NC State Extension Publications

Description and Biology

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The hickory spiral borer, Agrilus torquatus, is an elongate, shiny brownish copper beetle about 3/8 inch long (female) or a slender beetle with a reddish copper pronotum and black wings that reflect iridescent greens, blues, and violet colors (males). Males are smaller than females. Adults appear from spring throughout the summer. Females lay tiny flat, disk-like eggs under bark scales and crevices of the main trunk or branches or glued firmly to smooth bark of twigs by a transparent secretion. From the eggs hatch slender, segmented, legless grubs that eventually grow into 3/4 inch flatheaded wood borers. These grubs bore into the bark and feed in the outer sapwood. Hickory spiral borers develop for two growing seasons. In late autumn, grubs begin spiral burrows inside the stem, encircling the stem until reaching the center. They typically infest stems that are 1/2 to 13/4 inches in diament. Grubs then pupate in cells hollowed out in the pith. Infested stems usually break off at the spiral gallery. New adults emerge the third year and chew large irregular holes in the leaves.

Hickory spiral borer

Hickory spiral borers are dark, slender beetles.

Grubs of hickory spiral borers

Grubs of hickory spiral borers tunnel just under the bark.

Infested pecan and hickory stems

Infested pecan and hickory stems often break at the spiral excavation.

Host Plants

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Hickory spiral borers infest pecan and hickory. Repeated attacks on young trees may leave them stunted, misshapen, or crooked. Flatheaded borers are especially destructive to newly planted trees and trees weakened by drought, defoliation, or other adverse factors. Most borers do not attack perfectly healthy trees.

spiral tunnel

The spiral tunnel chewed by hickory spiral borers is diagnostic for this species.

Residential Recommendations

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Five species of small parasitic wasps attack hickory spiral borers. This may explain why it is usually not a common pest. Collecting and destroying the fallen stems destroys the grubs inside. Small trees can be pruned to remove dead branches and terminals as soon as leaves develop in spring. Be sure to cut off small dead twigs that have contain first winter larvae. To be sure of removing the tunneling grubs, infested twigs should be clipped 3 or 4 inches below the dead part. Once grubs are tunneling inside the stem, they are very difficult to control with insecticides.

Other Resources

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This Factsheet has not been peer reviewed.

Author

Professor Emeritus
Entomology & Plant Pathology

Publication date: March 23, 2020
Revised: March 23, 2020

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