|Abstract or Summary
- The study (1) describes the life cycle of P. nebulosus,
(2) examines stimuli that may cause the beetles to locate brood
material, and (3) establishes the relationship between moisture
stress in Douglas-fir and colonization by P. nebuZosus.and other
(1) Development of P. nebulosus goes through the egg stage,
three larval instars, and the pupal and callow adult stages. Teneral
adults emerge from late spring through fall, disperse, and feed in
tissues of live Douglas-fir twigs before attaining sexual maturity.
Progeny initiated in early spring may become capable of reproduction
and initiate colonization of susceptible host material in fall.
P. nebulosus overwinters in all stages except the egg and pupal
stages, hibernating in feeding tunnels or galleries of newly colonized
breeding sites. The main breeding period is the early spring. Females
begin gallery construction.
(2) The flight of immature beetles is governed by temperature,
but is induced by light and appears primarily oriented toward light.
Positive photic response appears to overpower response to vegetative
Temperature induces a reversal in the beetle's photic response
at two thresholds (15.5°C and 34°C). Decrease of light intensity
to 17 f.c. induces a light negative and thigmotactic response. Two
temperatures (19°C and 34°C) limit the range for flight take-off,
which occurs only under conditions inducing positive phototropism
in the beetle.
Field tests failed to attract flying immature beetles to
materials known as highly attractive to mature beetles. In laboratory
tests, walking beetles responded to ethanol vapors in darkness
but were unresponsive in the light. Circumstantial evidence suggests
that the beetles alight into Douglas-fir crowns by chance and respond
to feeding stimuli under condition-favoring negative phototaxis and
The flight of mature P. nebulosus differs from the flight of
immature beetles primarily in orientation. Field tests showed
strong response by mature P. nebulosus to fresh Douglas-fir twigs
and stem sections as well as to certain monoterpene solutions and
Ethanol was also attractive to Pseudohylesinus grandis Sw.;
Dendroctonus pseudotsugae Hopk.; Hy1astes nigrinus (Mann); Gnathotriohus spp.; Trypodondron linoatum Oliver; and Anisandrus
pyri (Peck). This suggests the possibility that ethanol is a
constituent of primary host-attraction also to other scolytids
besides timber beetles as suggested by others.
The bimodal flight pattern of mature P. nebulosus and its
possible causes are discussed. It is possible that trees under
midday moisture deficits may become a temporary attraction source
and 'absorb' part of the airborne population.
(3) Diurnal and seasonal changes in moisture relations of
Douglas-fir were determined by Scholander's pressure chamber
technique of measuring plant moisture stress (PMS) in vascular
plants. The ecological interpretation of PMS, determined at dawn
and at midday, is discussed.
The relationship between PMS in Douglas-fir and colonization
by meristem insects was established by correlating the infestation
status of a tree with its PMS (diurnal minimum) condition. PMS
thresholds were found which indicate whether a host tree is in a
condition (a) inducive; (b) tolerant, or (c) abortive to initial
colonization by P. nebulosus, Scolytus unispinosus Lec.,
D. pseudotsugae (Scolytidae), and Melanophila drummondi Kirby
(Buprestidae). These PMS thresholds appear specific for each insect
species. The relation between the cardinal PMS condition in the
insect-host relationship and other physiological parameters of the
tree, such as photosynthetic activity, cell division, etc., are
discussed. Only trees under PMS levels indicative of physiological
condition with curtailed integrative processes become inducive to
attacks by meristem insects. Ecological, silvical and entomological
conclusions based on the obtained results may be important in forest
management considerations regarding reduction of damage caused by