910 I III V I rwnc.3i 



AND REFER TO I 



o M IF 



_ 



UNITED STATES DEPARTMENT OF AGRICULTURE C C"D I 

 FOREST SERVICE 



DISTRICT 5 Division of Forestry 



Umversity of California 



114 SANSOME STREET 



SAN FRANCISCO. CALIF. 



O 



Insect Control September 15, 1915, 



(To be attached to Manual of Insect Control, 

 issued November, 1914) 



Attention is called to the upper half of page 59 ir 

 Mr. Hopping' s "Manual of Insect Control". The statement in 

 regard to the Barkhouse Project on the Illarnath National For- 

 est was based on a somewhat premature examination during the 

 fall of 1913. A more' recent examination this spring has show: 

 that the infestation has greatly decreased and that the con- 

 trol work was evidentljr successful. 



Recent field end office conferences with members o: 

 the Bureau of Entomology have revealed a misunderstanding re- 

 garding the meaning and application of the so-called 75$, or 

 percentage, theory of insect control. It has been our idea 

 that' this theory involved the cutting of J*r/o ojf the infested 

 trees in each gr_ijvvp, o_j^ othejr fractional unit_, of the general 

 cojitrol a.rea upon which work was bein^ conducted. In reality- 

 it -involves the cutting of a percentage of infested trees, pj 

 the entire control re_a_, sufficient to destroy about 75% of 

 the infesting beetles. This is generally accomplished by 

 cutting heavily infested single trees and groups of trees, anci 

 leaving weakly infested individual trees that are isolated. 



In our control work in the past we seem to have ap- 

 proached the problem from the standpoint of the individual 

 tree, or 'group of trees, often representing a little nucleus 

 of infestation. The Bureau of Entomology, on the other hand 

 has approached it from the broader standpoint of the whole 

 "infestation unit". 



Apparently there is an interdependence and correla- 

 tion bet'.v.een the various parts of en infestation which make 

 this latter method of approach more logical and effective. 

 This inter-relationship between the various parts of an infes 

 tation is accounted for by the theory that the insects when 

 they emerge mingle in the air and fly for considerable distan- 

 ces before attacking new trees. Thus, reinf estation in the 

 vicinity- of old work may be partly attributable to beetles 

 that have emerged from distant trees and not entirely to thos 

 from neighboring trees. 



