

OP THE ^ 



UNIVERSITY 1 



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FOREST PROTECTION 23 



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insects from their winter quarters, may be disastrous to large 

 numbers of certain species, particularly during the molting 

 periods of the larvae. 



V. INSECT PLAGUES. A succession of favorable springs, free from 

 late frosts and wet spells, is apt to result in an anomalous mul- 

 tiplication of a species. Hence, according to European re- 

 cords, insect plagues, like successions of favorable climatic 

 conditions, occur and recur after periodic intervals. The ef- 

 fects of parasitism however, are very likely to be confused with 

 climatic effects in these records, and too much dependance 

 should not be placed on them. These periodic plagues of in- 

 sects are very likely to occur in spite of all human ingenuity. 

 But experience teaches us that, in the great majority of cases, 

 nature may be trusted to restore the balance that has been 

 so disturbed. An abnormal increase in the numbers of a given 

 species not only is likely to reduce the natural food supply of 

 such a species so that many individuals will die of starvation, 

 but the parasitic and predaceous enemies of the species also 

 enormously increase in numbers, being encouraged to do so 

 by the abundance of the food on which they exist, and by the 

 ease with which it may be obtained. For the same reason, 

 bacterial and fungous diseases have a better opportunity to 

 spread from one individual to another. The years following 

 an insect plague are, therefore, very likely to be exceptionally 

 free from the particular species involved. Consequently, a 

 plague of this sort usually lasts for but one or two years, al- 

 though in exceptional cases it may last for three or four years. 

 In the forest, an insect plague, in which several species are 

 often involved, is likely to follow in the wake of a destructive 

 fire or storm, or of an attack by fungi. In any case where 

 such a plague has swept through the forest the dead trees should 

 be marketed immediately if the conditions are at all favorable. 

 Otherwise the resulting loss will be much more serious. 



The amount of damage done by a serious outbreak of insects 

 in a forest will depend very largely on the nature of the species 

 involved. If the species is "monophagous," that is, depen- 

 dent for its food supply only on a single species of tree, it is 

 likely to cause serious losses only in localities where pure stands 

 of the particular tree occur, or, at least, where the trees of 

 that species are not so scattered through the forest as to make 

 it difficult for the adult females of the injurious insect to find 

 a suitable place for oviposition. Polyphagous insects, on the 

 other hand, affect many host trees; and while they are likely 

 to distribute their injuries, so that their effect on the forest 

 is less noticeable, still the ultimate losses extending over a 

 period of years, may be very great. A species imported ac- 



