Agrilus biguttatus (Fabricius) - Two spotted oak borer

Coleoptera: Buprestidae

IDENTIFICATION

Adults are cylindrical, metallic beetles, 8 to 13 mm long, ranging in colour from golden-green, green, blue-green, blue to violet.^{42, 129} The elytra are slender and have two white spots near the posterior end of the elytral suture.^{42, 129} The frons is wide and flat, with a shallow depression.¹²⁹ The length of the pronotum is one and a half times the width.¹²⁹

HOST TREES

Quercus (main host), Fagus and Castanea.^{98, 130} Quercus rubra is a known host tree occurring within Canada.⁹⁸

LOCATION OF INFESTATION WITHIN THE TREE

Larvae feed on the inner bark and sapwood along the entire bole (including stumps) and larger diameter branches.^{120, 129, 130} Immature beetles maturation feed on host tree leaves.^{42, 98, 120}

HOST CONDITION

Standing trees (especially if weakened by defoliation, frost damage or water stress) or recently downed trees and stumps.^{98, 120, 129, 130} In it's native range, larvae will not develop in dried dead trees.¹²⁰ Healthy trees are also susceptible to attack.^{98, 120} Although beetles prefer larger (i.e. greater than 30 mm) and older (i.e. 50 to 120 years) trees, attacks have also been recorded on 20 year old hosts.^{98, 120}

DISTRIBUTION

Europe, Middle East, Siberia and northern Africa.^{42, 98, 120}

SIGNS AND SYMPTOMS

Females oviposit clusters of 5 to 6 eggs on the bark of host trees.^{42, 120, 129} Larvae bore through the bark and excavate frass filled, zig-zagging galleries which can reach lengths of 155 mm.^{42, 98, 120} Early instar larvae generally make their galleries along the wood grain while older instars produce galleries in irregular twisting transverse directions.^{42, 98, 120} These galleries are not as distinctly "S-shaped" as those created by the emerald ash borer, Agrilus planipennis. Dark cracks and discolouration of the bark and cambial tissue may occur over the galleries as the tree tries to defend itself from the invasion.^{98, 120}

Larvae overwinter in 10 to 14.8 mm by 3 to 4.5 mm chambers within the inner bark.^{42, 120} Woodpecker activity on living trees may indicate overwintering larvae.¹²⁰ Beetles emerge from "D-shaped" exit holes that are about 2.5 by 3.0 mm and are found from ground level up to a height of 10 m (i.e. lower portion of the crown).^{42, 98, 120}

Larval galleries can girdle the host tree resulting in twig and branch die-back, changes in foliage colour from green to brown, thinning of foliage within the crown, development of epicormic shoots along the stem and tree mortality in heavily infested trees.^{42, 98, 120}

A - Adult A. biguttatus (8-13 mm long). Note white spots near elytral suture.

B - Zig-zag shaped larval galleries (up to 155 cm long).

C-Branch and twig dieback caused by A. biguttatus.

D - A. biguttatus larva just prior to pupation.

E - Mature A. biguttatus larva.

F - D-shaped exit hole (2.5 by 3.0 mm) of A. biguttatus.

G - Early instar larval galleries. Note zig-zag shape.

Photo credits

• A Leen G. Moraal, Alterra, Researchinstituut voor de Groene Ruimte
• B Louis-Michel Nageleisen, Département de la Santé des Forêts, France, Image 2515035, www.invasive.org, Feb. 5, 2004
• C Louis-Michel Nageleisen, Département de la Santé des Forêts, France, Image 2515036, www.invasive.org, Feb. 5, 2004
• D Louis-Michel Nageleisen, Département de la Santé des Forêts, France, Image 2515038, www.invasive.org, Feb. 5, 2004
• E Louis-Michel Nageleisen, Département de la Santé des Forêts, France, Image 2515032, www.invasive.org, Feb. 5, 2004.
• F C. Bystrowski
• G Louis-Michel Nageleisen, Département de la Santé des Forêts, France, Image 2515039, www.invasive.org, Feb. 5, 2004