384 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1941 



This same picture applies more or less to all bark beetle outbreaks 

 in all parts of the United States. 



It is obvious that any satisfactory explanation for the susceptibility 

 of trees to bark beetle attack must also explain all the observed facts 

 during the ups and downs of these outbreaks. It is the writer's 

 opinion that the condition is induced primarily by lack of moisture, 

 chiefly from drought but also from excessive competition within 

 the stand. It is probable that in the case of ponderosa pine, as with 

 other trees, the roots are first affected.^ Then, in turn, the foliage 

 thins out to compensate for a reduced root system. The topmost 

 branches often die. Certain insects that kill the twigs (such as scale 

 insects and the birdseye pine midge, Retinodiplosis sp., pi. 8, fig. 1) 

 may bring about additional defoliation. This all results in the manu- 

 facture of less food and consequently narrower rings, which in turn 

 may reduce the volume of conduction, all of which contributes to 

 lowered vigor to the point where many trees cannot recover. 



Thus it could be that under adverse conditions many trees die 

 from the effects of the physical environment without insect attack; 

 others are attacked by borers which in themselves are not very 

 important — at least no sound physiological explanation of the effects 

 of their attack has been offered — but are only symptoms of a deeper 

 malady. These "predisposing insects" of Graham (1939, p. 226), 

 Keen and Salman (1941), and West (1941), with their special senses 

 can recognize this moribund condition before we can, and act 

 accordingly. Such insects appear to be the principal factor only 

 because they are immediately associated with visual symptoms of 

 death. Others of these trees in slightly better condition do not 

 fade (die) until conduction is completely and quickly cut off by 

 blue-staining fungi introduced into the sapwood by bark beetles 

 (Nelson and Beal, 1929). This theory implies that many trees go 

 out of the stand from the effects of an adverse physical environment 

 and may serve as hosts to numbers of borers and other insects, 

 whereas other trees less seriously affected by drought serve as suitable 

 host material for breeding up large populations of bark beetles. It 

 is conceivable that the lowered water content and possibly higher 

 oxygen content of the wood associated with drought (Nelson, 1934; 

 Caird, 1935) are necessary for development of the blue stains, which, 

 in turn, condition the tree for optimum development of bark beetle 

 broods (Leach, Orr, and Christensen, 1934). 



From this it should by no means be inferred that these bark 

 beetles cannot by their attack kill healthy trees. The aggressiveness 



* Secrest, MacAloney, and Lorenz (1941) have shown that the foliage of drought-affected 

 hemlocks may remain green several years even when the tree has a dead root system 

 (apparently sustained by stored food in the trunk and moisture received through the 

 dead roots). 



