Pityogenes chalcographus L.
The smaller European spruce bark beetle or six-spined spruce bark beetle is represented in the picture at left. P. chalcographus is found in Europe and northern Asia where it is a pest on Norway spruce, Picea abies. Males have 3 spines on the backend of each elytron (or 6 spines in total as these beetles have two elytra as do all
beetles). Females have very small spines compared to males and they also have a large depression in the middle of their face (frons).
Compared to a human head, this depression is about the size of a tennis ball. The
depression is called a "fossa" and may be used to carry fungal spores that are introduced into the host tree and germinate and
later grow and are fed upon by the beetle's larvae. The beetle also introduces fungi that help paralize the tree's ability to produce resin
that is used by the tree to repel the beetles when they are trying to bore into the tree (called attacks). The beetles must attack a standing tree in large numbers to insure that enough
fungi are introduced to kill the tree before it can produce resin and repel/kill the beetles. These species and many other bark beetles use aggregation pheromone
to attract more individuals of the same species to the tree for the purpose of killing the tree and for mating. In P. chalcographus, two chemicals
(chalcogran and methyl decadienoate) comprise the aggregation pheromone that is produced
by the male. The pheromone attracts both sexes. The attracted males want to join the attack and secure an area for his and
several female's young, while the females want to find a male-dug hole and room (called "nuptial chamber") beneath the bark where she can begin a tunnel in which to lay eggs along the sides.
The tunnels are excavated only in the thin phloem layer just under the thin bark of Norway spruce. The phloem layer is only about 2-4 mm thick in Norway spruce and is rich in sugars and nutrients since this is
the layer that transports photosynthate (sugars/amino acids made by photosynthesis) from the needles to the roots. In all species of Pityogenes
there are several females that join the single male in his nuptial chamber. Sometimes there can be as many as 10 females but usually not more as it becomes too
crowded and not such a good place for later arriving females as their larvae will be smaller and at a competitive disadvantage with the older and larger
larvae of previous females - therefore, later arriving females must look further for a male with less females. Since there is a 1 to 1 (male:female) sex ratio of larvae and emerging adults,
this means that many males never have young during their lifetime. This is also true of other polygynous
bark beetles such as those in the genus Ips, while many bark beetles in the family Scolytidae (but in the other subfamily) have a monogamous mating system in which
the female usually attacks the tree first and is joined later by a male (for example the genus Dendroctonus..
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The scanned picture above is taken from S.L. Wood (1982) who got it from Blackman (1915). It is probably not possible to see the
difference between P. chalcographus of Europe and two North American species, P. fossifrons, and P. hopkinsi using drawings.