As if you folks haven't had enough, you are all going to become experts on the
Spruce Beetle (Dendroctonus rufipennis) and whether a .6 degree C increase in temperature
makes the little buggers more dangerous. And no, I don't have anything better to do. ;>)
The spruce beetle ... is the most serious insect pest of ... spruce. It is restricted
largely to, mature and overmature spruce, and epidemics have occurred throughout recorded
history. One of the most damaging out breaks was in Colorado from 1939 to 1951, when
beetles killed nearly 6 billion board feet of standing spruce. Damaging attacks have
been largely associated with extensive windthrow [i.e. trees blown over by the wind -cd],
where downed trees have provided an ample food supply for a rapid buildup of beetle
populations. The beetle progeny then emerge to attack living trees, sometimes seriously
damaging the residual stand. Spruce beetles prefer downed material to standing trees, but
if downed material is not available, then standing trees may be attacked.
Source: Minneapolis Area Office - Bureau of Indian Affairs
http://199.128.173.21/spfo/pubs/silvics_manual/Volume_1/picea/engelmannii.htm
Outbreaks can develop any time in mature spruce stands throughout the Northern Region
following blow-down, winter damage, or improper treatment of logging residuals. The last
major epidemics in this Region occurred from 1967 to 1969 and from 1980 into 1984.
Source: SPRUCE BEETLE published by the USDA Forest Service
http://www.fs.fed.us/r1/foresthealth/bark/sbtxt.htm
The life cycle of the Spruce beetle is generally 2 years, in colder conditions it can be
as long as 3 years.
The length of the spruce beetle life cycle is highly dependent on the rate of heat
accumulation. In warm summers, spruce beetle can complete its life cycle in 1 year.
Source: the Bark Beetle Management Guidebook published by Gov't of British Columbia
http://www.for.gov.bc.ca/tasb/legsregs/fpc/fpcguide/beetle/table2.htm
In other words when it gets warm they hatch, reproduce, do their damage and die all in
one year instead of two. This probably doesn't prove anything because it seems that in
either case they do their damage in a single season, but perhaps if their life cycle is
crammed into one season they have less time to attack trees because they also have to do
all their other functions in a single season.
The hibernation habit of adults may have evolved in response to cold winter temperatures.
When beetles leave the upper bole and reenter the bark near the base, their survival is
enhanced because under normal conditions 6 feet or more of snow accumulates on the ground
in winter in the sprucefir forest and covers the bases of the trees. Below this snow
line, temperatures are near 32'F while above the snow line ambient air temperatures
exist. Thus, beetles in the bark below the snow line are not subjected to lethal
sub-freezing temperatures.
Source: SPRUCE BEETLE
So, in response to cold weather conditions, the beetles are hibernating in the tree near
ground level, where, because of the snow cover, fluctuations in air temperature will have
no effect.
Management Strategies:
Physical Methods: Solar heat involves exposing infested logging residuals or windthrow to
direct sunlight to kill inhabiting larvae. To maximize brood mortality, all host
material greater than eight inches dbh should be cut into 5-foot lengths. All branches
and debris shading the host material should be removed. The infested material should be
rotated at 2- week intervals during the summer to expose all surfaces. While using solar
heat is effective on some sites in the Rocky Mountains, it is not effective in Alaska,
because summer temperatures are not warm enough.
Source: USDA Forest Service
http://www.na.fs.fed.us/spfo/pubs/fidls/sprucebeetle/sprucebeetle.htm
Since the larvae are inside the tree bark, I suspect that it is not the ultraviolet rays
of the sun that kill them, but the increased temperature. So increased temperatures most
likely have an adverse impact on the Spruce beetle - except in Alaska where it doesn't
get warm enough.
Chuck Dolci
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