Agrilus difficilis, commonly referred to as the honeylocust borer or flatheaded locust borer, is a species of wood boring beetle that attacks honeylocust (Gleditisa triacanthos) trees. It is considered a secondary pest that develops in host trees with trunk wounds, canker growth, or under severe stress. Adults of A. difficilis are 7-13 mm (~1/4-1/2 inch) long with large prominent eyes, short antennae, a flat dorsal body shape, black bodies with metallic areas of green or purple, and yellow spots below the wings. The larvae are elongated, flat, and creamy white with an enlarged head region. One notable feature of A. difficilis larvae is that they have rounded end segments, unlike the larvae of certain closely related species such as emerald ash borer (Agrilus planipennis), who have bell-shaped rear abdominal segments.
- Agrilus difficilis is a species of wood boring beetle that attacks diseased or stressed honeylocust trees. Repeated infestations can lead to dieback of twigs and branches in the crown and a general decline in tree health.
- Symptoms of an infestation include large quantities of gum oozing from bark wounds and the presence of D-shaped exit holes in bark. Removing bark will expose frass-filled galleries on infested trees.
- Practices that promote tree vigor can be an effective preventative approach against A. difficilis. Removing dead limbs or trees is also recommended. Preventative insecticide sprays can protect susceptible trees against egg laying adults.
Lateral (top) and dorsal (bottom) views of A. difficilis. Note the black body and yellow spots on the abdomen underneath the wings, as well as the metallic green color visible on the sides of the abdomen. This specimen was captured in Boulder County, CO. Image credit: Hanna Royals, Screening Aids, USDA APHIS PPQ, Bugwood.org
Galleries with larvae of Agrilus spp. Image credit: James Solomon, USDA Forest Service, Bugwood.org
D-shaped exit hole produced by emergence of Agrilus spp. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org
Life history and habits
Agrilus difficilis overwinters as a larva under bark or in sapwood. Mature larvae construct pupation chambers in the xylem of a host tree. In Colorado, adults begin emerging in late-May and feed on foliage for a couple weeks before laying eggs singly or in clusters within bark crevices, and the eggs are covered with a frothy substance that gradually solidifies. Bark scars and recent wounds around canker edges and trunk crotches are particularly attractive egg laying sites. Large and small trees are susceptible to attack, in addition to branches with a diameter of 5 cm (2 inches) or greater. Newly hatched larvae begin excavating tunnels under the bark, leaving galleries packed with frass. Typically, galleries are mostly concentrated on larger branches and the trunk in areas affected by sun scalding, wounds, or cankers. One generation is produced each year, and the spread of A. difficilis likely occurs through the transport of infested nursery trees or firewood.
The first symptoms of injury often include wet areas with large quantities of sap oozing around bark holes. Exposure to air will cause the sap to harden into a mass of gum. Larvae excavate serpentine galleries underneath tree bark, which can restrict nutrient transport in the host tree and can cause progressive canopy thinning. Branches that are larger than about 1.5 cm (~½ inch) in diameter may dieback from feeding of larvae. After emerging, adults leave small D-shaped exit holes in bark. As adults feed on foliage, they leave notches on the leaf margins.
Inspect trees for bark wounds with oozing secretions. On infested trees, removing portions of bark will reveal galleries filled with frass. The presence of D-shaped exit holes in bark is an indication that the honeylocust tree has been attacked by A. difficilis.
Practices that maintain tree health are recommended to help prevent infestations of A. difficilis since this pest tends to target trees with trunk wounds, canker growths, or under severe growing stress. Proper trimming and storm drainage can help maintain tree health, and mechanical or environmental injury to trees should be avoided when possible. Dead limbs or trees should be removed promptly, and the bark should be removed from felled trees.
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