|Abstract or Summary
- Factors considered were phloem moisture, sour-phloem (which
may be furthered by the presence of ample phloem moisture), oleoresin of the host, and the presence of unmated female beetles in
suitable host material.
Levels of phloem moisture present in variously prepared logs
in the field ranged from about 100 to upwards of 250 percent of dry
weight. Observation of such host material during the main season
of beetle flight supported the conclusion that the Douglas-fir beetle
attack volume was not related to moisture level. Experimentation
with short logs which had either been soaked or not soaked verified
this conclusion; at the same time it was demonstrated that Gnatho-trichus
sp., Dryocetes autographus (Ratzeburg), and Hylastes
nigrinus (Mannerheim) were attracted to the logs which had been
Within the range observed in the field, moisture level did not
influence brood success. The case where moisture implements an air-tight seal, its effectiveness being indicated by sour-phloem
development, is an exception.
Sour-phloem is a decay condition which develops under anerobic
circumstances. Moisture is viewed as an accessory to its formation
in that it implements an air-tight seal of intact bark. At the
time of attack only incipient sour-phloem was present in various of
the pieces of host material in this study and in this stage of development
did not influence attack by the Douglas-fir beetle. Sour-phloem,
however, was associated with restricted brood development, more
likely as an indicator of low oxygen levels rather than as a detrimental
habitat factor in itself.
Minimal oleoresin exudation pressures (1 to 17 pounds per
square inch) were measured in fallen Douglas-firs which, observed
through the attack season and later analyzed, were seen to have an
appreciable number of incidences where oleoresin was judged to
have interfered with beetle entry. Similarly, brood size and success
was determined to have been limited by oleoresin.
The introduction of unmated female beetles was seen to so
strongly prompt other females to attack a log that it not only affords
an explanation of how beetles are attracted en masse but also explains
why there can be beetle indifference to otherwise suitable
host material which happens to lack prior entered females. MaIe
beetles are also attracted by the unmated females. Beetle response
is to odor; air blown over a properly treated and hidden log attracted beetles.
The intensity of mass attack relates to the intensity of the
earliest attack. The species demonstrates a gregarious habit which,
it is argued, would facilitate mating, shatter the tree's defensive
oleoresin exudation, enable mass tunneling of the inner bark so
that adequate ventilation would be afforded the brood, and serve to
insure that it will place economic demand on the population of its
The strong displacement effect of logs having female beetles
warrants strong consideration when studies are made seeking to
identify attractive factors that belong to the host per se. When
forced entry of females was made into irregular host material, it
received subsequent attack.
Intensity of attack, as judged from the experiment of one
season, is independent of host felling dates when the range of these
is less than a year.
Air temperature observation in connection with the studies
provided insight with regard to the predicability of attack in suitable
host material as spring warming continued.