of water are placed 

 in an earthenware vessel and three ounces of sulphuric acid 

 added; then three ounces of potassium cyanide are dropped 

 into the liquid and the tent quickly closed as the gas that is 

 immediately generated is deadly to all animal life. The 

 amount of gas necessary to kill all the insects on the tree varies 

 with the species of insects and the size of the tree. Careful 

 tables are given in government and state bulletins relating to 

 this subject. 



Nursery trees that are infested with scale-insects or other 

 pests may be placed in a tight bin or a fumigating house and 

 given a thorough treatment with this gas. It is also some- 

 times used in greenhouses for scale-insects, aphids, etc., and in 

 mills that have become infested with the Mediterranean flour 

 moth or other pests. This gas is sometimes used to kill 

 household pests such as bedbugs, cockroaches, etc., but as it is 

 very dangerous it should never be used except by some ex- 

 perienced person. 



Carbon bisulphide is used in killing insects in stored grain 

 and sometimes also in treating subterranean insects. It is 

 bought and used as a liquid which volatilizes into a heavy, ill- 

 smelling gas which is not quite as deadly as hydrocyanic gas 

 but is so strong that large doses prove fatal to all animal life. 

 It can be used in closets infested by clothes-moths by exposing 

 some of the liquid in a saucer on the floor, (better on a shelf as 

 the fumes are heavier than air and sink rather than rise) and 



