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Bark beetles
Mountain pine beetle,
Dendroctonus ponderosae
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Arthropoda
Class: Insecta
Order: Coleoptera
Family: Curculionidae
Subfamily: Scolytinae
Latreille, 1804
Tribes

Bothrosternini
Cactopinini
Corthylini
Cryphalini
Crypturgini
Ctenophorini
Dryocoetini
Hylastini
Hylesinini
Hypoborini
Ipini
Phloeosinini
Phloeotribini
Pityophthorini
Polygraphini
Scolytini
Scolytoplatypodini
Xyleborini
Xyloterini

A bark beetle is one of about 6,000 species in 247 genera of beetles in the subfamily Scolytinae.[1] Previously, this was considered a distinct family (Scolytidae), but is now understood to be specialized clade of the "true weevil" family (Curculionidae).[2][3] 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.[4][5] However, the subfamily also has many species with other lifestyles, including some that tunnel into wood, twigs, fruit, seeds, or herbaceous plants.[1] 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.[2] The group is often referred to more broadly as bark and ambrosia beetles[6][7], or simply scolytines.

Description

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Adult Beetles

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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.[6] 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.[2][1][6] 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.[2][6]

Larvae and Pupae

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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).

Sex Differences

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Ecology

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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.[8][9] 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.[1] 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.[10]

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.[1]

Phloem Feeders (True Bark Beetles)

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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.[1] This has been termed phloeophagy, and it is apparently the basal behavior of the lineage.[5] Thus, phloeophagous scolytines are sometimes referred to as "true bark beetles."[11][12] 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.[1]

This guild includes many species that are important pests of conifer trees, particularly bark beetles in the genera Ips, Dendroctonus, and Scolytus.[8] 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.[8] 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").[8]

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.[1] 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).[13]

Fungus Farmers (Ambrosia Beetles)

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Wood Feeders

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Pith Feeders

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Fruit and Seed Feeders

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Herbaceous Plant Feeders

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Fungus Feeders

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    Bark beetle gallery engraving the sapwood
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    Bark beetle galleries with bark showing exit holes
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    Some species produce single winding tracks
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    Bark beetle galleries on a dead American elm
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    Bark beetle trap
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    Manually decorticated trunk of a spruce as protection to bark beetles

Other "Bark Beetle" Taxa

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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:


See also

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References

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  1. ^ a b c d e f g h Kirkendall, Lawrence; Biedermann, Peter H.W.; Jordal, Bjarte (2015). "Chapter 3: Evolution and Diversity of Bark and Ambrosia Beetles". In Vega, F.E.; Hofstetter, R.W. (eds.). Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press.
  2. ^ a b c d Hulcr, Jiri; Atkinson, Thomas H.; Cognato, Anthony I.; Jordal, Bjarte H.; McKenna, Duane D. (2015). "Chapter 2: Morphology, Taxonomy, and Phylogenetics of Bark Beetles". In Vega, F.E.; Hofstetter, R.W. (eds.). Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press.
  3. ^ Crowson, R.A. (1967). The natural classification of the families of Coleoptera (PDF). Middlesex: E.W. Classey LTD. p. 214. Retrieved 14 November 2019.
  4. ^ Sequiera, A.S.; Normark, B.B.; Farrell, B.D. (2000). "Evolutionary assemblage of the conifer fauna: Distinguishing ancient from recent associations in bark beetles" (PDF). Proc. R. Soc. Lond. B. 267: 2359-2366. Retrieved 21 November 2019.
  5. ^ a b Farrell, Brian D.; Sequiera, Andrea S.; O'Meara, Brian C.; Normark, Benjamin B.; Chung, Jeffrey H.; Jordal, Bjarte H. (2001). "The evolution of agriculture in beetles (Curculionidae: Scolytinae and Platypodinae)". Evolution. 55 (10): 2011-2027. Retrieved 21 November 2019.
  6. ^ a b c d Arnett, Jr., Ross H.; Thomas, Michael C.; Skelley, Paul E.; Frank, J. Howard (2002). American Beetles Volume 2: Polyphaga: Scarabaeoidea through Curculionoidea. Boca Raton, FL: CRC Press LLC. ISBN 0-8493-0954-9.
  7. ^ "Subfamily Scolytinae - Bark and Ambrosia Beetles". BugGuide. Iowa State University. Retrieved 14 November 2019.
  8. ^ a b c d Fettig, Christopher J.; Hilszczanski, Jacek (2015). "Chapter 14: Management Strategies for Bark Beetles in Conifer Forests" (PDF). In Vega, F.E.; Hofstetter, R.W. (eds.). Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press.
  9. ^ Katz, Cheryl. "Small Pests, Big Problems: The Global Spread of Bark Beetles". Yale Environment360. Yale School of Forestry & Environmental Studies. Retrieved 18 November 2019.
  10. ^ Raffa, K.F.; Lindgren, B.S. (2013). "Evolution of tree killing in bark beetles (Coleoptera: Curculionidae): trade-offs between the maddening crowds and a sticky situation" (PDF). Canadian Entomologist. 145: 471-495. Retrieved 18 November 2019.
  11. ^ Hulcr, Jiri; Mogia, Martin; Isua, Brus; Novotny, Vojtech (2007). "Host specificity of ambrosia and bark beetles (Col., Curculionidae: Scolytinae and Platypodinae) in a New Guinea Rainforest" (PDF). Ecological Entomology. 32: 762-772. Retrieved 21 November 2019.
  12. ^ Atkinson, Thomas H.; Peck, Stewart B. (1994). "Annotated checklist of the bark and ambrosia beetles (Coleoptera: Platypodidae and Scolytidae) of tropical southern Florida" (PDF). Florida Entomologist Online. 77 (3): 313-329. Retrieved 21 November 2019.
  13. ^ Harrington, T.C. (2005). "Chapter 11: Ecology and evolution of mycophagous bark beetles and their fungal partners" (PDF). In Vega, F.E.; Blackwell, M (eds.). Ecological and Evolutionary Advances in Insect-Fungal Associations. Oxford University Press.
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Category:Woodboring beetles Category:Insect pests of temperate forests Category:Insect vectors of plant pathogens

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