Ips typographus
The spruce engraver is probably the most important pest of Norway spruce in Europe. The beetles are termed "aggressive" by forest entomologists because hundreds to thousands of beetles cooperate in an attack of one or a few adjacent spruce trees until they kill them. Both male and female bark beetles aggregate in response to a pheromone, consisting of methyl butenol and cis-verbenol that is produced by feeding males. It is necessary for the beetles to kill the tree because otherwise the tree produces resin that eventually repels or kills the beetles and their larvae. The adults overwinter in both the dead tree, just under the bark, and in the forest litter (also called duff). When they hibernate in the duff, they walk down the tree in autumn and out one or two meters from the base and then settle down about 10 cm below the surface of the dried needles. In the spring (usually from May 15 to June 15) in Värmland (middle) Sweden, beetles crawl upward and perch on twigs to get a better position for opening their wing covers. The folded wings are transparent, like a flies, but unlike a fly, the wings actually unfold and become more than twice as long as the hard wing covers (called elytra). Within a few milliseconds, the beetle attempts to fly away. Often several attempts must be made, where the beetle sputters down into the duff and must climb back again and prepare to fly. The temperature is critical, all beetles can fly only when the temperature is above 18 degrees C (64.5 degrees F).
duff temperatures
survival and take-off
image (C) 1995 by John A. Byers