Värmland, SWEDEN [Click here] for a full-size picture. Here I am conducting bark beetle research in the
Spring (May 20 to 15 June) in the province of Värmland, Sweden.
The tall trees in the background are Norway spruce (Picea abies) that can be attacked and killed by two species of bark beetle,
Ips typographus and
The black and white "things" on the car roof (a Swedish Volvo) are plastic pipe traps (not the telephone pole) used for catching bark beetles when they are attracted to
chemicals released from vials inside
the traps. The chemicals (usually two or more pure chemicals) must be released together to mimic the natural pheromone - which is
the odor one sex (in this case the males) use to attract females to their "burrow" in the tree. Other males flying around in the forest
are also attracted to the pheromone odor since they "want" to find a place nearby that is able to support young larvae. In these two species
only the males bore into the tree and wait for several (1 to 4 or more) females to join them in their chamber under the bark. After mating, the females tunnel away
from the central chamber and lay eggs along the sides of the tunnels (for several centimeters or inches). The eggs hatch in a few days and the young larval
worms feed in very small side-tunnels for several weeks. All tunneling and feeding is done in a very thin layer under the bark, called the
phloem layer (also including a very thin layer of cambium). this layer is only a few mm thick and is just a few mm under the surface.
Later in the summer the larvae go into the pupal stage (like inside a cocoon, which may last all winter). They metamorphosize to soft yellow adults (called callows).
After some more weeks of feeding they bore out of the bark and fly away. In middle and northern latitudes of Sweden the young adults leave the
tree in the early autumn but it is too cold to fly so they crawl down the tree to the ground and dig into the duff for a few centimeters or inches and spend the winter there. In the spring
they fly on a day above 19 degrees C (about 66 degrees F). For a few hours each day for several days they search for suitable trees, usually by orienting to the odor of pheromone from a tree
already under colonization by other beetles of the same species. We really don't know how much flying they do during the searching
period, but they can fly up to 45 kilometers (28 miles) on a flight mill (a small roter suspending a flying beetle). The beetle may find a tree under attack by other beetles
but if the tree is covered with attacks the beetle may decide there is too much
competition and no place to have a good home. This is kind of like how people find a place to put a blanket on the beach - we like to be sort of near others, but not too near,
and we don't usually like to be really far away from the crowds (so we can look). If it is really crowded we may go to another beach.
If the beetle begins to become exhausted from flying, it probably will try to attack a tree that so far has
no beetles trying to eat it. A tree attacked by beetles attempts to defend itself by making resin and pitch so that the beetles are both poisoned and
stuck up. However, the beetles have through evolution become rather well adapted to this defence, but not compeletly. It seems like there
is a continual "evolutionary war" between bark beetles and tree, much as there is one between humans and our diseases.
There are many parallels between bark beetles and other forms of life including humans, and that is why it is so interesting to study them (bark beetles I mean).