Hickory bark beetle (Scolytus quadrispinosus)

Description:

Hickory bark beetle (Scolytus quadrispinosus) is a small, native bark beetle of eastern North America that bores into the bark of hickory and pecan trees [1]. MISIN tracks it because, during droughts and in stressed trees, it can build to outbreak numbers and kill hickories, and it takes part in the "hickory decline" that has damaged hickory stands in the Midwest [1][2].

Identification:

The hickory bark beetle is tiny ? the adult is a stout, shiny black beetle only about 1/5 inch (5 mm) long, roughly the size of a grain of rice ? so you are far more likely to notice its damage than the beetle itself [1]. The clearest sign is the tree's crown: the leaves of a heavily attacked hickory turn yellow, then red within a few weeks, and finally brown as the tree dies [1]. Look closely at the bark of the trunk and larger branches for many small, round exit holes about 1/8 inch (3 mm) across ? like fine shot holes ? which are often the only sign in fall and winter [1]. Peeling back the bark reveals winding tunnels (galleries) carved between the bark and the wood [1]. Attacks are easiest to notice in mid- to late summer, when adults are active and stressed trees begin to fade [1].

At-a-glance cues:

1. Hickory or pecan leaves turning yellow, then red, then brown over a few weeks [1]
2. Many small, round exit holes (about 1/8 inch) scattered over the trunk and branch bark [1]
3. Winding tunnels between the bark and wood when the bark is peeled back [1]
4. A tiny, stout, shiny black beetle about 1/5 inch long [1]

Hosts:

The hickory bark beetle is native to eastern North America, from Quebec south to the Gulf and west to Texas [1]. Its hosts are hickory and pecan trees (the genus Carya) ? it prefers hickories such as shagbark and bitternut hickory and is considered the most damaging insect pest of this group of trees [1]. It attacks the trunk and larger branches [1]. It goes after trees that are already stressed ? by drought, injury, fire, storm damage, or disease ? and rarely attacks healthy, vigorous trees [1].

Life Cycle:

Adult beetles appear in early summer [1]. They first feed at the bases of leaf stalks and on young twigs, then bore through the bark of the trunk and larger branches [1]. Under the bark, each female tunnels an egg gallery and lays about 20 to 60 eggs along it [1]. The grubs that hatch tunnel outward, feeding between the bark and the wood, and when many are present their tunnels ring (girdle) the branch or trunk and kill it [1]. The beetles spend the winter as larvae under the bark and turn into adults the next spring, and there are about two generations each year [1]. People move the beetle to new places mainly by moving infested wood ? cut logs and firewood with beetles still under the bark [1].

Impact and Damage:

For homeowners, the hickory bark beetle is a threat to hickory and pecan trees, especially during dry years [1]. It rarely bothers a healthy, well-watered tree, but a tree weakened by drought or injury can be attacked and killed, and once the crown is dying there is usually no saving that tree [1]. The beetle is not a danger to people or pets [1].

In woodlands, outbreaks of the hickory bark beetle have long been blamed for widespread hickory die-offs during extended droughts [1]. In the Midwest, the beetle is also tied to "hickory decline," in which the beetle together with associated canker fungi damages and kills hickories across a stand [2]. Losing mature hickories reduces the nuts and habitat that wildlife depend on [1].

Similar species:

Hickory bark beetle is one of several small, similar-looking bark beetles in the genus Scolytus, and the surest clue is the tree under attack: hickory bark beetle bores into hickory and pecan, so a small bark beetle in an elm, oak, or ash is a different species [1].

Smaller European elm bark beetle (Scolytus multistriatus) is a closely related bark beetle of about the same size. It attacks elms rather than hickories and is best known for spreading Dutch elm disease, so if the affected tree is an elm and not a hickory, it is not the hickory bark beetle.

Monitoring and rapid response:

Signs to watch for:

1. Hickory or pecan crowns fading from yellow to red to brown, especially in a dry summer [1]
2. Many small round exit holes over the trunk and branch bark, most visible in fall and winter [1]
3. Winding tunnels between the bark and wood, and dying branches high in the tree [1]

What you can do:

1. Keep hickory and pecan trees healthy ? water them during droughts and avoid wounding the trunk and roots ? because vigorous trees are rarely attacked [1].
2. Do not move firewood or logs from an infested tree; beetles under the bark ride along and start new attacks elsewhere [1].
3. If a tree is infested, remove and destroy the wood before the next generation of beetles emerges in spring ? chip it, debark it, burn it, or bury it while the larvae are still under the bark [1].
4. For a woodlot, a crew can thin crowded, stressed stands to improve tree vigor and promptly remove and destroy dying hickories to lower beetle numbers across the site [1].

When to call a professional:

1. Large or high-value hickory and pecan trees, or trees with beetles high in the canopy, are best assessed by a certified arborist or forester [1].
2. Widespread hickory dieback across a woodlot may signal a drought-driven outbreak or hickory decline and warrants professional advice [1][2].
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. USDA Forest Service. "Hickory Bark Beetle (Scolytus quadrispinosus Say)" ? Forest Insect & Disease Leaflet. wiki.bugwood.org
2. Juzwik J, et al. "Fungi associated with stem cankers and coincidental scolytid beetles on declining hickory in the upper Midwest." USDA Forest Service Research. research.fs.usda.gov

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