14 MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



away from the wood and sloughs off. Such damage often occurs 

 when dense stands are opened up by cutting or where young trees 

 are grown in exposed plantations. 



Excessive quantities of dust in the air, as along dirt roads, 

 causes a clogging of the stomata, or breathing pores, of leaves and 

 results in partial suffocation of trees. In the Western States such 

 injury is frequently followed by an attack of scale insects, which 

 add to the injury and sometimes kill many young trees. 



Trees are sometimes injured by smelter smoke and chemicals or 

 oils deposited on the ground. This injury leads to attack by many 

 species of insects. Flooding of trees usually kills them. 



Trees may suffer mechanical injury from logging operations, 

 lightning, road building, packing of soil or exposure of roots (as 

 in camp grounds), or from the work of animals such as bears, 

 beavers, and porcupines, and from sap-sucking birds. Such injury 

 usually has little effect on a forest as a whole, and trees show 

 remarkable power of recovery from limited mechanical injury, 

 if it is not followed by the entry of insects or fungi. 



Usually insect damage is readily apparent from the very start, 

 but it is well to make certain whether other conditions are partly 

 responsible before taking steps to control the insect pests. If they 

 are not the primary cause, little benefit can be expected from the 

 effort to control them. 



A forest officer should become familiar with the appearance and 

 characteristics of those insects capable of killing or injuring trees 

 and destroying wood products on the area under his care. The 

 insects he really needs to know are comparatively few, but ability 

 to recognize the injurious forms comes only after considerable 

 study, not only of the insect stages but also of their typical work, 

 whether it be markings on the bark and wood, tunneling of 

 needles, or deforming of terminals. In the following discussions 

 special emphasis is placed on the habits and typical work of the 

 most injurious forms; for it is through these that the forester 

 first becomes acquainted with the destructive species, and only 

 after considerable experience does he learn to recognize insect 

 adults and larvae dissociated from their work and from typical 

 host trees. 



Adult insects can be distinguished from other small invertebrate 

 animals by the fact that they have jointed bodies of three parts 

 (head, thorax, and abdomen) , breathe through tracheae, and have 

 one pair of antennae and three pairs of legs. 



The larva is the form most frequently encountered by the for- 

 ester ; but unfortunately it is difficult to distinguish insects in this 

 form by any simple characters. Usually it is sufficient for the 

 forester to be able to recognize the larvae as belonging to a certain 

 group. The forester easily acquires the ability to recognize some 

 of the more common forms through becoming familiar with their 

 work. 



The insects most important to forestry are included in seven 

 main groups, or orders, under the large class Hexapoda, or Insecta 

 U7). 1 These common groups (fig. 4) include the beetles (Coleop- 

 tera (99)), butterflies and moths (Lepidoptera), wasps (Hymen- 

 optera), fl ies (Diptera), scales and aphids (Homoptera), bugs 



1 Italic numbers in parentheses refer to Literature Cited, p. 246. 



