Bark beetles in the broad sense include true bark beetles, which breed and feed in the phloem (phloemophagous species), and ambrosia beetles (some bark beetles all pinhole borers), which bore into the wood and feed mainly on ectosymbiont ambrosia fungi in the tunnels (xylemycetophagous species) (Vega & Hofstetter 2015). Some bark beetles develop in seeds and fruits (spermophagous species), in the pith and other thinner stems, or in the petioles of fallen leaves (myelophagous species). More broadly, bark beetles and pinhole borers are phytophagous and mycophagous. When discussing the feeding of these two groups, we use the name ‘bark beetles’ to cover all categories, whatever their specificities.

However, bark beetles and pinhole borers have a much more diverse diet throughout their range (Koch 1992, Mihajlović 2008, Vega & Hofstetter 2015). That’s how we distinguish:

• Phloemophages are species that feed in the phloem tissue (the inner part of the bark), although the larvae of some species do burrow into the outer tissues of the sapwood and can (but not often) carry fungi that increase the nutritional value of the substrate. They introduce fungal spores into the host at random. Their castings are reddish brown (terracotta colour). These are all species of the genera Ips, Dendroctonus, Tomicus, and some from the genera Scolytus and Pityophthorus.
• Xylomycetophages (ambrosia beetles) feed on “cultivated” ectosymbiont fungi that grow in wood, and the larvae of some species of this group of bark beetle also consume wood. Schedl’s (1958) definition says: “the larvae… feed on the mycelia of fungi which they grow on the walls of their tunnels”. The bark beetles have specially designed structures to transport fungal spores – mycangia. They push light brown, almost white castings out of their tunnels.
• Spermatophages feed on large hard seeds and the enveloping tissues of fruits, such as the developing seeds of cones, the supporting structures of legume seeds, maize seeds and macadamia seeds. Hypothenemus hampei feed on the developing seeds of coffee.
• Myelophages are species that feed on the pith of twigs, small branches or thin trunks of various tree species, including young vines. This is characteristic of some species of the genera Pityophthorus, Hypothenemus and Scolytus.
• Xylophages feed on xylem (sapwood, never heartwood) but do not “grow” fungi in their tunnels. These are species of the genera Phloeoborus, some of the genera Lymantor and Scolytus.
• Mycophages feed on fungi (which they do not “grow” in their tunnels), mainly on the sorus of ascomyota in dry twigs or bark (genus Lymantor).
• Herbiphages (herbivores) feed on clover roots (e.g., Hylastinus obscurus) stems of milkweeds from genus Euphorbia, the Xylocleptes bispinus feeds in the non-woody tissues of leather flower from genus Clematis. Some species are phyllophages, i.e., they feed on leaf petioles (Scolytus spp.). Some also feed on fresh or dry fleshy plant tissues, including herbaceous plant stems, leaf stalks, cacti, grass stems and mangrove viviparous shoots.

Thus, in the broad sense of the word, bark beetles and pinhole borers are phytophagous (phloemophagous, phloemomycetophagous, xylomycetophagous, spermatophagous, myelophagous, xylophagous, herbiophagous-herbivorous and phyllophagous). Occasionally, some species are mycophagous.

Bark beetles usually live in scattered habitats that are suitable for one generation of beetles to survive. This means that new generations of adults disperse to find new locations for their offspring. These two characteristics of bark beetles and ambrosia beetles means they have a large variety of spatial and temporal requirements. Most species develop on dead or dying tissues of woody and sometimes herbaceous plants and are not always economically important. Indeed, such species may become economically important if their tunnels are in wood used for furniture or veneer, or if they are vectors of pathogenic fungi and colonise healthy trees. They are typical forest dwellers, dendrobionts. Relatively few species attack healthy trees, seedlings, saplings or seeds of commercially important crops (e.g. developing coffee fruits, young cones, palms, macadamias). When that happens in warmer climates they can cause significant economic damage, in particular in monoculture plantations of such crops, when rapid population increases can occur following extreme weather events that damage or destroy plantations (Lieutier et al. 2004).

The local distribution of bark beetles depends mainly on temperature conditions (they are poikilothermic or ectothermic animals), the presence of suitable habitat for their development, and the amount of food present. Higher temperatures and a sudden increase in the trophic capacity of the environment (after windthrows, snow damage or droughts) can cause the population of bark beetles to increase so much that a gradation or calamity occurs within 2-3 years due to increased female fertility. This is important because a large population of bark beetles allows them to overcome host resistance and successfully attack and colonise even healthy, living trees.

Pinhole borers are species that are ecologically similar to some members of bark beetles in the genera Anisandrus, Xyleborus and Trypodendron. Morphologically, pinhole borers are quite distinct from the species of the subfamily Scolytinae. The larvae of the pinhole borers live in wood which contains various species of ectosymbiontic fungi that provide food for the larvae.

We classify bark beetles and pinhole borers as saproxyles, organisms that are attached to dead or dying wood, dying or dead trees, wood fungi or the presence of other saproxyles during certain stages of their development cycle. They participate in wood decomposition as primary saproxyles (during the colonisation phase when they colonise intact trees / wood) or as secondary saproxyles that participate in decomposition (joining primary saproxyles) and decompose decaying wood, followed by humification. Saproxyles facilitate and increase the rate of wood decomposition and metabolize large amounts of nutrients through multitrophic interactions (Jurc et al. 2010, 2010a). Tertiary saproxyls are species that inhabit dead, fallen and felled trees only after a certain period of time.

The Scolytinae differ greatly in their ecology and biochemical adaptations to host plants although they can occur simultaneously in the same habitat. Bark beetles that feed on the phloem (the relatively thin layer of bark beneath the dead outer part of the bark) are usually restricted to one or only a few host plants. Ambrosia beetles (and pinhole borers), which introduce symbiont fungi into the wood for larval feeding, usually inhabit a larger number of host plants. Most of the biological knowledge on phloemophagous and xylomycetophagous ambrosia beetles is based on studies of only a few genera such as Dendroctonus, Ips, Scolytus, Xyleborus, Trypodendron, Tomicus, Pityogenes, Pityophthorus, Hylastes and Gnathotrichus. Most of the species in these genera are obligate and facultative “tree-killing” bark beetles and represent only about 10% of the bark beetle species (data for the USA and Canada) (Byers 2004). In Slovenia, this share is around 4%. Researchers believe that the biology of bark beetles that are adapted to only one or a few hosts (and these are our economically important species on spruce, red pine or white fir) is the result of natural selection on the variability of plant biochemical substances in the tissues. It is believed that each plant species in coevolution with the bark beetles has developed chemical substances against the “tree-killing” bark beetles. They conclude that semiochemicals (signal chemical compounds or chemical substances that modify the behaviour of bark beetles) are composed of chemical substances from the host trees and the bark beetles. Host and non-host signal compounds may be attractants, repellents, toxic to the understorey or food for the understorey. Signalling substances (compounds) affect: (1) host tree detection and acceptance, (2) feeding stimulation and deterrence, (3) plant resistance, (4) pheromone / alomone biosynthesis and communication, (5) attraction of predators, parasites and competitors of bark beetles. Synthetic pheromones such as Pheroprax®, Chalcoprax®, Linoprax® and others are used in the monitoring of populations of some economically important bark beetles. Knowledge of the chemical ecology of bark beetles and tree-bark beetle relationships provides the basis for managing bark beetle populations and reducing damage to trees.