ENTOMOLOGY 363 



For these insects direct poisons, such as the arsenicals, which 

 may be safely applied to the leaves or other parts of the plant at- 

 tacked, and which will be swallowed by the insect with its food, fur- 

 nish the surest and simplest remedy, and should always be employed, 

 except where the parts treated are themselves to be shortly used for 

 the food of animals or of man. 



Injury from Sucking Insects, The sucking insects are those 

 which injure plants by the gradual extraction of the juices from the 

 bark, leaves, or fruit, and include the plant-bugs, aphides, scale in- 

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

 of biting jaws, sucking beaks or bristles, 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 insects the application of poisons, which pene- 

 trate 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 as a caustic, or will smother or stifle them by closing 

 their breathing pores, or will fill the air about them with poisonous 

 fumes. Of value also as repellents are various deterrent or obnoxious 

 substances. "Where it is not desirable to use poisons for biting insects 

 some of the means just enumerated may often be employed. 



Groups SuBject to Special Treatment. The two general groups 

 outlined above comprise the species which live and feed upon the 

 exterior of plants for some portion or all of their lives, and include 

 the great majority of the injurious species. Certain insects, however, 

 owing to peculiarities of habit, inaccessibility, or other causes, require 

 special methods of treatment. Of these, two groups properly come 

 within the scope of this bulletin : (1) Those working beneath the soil, 

 or subterranean insects, such as the white grubs, root maggots, root 

 aphides, etc.; and (2) insects affecting stored products, as various 

 grain and flour pests. 



The classification of insects outlined above, based on mode of 

 nourishment, and indicating groups amenable to similar remedial 

 treatment, simply stated, is as follows: 

 I. External feeders: 



(a) Biting insects. 



(b) Sucking insects. 

 II. Internal feeders. 



III. Subterranean insects. 



IV. Insects affecting stored products. 

 V. Household pests. 



VI. Animal parasites. 



THE ARSENICALS. 



Pan's Green. Paris green is a definite chemical compound of 

 white arsenic, copper oxide, and acetic acid, and is known as the 

 aceto-arsenite of copper. Properly compounded and washed, it should 

 be substantially uniform in composition and nearly free from uncoin- 

 bined soluble white arsenic. It is a rather coarse powder, or, more 

 properly speaking, crystal, and settles rapidly in water, which is its 



