SHADE TREE INSECTS 3 



sawflies, and beetles have two pairs of wings. In the beetles the forewings are 

 hardened and are called elytra. These fold over the hind wings and cover most 

 of the abdomen. The larvae of many moths, butterflies, and sawflies bear on the 

 under side of the abdomen appendages which serve for support and are called 



prolegs. 



The Abundance of Insects. The common expression, "There are many more 

 insects now than there were in grandfather's day," is true. There have always 

 been insect outbreaks, however. Frequently insect pests native to a region become 

 abundant, cause considerable damage for perhaps a few years, and then become 

 scarce. These outbreaks, followed by periods of scarcity, are due largely to 

 climatic conditions but also to the natural enemies of the insect. When an insect 

 pest is abundant, the enemies which attack it also become abundant and finally 

 increase to such numbers that most of the host insects are killed off. When the 

 insect pest is almost wiped out, many of its enemies starve to death, allowing the 

 few remaining host insects to increase in numbers and again assume epidemic 

 proportions before its enemies can multiply sufificiently to bring it under control. 



To this local cycle, man through his travel and transportation activities has 

 accidentally introduced from other parts of the world many pests which were not 

 accompanied by their native enemies. When such an introduced species finds a 

 favorable climate in the new locality and no natural enemies to keep it under 

 control, it spreads unmolested and becomes a much worse pest in the new locality 

 than many of the native pests and often becomes more destructive than it was in 

 its old habitat. For this reason various governments collect preda.tory and 

 parasitic enemies of an insect pest in the original habitat of that species and 

 introduce them into the new locality with the hope that eventually these enemies 

 will bring the pest under control. 



Man has also brought about conditions favorable to more serious insect out- 

 breaks through his tendency to produce pure stands of certain species of trees, 

 that is, areas containing but one species of tree. Many of these stands are the 

 result of direct seeding or planting or are pure stands coming in naturally after 

 clear cutting or fires in the original forest. In many cases the original forests con- 

 tained mixtures of several species of trees so that many injurious insects which 

 fed on one or only a few species experienced some difificulty in finding their desired 

 food. With the increase in pure stands, these insects had much easier access to a 

 decidedly larger food supply where they could build up much larger populations 

 than before. The fact that insects are prolific breeders aggravated this condition. 



Because of the increase in damage caused by insects and the importance 

 of some of our shade and forest trees, it is only natural that artificial means 

 should be used to protect them from attack whenever insect pests become abun- 

 dant. Protection against leaf-feeding insects is very desirable since defolia- 

 tion of a tree weakens it and thus makes it more susceptible to attack by bark 

 and wood-boring insects as well as by organisms which do not usually attack 

 healthy trees but which will hasten the death of weakened trees. Leaf-feeding 

 insects alone may kill a thrifty, broad-leaved deciduous tree by completely 

 defoliating it for three years in succession. One complete defoliation maj' kill 

 a needle-bearing evergreen. 



Insecticides. The usual method of controlling leaf-chewing insects which feed 

 on the surface of the leaves is to apply a thorough coating of a poison to the leaves 

 so that the insects must either eat the poisoned leaves, go without food, or look 

 elsewhere for it. For those insects which feed on both sides of the leaves, de- 

 vouring the entire leaf surface, a poison application on either surface will sufilice. 

 If, however, the insect feeds on the tissues of only one surface (e. g., the lower, as 



