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A bark beetle is one of about 6,000 species in 247 genera of beetles in the subfamily Scolytinae. Previously, this was considered a distinct family (Scolytidae), but is now understood to be specialized clade of the "true weevil" family (Curculionidae). The common name refers to the fact that most species feed and develop in the inner bark (phloem and cambium layers) of tree stems, and the group probably originated from ancestors that fed in the inner bark of conifers. However, the subfamily also has many species with other lifestyles, including some that tunnel into wood, twigs, fruit, seeds, or herbaceous plants. Thus, the term "bark beetle" may refer to the taxonomic group (any species in the subfamily Scolytinae) or more narrowly to those species in Scolytinae that feed in the inner bark. The group is often referred to more broadly as bark and ambrosia beetles, or simply scolytines.

Adult Beetles
Although they are placed in the weevil family, adult scolytines lack the pronounced rostrum (snout) that is characteristic of most weevils. They are mostly quite small, ranging from less than 1 mm to 9 mm long, with most species between 1 and 3 mm. Their adult form generally reflects a lifestyle of constructing and moving through galleries (tunnels) in woody plant tissues, with bodies that are roughly cylindrical in shape, relatively short legs and antennae that can fit tightly against the body, large heads containing powerful muscles attached to large chewing mandibles, and tibiae that are often flattened and armed with spines or teeth to aid in excavation. Their antennae are elbowed and clubbed. Their tarsal formula is truly 5-5-5, but appears to be 4-4-4 because the fourth segment is reduced and not visible in most species.

Larvae and Pupae
Bark beetle larvae are generally grub-like, legless, C-shaped, and whitish in color except for brown or amber head capsules. Pupae are also whitish, similar in size to adults, and exarate (with free appendages, not encased in a cocoon).

Ecology
The bark beetle species that attack and kill healthy trees include some of the most economically and ecologically important pests of forests and shade trees around the world. However, it is estimated that less than 1% of bark beetle species frequently kill healthy trees, and probably less than 10% of them may occasionally do so. Most bark beetle species infest the stems of woody plants that are already dead, dying, or severely stressed from some other cause; they are typically attracted by volatile odors (kairomones) emitted from compromised host trees, which allows them to avoid the chemical and physical defenses employed by healthy, vigorous trees.

Bark beetles are a diverse group, both taxonomically and ecologically, and have evolved to exploit a wide array of habitats and host types. One aspect that they hold in common is that their lifestyle involves living and feeding in tunnels (known as galleries) as both adults and larvae, although the substrates and food sources vary. While many species defy neat categorization or utilize a combination of strategies, scolytine feeding behaviors generally fall into the following guilds.

Phloem Feeders (True Bark Beetles)
The largest proportion of bark beetle species feed in the inner bark of trees, primarily feeding on the phloem, which is the most nitrogen-rich tissue in tree stems. This has been termed phloeophagy, and it is apparently the basal behavior of the lineage. Thus, phloeophagous scolytines are sometimes referred to as "true bark beetles." The actively-dividing cambium layer is often consumed, and some species will score the surface of the sapwood as they feed. Larvae of some phloeophagous bark beetle species may also tunnel into the sapwood or outer bark layers, typically late in larval development.

This guild includes many species that are important pests of conifer trees, particularly bark beetles in the genera Ips, Dendroctonus, and Scolytus. While many bark beetles primarily target trees target trees that are stressed, dying, or recently dead from other causes, some of these species species are known to attack and kill apparently healthy trees under certain circumstances, sometimes causing widespread mortality in large-scale regional outbreaks. Such outbreaks are often associated with major environmental events or disturbances, such as storm damage, severe and extended droughts, and temperature trends related to climate change. These conditions allow the bark beetles to exploit weakened and stressed trees, and increase their populations to the point where they can overwhelm the defenses of healthy trees through sheer numbers (a strategy known as "mass attack").

Many phloem-feeding bark beetles have a close symbiotic relationship with one or more species of fungi, which are carried (often in specialized structures known as mycangia) by the beetles and spread through the tissues of the host plant, thereby enhancing the nutritional value of the material to the beetle, particularly by making nitrogen-containing compounds more available. This strategy has been termed phloeomycetophagy. In some species (such as the southern pine beetle, Dendroctonus frontalis), the insect may feed primarily on the fungal growth on the walls of its galleries or chambers at some stage in its life, a behavior that may represent an evolutionary transition between phloeomycetophagy and the "fungus-farming" behavior of ambrosia beetles (see below).

Other "Bark Beetle" Taxa
Some other groups of insects are also referred to as "bark beetles," but their common names are generally modified with additional adjectives to distinguish them from Scolytinae. These include:
 * Family Boridae - Conifer bark beetles
 * Family Botherideridae - Dry bark beetles
 * Family Cerylonidae - Minute bark beetles
 * Subfamily Colydiinae of Family Zopheridae - Cylindrical bark beetles
 * Family Cucujidae - Flat bark beetles
 * Family Laemophloeidae - Lined bark beetles
 * Family Passandridae - Parasitic bark beetles
 * Family Rhysodidae - Wrinkled bark beetles
 * Family Salpingidae - Narrow-waisted bark beetles
 * Family Silvanidae - Silvanid flat bark beetles
 * Family Synchroidae - Synchroa bark beetles

External links and further reading

 * American and Mexican Bark and Ambrosia beetles
 * Nordhaus, Hannah. Bark Beetle Outbreaks in Western North America: Causes and Consequences. University of Utah Press: Salt Lake City, 2009. ISBN 978-0-87480-965-7

 Category:Woodboring beetles Category:Insect pests of temperate forests Category:Insect vectors of plant pathogens