Lesser shothole borer

Distribution

Quebec, Nova Scotia, Ontario, British Columbia

Distribution elsewhere in the world

South Africa, Germany, Central Asia, Australia, Belarus, Belgium, Korea, Croatia, Spain, United States, France, Kazakhstan, Mexico, Moldova, Norway, New Zealand, Panama, Baltic states, Netherlands, Czech Republic, United Kingdom, Russia, Turkey, and Ukraine

Micro-habitat(s)

Bark, sapwood

Damage, symptoms and biology

The adult is small (the female is 2.0 to 2.4 mm long, and the male, 1.5 to 2.0 mm long),           cylindrical, and uniformly blackish brown or yellowish brown with short, scattered hairs. Tiny granules (swellings) are present on the elytra, at the apex and near the suture. Granules are absent on the declivity or sloped part of the elytra. The pronotum (or prothorax) and head occupy nearly a third of the insect’s total length. When the insect is viewed from its top surface, the head is not visible. The larva is pinkish white, cylindrical, legless and slightly curved.

Young adults overwinter or enter diapause within the wood. They huddle together at the back of the larval tunnels and emerge between April and August. Peak flight activity occurs from late June to mid-July. After mating, the female bores through the bark and excavates a narrow tunnel (1 mm) which typically extends 0.6 cm to 3.8 cm in a direction that varies with the tree species. The tunnel, or gallery, has a variable number of branches, which terminate in a leaf-shaped enlargement. This is a larval chamber, in which the female deposits about 40 eggs, on average. The larvae develop in these niches and feed gregariously on the mycelium of a fungus called Ambrosiella sulfurea, which is carried to the chamber by the female beetle. The fungus develops on a mixture of sawdust and frass and colonizes the various galleries, staining them dark brown or black. If the larvae do not consume enough fungus, the fungus may close off the galleries, imprisoning them. Since the female lays eggs over a period of several weeks, larvae of different sizes live together in the same gallery or in the different galleries making up the nest.

The insect’s life cycle is completed in 10 weeks and there is one generation, sometimes two generations, per year. Tiny, toothpick-like tubes of slightly compressed sawdust can sometimes be observed protruding from the bark; they are caused by the female pushing sawdust out the entry hole as she tunnels into the tree. The galleries are generally clean. The entry holes are circular, with a black margin, and about 1 to 3 mm in diameter. X. saxeseni can be distinguished from other bark borers by the very narrow galleries it excavates and the leaf-shaped enlargement, or chamber, in which they terminate.

The foliage of infested trees turns yellow, then brown and wilts completely. A severe infestation can kill trees. X. saxeseni is known to preferentially attack trees that have already been weakened or stressed by disease, external injuries or prolonged drought. The insects also infest logs lying on the ground. Tunnelling can cause fungal infections within the tree and compromise the quality of the wood.

Other information

X. saxeseni is the most widely occurring insect in the ambrosia bark beetle group. In some regions, however, it appears to pose a threat mainly to ornamental trees and fruit trees (apricot, cherry and plum). Since only the female beetle is able to fly, it is the female that sets off in search of a suitable host on which to breed. Luckily for the species, females far outnumber the males, with a female:male ratio sometimes as high as 15:1.

X. saxeseni occurs throughout the geographic range of Douglas-fir in northwestern British Columbia, where it capitalizes on natural disturbances such as windthrow and glaze ice. In Quebec, there has been an increase in X. saxeseni populations in southern areas affected by the ice storm of 1998. In France, it is the most common insect in forests damaged by the wind storm of 1999. Nonetheless, X. saxeseni is not considered an economically important pest.

In Canada, X. saxeseni is known to be a pest of dwarf hackberry, a threatened species that occurs only in southern Ontario.

Main host(s)

Amabilis fir, black walnut, Douglas-fir, dwarf hackberry, Manitoba maple, pines, plains cottonwood, sugar maple, western hemlock, white elm