Tetropium gracilicorne (Reitter) - Thin-antenna spruce borer

Coleoptera: Cerambycidae

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IDENTIFICATION

Adult beetles typically have a flat black body, 8 to 18 mm long, brown or red legs and relatively thin, short, brown or red antennae (half the length of body).^{2, 19, 24} The 2^nd through the 5^th antennal segments are enlarged.²⁴ There are different colour morphs within this species: black body, antennae and legs with light brown elytra or completely black beetles.¹⁹ The elytra have a broad covering of very fine hairs and are densely punctured.²

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HOST TREES

Larix.¹⁹

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LOCATION OF INFESTATION WITHIN THE TREE

Larvae feed on the inner bark and sapwood from root spurs to the top of the crown.²⁴

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HOST CONDITION

Stressed (e.g. defoliation, disease, fire, or wind), dying, and recently cut trees are preferred, but beetles also attack healthy, mature trees.^{19, 24, 49} Trees of all ages are attacked.²⁴

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DISTRIBUTION

Asia including Russia, Kazakhstan, northern China, northern Japan and northern Mongolia.²⁴

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SIGNS AND SYMPTOMS

Female lays eggs, often in groups of 3 to 5 in bark crevices.^{19, 24}

Larvae form irregular shaped, longitudinal galleries (10 mm wide) first in the inner bark and then later in the sapwood. Larval galleries are filled with granular frass.^{19, 42} Mature larvae bore horizontally into the sapwood to a depth of 4 to 5 cm and then parallel to the wood grain for 1.6 to 2.2 cm.^{19, 24} These "L-shaped" galleries end in a pupal chamber. If development occurs under thick bark, larvae may only bore beneath the bark without entering the sapwood.²⁴

Before pupation, larvae make oval emergence holes that are plugged with frass.²⁴ Pupal chambers are lined with frass.^{19, 24} Pupae develop with their heads toward the holes.¹⁹

Adults emerge through these oval holes.¹⁹ External signs and symptoms include peeling bark, frass at the base of infested trees, beetles visible on the trunks, and yellowing and wilting of needles.²⁴ Consecutive years of attack can result in tree mortality.⁴² When trees are not killed, infestations result in loss of vigor and reduced wood marketability due to bore holes.²⁴

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A - Adult T. gracilicorne (8-18 mm long). Note antennae are half the body length.

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Photo credits

• A Hiroshi Makihara, Forestry and Forest Products Research Institute. Agriculture, Forestry and Fisheries Research Council, Japan
• B Institut für Forstschutz, Österreichischer Pflanzenschutzdienst Holz
• C Institut für Forstschutz, Österreichischer Pflanzenschutzdienst Holz