102 STREET AND HIGHWAY PLANTING. 



have reached a certain state, cause it to become a pupa. This, like the 

 egg, is a resting condition. The pupa of butterflies and moths is known 

 as a cocoon or a chrysalis. While the insect is in the pupal state 

 internal changes take place in the insect's structure which lead to its 

 development as an adult insect. 



Some insects do not have all of these stages in their development, but 

 reach the adult condition by shorter periods; and certain forms, com- 

 monly some of the aphids, are born alive from the body to the parent 

 insect. Since these things are so, it is easily seen that the study and 

 life history of the insect will show just what time the insect will be 

 most readily injured by the agencies of man. For it would be foolish 

 to spray leaves with stomach poisons with the hope of poisoning adult 

 moths, or killing the eggs they might have laid. The adult forms could 

 not be hurt by the poison, as they have no way of eating the leaves, 

 and the eggs would rarely be injured by the poisons, because if the 

 poisons were strong enough to penetrate the egg cases or dehydrate the 

 contents, the foliage would suffer. Of course strong contact poisons 

 are used to kill eggs and adults of certain classes, as the scale insects, 

 but these sprays are usually applied when there is no foliage on the 

 tree. Since, then, it would be difficult to reduce the number of adults 

 directly, it must be done indirectly. This is easily accomplished by 

 having repeated sprayings at the time the larva 1 are emerging from the 

 eggs. As the larvae feed upon the foliage, stomach poisons are effective 

 and reduce the number of adults directly. 



In other cases where the adults feed upon parts that may be readily 

 poisoned, as in the case of certain leaf -eating beetles, stomach poisons 

 may be used, but here the difficulty lies in the fact that the tissues of 

 the body of the adult are very much less easily affected by the poisons 

 than the tissues of the body in the larval state. 



For the individual it would be both tiresome and purposeless to 

 assume the task of finding out the life histories of the different insects. 

 Most of them, certainly all of the important ones, have been carefully 

 worked out by scientists over the country, and any citizen will find 

 abundant expert information and advice at his command by applying 

 to the state entomologist, or to the entomologists at the state experiment 

 stations. 



Because of the fact that the insects troubling shade trees, as well as 

 other trees, are so ably handled by bulletins of this and other states, 

 it is not intended to offer any discussion of the insects themselves, species 

 by species. A few hints as to the intelligent handling of insects may be 

 useful. 



Insects fall into three general classes, according to the kind of injury 

 they inflict. 



