228 The Spraying of Plants. 



and caterpillars are destroyed by means of poisons which are 

 eaten, though they yield to the other treatment equally well. 



The food of many insects, however, cannot be poisoned, since 

 they feed upon the juices of plants and do not eat the external 

 coverings. It is fortunate that most of these insects have soft 

 bodies, so that they yield readily to treatment if the poison 

 comes in contact with them. Their mouth parts are formed for 

 penetrating the external coverings of plants to a depth suffi- 

 cient to reach the sap ; just as the mosquitoes' bills are in the 

 habit of penetrating human kind. All aphides belong to this 

 class, as well as the true bugs, these having mouth parts which 

 are adapted to suck, but not to chew. The utter uselessness of 

 covering a plant with poisons to protect it from these pests 

 will readily be seen. No matter how thick the poison may be, 

 the insect's beak will penetrate this poisonous layer, and it 

 will take no food until the beak has passed the limit of the 

 poison and is deeply buried in the tissues of the plant. 



From the above it will be seen that : 



(a) To destroy chewing insects, such as the potato beetle, 

 poisons must be evenly distributed over those parts upon which 

 the insects feed, and this may in some cases be done even before 

 the insect is present, or is visible. Only those poisons which 

 cause death after being eaten should be used. 



(ft) To destroy sucking insects, such as plant lice, the ma- 

 terials must be distributed upon the insects as evenly as possi- 

 ble, and it is useless to make any application before the insect 

 has appeared. Only those poisons which kill by coming in con- 

 tact with the insect's body should be used. 



First determine what kind of a pest it is that needs treat- 

 ment, then select the proper material from among those men- 

 tioned in Chapter IV. 1 



II. UPON FUNGI. 



Among fungi we find many serious enemies. It is difficult to 

 tell just what a fungus is, but some of the principal character- 

 istics may be mentioned. A fungus is a plant ; but unlike 



1 The complete transformations which many insects undergo before maturing 

 are as follows : (a) the egg ; (b) the larva, grub, or caterpillar ; (c) the pupa or 

 chrysalis ; (d) the imago or matured adult insect. 



