Ambrosia beetles

Ambrosia beetles are small (usually less than 0.2 inch (5 mm) long), cylindrical, and reddish to dark brown. They bore directly into the wood of recently cut logs or damaged areas of limbs and tree trunks, for example, old tap holes. Ambrosia beetles do not kill trees or prevent them from recovering from stress, but their presence indicates physical damage or severe tree stress.

The adults and white grub-like larvae do not eat wood. Rather, they feed on a fungus (“ambrosia”) that grows in the tunnels. Fungus spores are carried by the beetles and introduced into the wood during tunnel construction. As the fungus grows, it discolors or stains the wood immediately adjacent to the tunnel (Fig. 63). In some instances, the wood is stained for several inches (several centimeters) above and below the tunnel.

The most reliable indications of attack are small entrance holes (less than 0.1 inch or 2 mm in diameter) and the presence of fine, light-colored wood dust in bark crevices beneath each hole (Fig. 64). When an infested piece of wood is split open, the simple, branched or compound dark-stained tunnels are obvious (Fig. 63).

One species (Xyloterinus politus) frequently associated with declining or wind-thrown sugar maple produces a cylinder of tightly packed boring dust that produces up to 1.0 inch (25 mm) from the entrance hole (Fig. 65). Another species, the pitted ambrosia beetle (Corthylus punctatissimus), attacks and kills seedlings of sugar maple and other hardwoods. Adults penetrate stems below ground level, tunnel into the outer wood, and eventually girdle the seedlings. There usually is a small pile of white wood chips (Fig. 66) in the soil adjacent to the entrance hole. An infestation is easily detected because seedlings wilt and turn brown during midsummer (Fig. 67). Heavy infestations have been reported in southeastern Canada and North Carolina. Fortunately, sugar maple reproduction is so abundant in most stands that even heavy infestations of the pitted ambrosia beetle are not important.

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