CHAPTER XVIII 



INSECTS 



Insects which prey upon the apple tree or its products are very 

 numerous. In 1894, before a Western horticultural society, a list 

 of 281 species was discussed, to which might have been added 

 at least 50 others, making a total of over 300. To make the 

 list complete there must, of course, be added such others as have 

 been reported during the twenty years since 1894. Although this 

 list covers a large territory and includes many species not known 

 to be seriously injurious, it is suggestive of one of the dangers 

 to the apple crops of the country. 



Besides the common harmful insects with which every orchardist 

 is familiar, there are others, which, although less generally destruc- 

 tive, of less general distribution, more given to seasonal fluctua- 

 tions, and usually accorded but slight attention, are nevertheless 

 causing considerable annual losses. These are sufficiently injurious 

 to warrant special attention outside the usual, general spraying. 



Many injurious insects fluctuate in numbers through more or less 

 definite cycles ; they gradually increase to a maximum of destruc- 

 tiveness, which may extend over two or three years, and then, 

 through the increase of parasites or through weather conditions 

 unfavorable to them, they rapidly decrease in numbers until they 

 cease to attract notice for a time, possibly for several years. Some 

 insects, like the codling moth, appear to be perpetually in evidence ; 

 but even with this insect there are marked seasonal differences in 

 numbers, which are sometimes attributed to the maximum devel- 

 opment of parasites or to climatic extremes, but which often cannot 

 with certainty be assigned to any definite cause. 



As destructive insects fluctuate between minimum and maxi- 

 mum destructiveness, so their parasites fluctuate between mini- 

 mum and maximum efficiency, maintaining a balance that prevents 

 perpetual ascendancy on the part of either host or parasite. An 



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