22 



Insecticides. 



or fruit ; they include plant-lice or aphides, scale insects, 

 thrips, and plant-feeding mites. These insects possess, 

 instead of biting- jaws, sucking beaks, which are thrust down 

 through the outer layers of the bark or leaves into the soft, 

 succulent tissues beneath, and used to extract the plant 

 juices; with a resulting injury not so noticeable as in the 

 first group, but not less serious. 



For this class of injects the application of poisons, which 

 penetrate little, if at all, into the plant cells, is of trifling 

 value ; and it is necessary to use substances which will act 

 externally on the bodies of these insects, either as a caustic, 

 or to smother or stifle them by closing their breathing pores, 

 or to fill the air about them with poisonous fumes. Various 

 deterrent or obnoxious substances are also of value as 

 repellents. Whenever it is desirable not to use poisons for 

 biting insects, some of the means just enumerated will often 

 be available for these also. 



Besides these two large groups of insects, insecticides can 

 be used for those working beneath the soil, or subterranean 

 insects ; and for those attacking stored products, such as 

 various grain or flour pests. Other groups, including species 

 which require other methods of treatment, and. for which 

 insecticides are not usually of much avail, are : internal 

 feeders, such as wood, bark, andst9m borers, leaf miners, gall 

 insects, etc., household pests, and internal animal parasites. 



For combating the external biting insects the use of 

 arsenical compounds has in America, it is stated, practically 

 supplanted all other substances. The two arsenicals in most 

 common use are Paris green and London purple, while 

 Scheele's green and arsenate of lead are also used to 

 some extent. Hellebore (another poison) is sometimes used 

 in the form of powder, instead of arsenicals, when small 

 quantities only are required ; it is stated to be particularly 

 effective against the larvae of sawflies. In Great Britain it is 

 used as a wash. 



The most usual method of employing arsenicals is by 

 spraying the plants which require treatment with a mixture 

 of the poison with water. Paris green and London purple 

 are used in America at the rate of i lb. to 100 — 250 gallons of 



