INJECT LOSSES TO SUSTAINED YIELD 409 



Hills in the late nineties and the early part of the twentieth century, 

 as this infestation in its severity and extent seems to have been a law 

 unto itself, and there is no record of any other one comparable to it. 

 It is interesting to note that for Dendroctonus Brevicomis infestation 

 in western yellow pine in California and Oregon it has been tentatively 

 demonstrated that the cycle of infestation is eight years. That is, given 

 an endemic infestation in a stand, the amount of infestation will in- 

 crease greatly for a period of four years, and then recede for a like 

 length of time. Observations on specific stands have not been conduct- 

 ed long enough to determine whether there is a regular progression of 

 these maximum and minimum infestations, or whether after having 

 completed a cycle of eight years the endemic or normal infestation may 

 continue for a variable number of years before progressing to a new 

 peak. Miller,^ as the result of observations on the Rouge River Area 

 in southern Oregon, says: "The annual loss may run as high as 3^ 

 per cent of the stand at the height of the epidemic, and does not exceed 

 one-half of 1 per cent of the stand during the low point." My obser- 

 vations in southern Colorado lead me to believe that our maximums 

 are higher than the above figures, and our minimums lower, or in 

 other words, that our losses when they do occur, are much more likely 

 to be concentrated within a period of a few years of great intensity. 



One other point in regard to the habits of Dendroctonus species of 

 great importance is the distance of flight of the swarming broods. In 

 spite of the fact that they have been long under observation, this is 

 still a mooted point, there being two widely divergent views held by 

 entomologists, the first that their flight is rather limited, and the other 

 that they may fly 20 miles or more. 



Speaking generally, much of the control work undertaken to date 

 has not been entirely successful. Areas treated for a year or two and 

 then abandoned have in the course of years become reinfested to a 

 degree equal to the original infestation that caused the institution of 

 control measures. Two reasons are offered for this — the first, that 

 the area covered was not sufficiently large, so that reinfestation took 

 place from the sides and surrounding country, and the other, that rein- 

 festation took place from small centers of infestation left within the 

 area, such centers consisting of individual or small groups of trees 

 which were left through the control operations not extending over a 



'J. M. Miller, Ashland Experiment Station, Bureau of Entomology, Ash- 

 land, Oregon. 



