gallery pattern of Pityophthorus micrographus: example of a polygamous species (from Rell, Knížek & Galko (2020a).
Generally the development is as follows. The male gnaws a cavity in the bark. i.e. the tissue between the dead, corky outer layer and the sapwood (xylem) of a branch or stem. Using pheromones the male attracts females to this “nuptial chamber”; in monogamous species but one, in polygamous species two or more. Each fertilised females, starting from the nuptial chamber makes an brood tunnel, where at some distance from each other individual eggs are deposited in individual niches. The emerging larva make individual tunnels, ending in somewhat widened pupal chamber, where pupation takes place. The emerged beetjes work themselves out through the bark and cork. Although the entire process takes place in the bark (phloem), also the underlying wood (xylem) is somewhat damaged. When later the branch dies and the bark peels off the gallery pattern, often characteristic for the species, remains recognisable in the naked wood.
Contrary to the xylem, the phloem is highly nutritious. While wood-boring insect larvae generally take several years for their development, bark beetles often have several annual generations.
Bark beetles mostly occur in stressed or dying trees. Because their gallery systems interrupt the down-flowing sap stream, especially high numbers from a threat to the tree. More importantly, bark beetles often introduce pathogenic fungi into the plant (e.g. Christiansen, Kolařík ao).
The larvae of several species in the sapwood of recently downed conifers; they have a symbiotic relationship with specialised fungi. These so-called ambrosia beetles are not treated here (Trypodendron, etc.)
Christiansen (1991a), Kolařík, Kostovćík & Patoužová (2007a), Lieutier, Mendel & Faccoli (2016a).