An ordinary infestation, furthermore, is character- 

 ized by "points of concentration". A point of concentration 

 may be a single tree or group, or a small area much more 

 heavily infested than the immediately surrounding areas. The 

 insects are thus more or less congregated at centers of at- 

 traction. This fact, together with the inter-relationship 

 between different parts of the infestation, furnishes the 

 key to the percentage, or partial cutting, method of insect 

 control. Just as new centers of concentration attract in- 

 sects from other parts of the unit, so an effective attack 

 on certain points of concentration will influence, to a 

 greater or less degree, the rest of the infestation unit. 

 The first aim of control, therefore, is to attack the points 

 of concentration, striking the insects where they are most 

 numerous. Because of the relationship between different 

 parts of the infestation unit, the destruction of large 

 broods at concentrated points affects the rest of the infes- 

 tation; and when the reduction of the insects in number has 

 been carried to a certain point, the backbone of the infes- 

 tation is broken and the insect attacks become scattered. 



The first mistake of the forest Service was failure 

 to grasp the conception of the infestation unit, its view 

 being limited to the operations of a single year, which real- 

 ly formed but part of a whole unit; The removal of every 

 infested tree on the area laid out- for a year's work, which 



prises only one-half of the entire infestation unit, is 

 equivalent to removing only fifty per cent, of the infested 



ea on the //hols unit. I-Ienco even the removal of every in- 

 fested tree in the work of a single year might not be incom- 

 patible with the percentage theory, if only part of the en- 

 tire infestation unit is covered. 



The second mistake has been made by the Forest Ser- 

 vice in its conception of the application of the percentage 

 theory to the area covered by one season's operations. The 

 system does not necessarily require that literally only 

 three-fourths of the trees in a given group or patch forming 

 part of an infestation unit should be cut and the balr.nce 

 left. The purpose is to strike a blow at the worst points 

 of concentrated infestation, and to strike just as mnny of 

 these points as quickly ao possible. All the "good bug trees 

 would be treatedthat is, all infested trees favorable to tl" 

 breeding of large quantities of insects. Under ordinary cir- 

 cumstances this would not amount to more than about 75 -er 

 cent, of the area of infested bark; that is, about 75 per 

 cent, of the broods would be destroyed. It might well happer 

 that every tree in any given infested group would be cut in 

 striking hard a center of infestation. The trees left stand- 

 ing would be smaller individuals of such a character, from 

 the standpoint of infestation, that relatively little damage 



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