At least two species of ambrosia beetles attack black walnut trees but the most serious is Xylosandrus germanus (Blandf.). This beetle occurs throughout most of the northeastern and north-central walnut growing regions.

    The adult female beetle is dark brown to black and about 1/8 inch long. The immature stages are rarely seen because they occur in tunnels made inside the wood by the adult female. External entrance holes to these tunnels are about 1/32 inch in diameter, and are sometimes referred to as pinholes.

    Young walnut trees up to 8 years old are most often attacked.

    A Xylosandrus germanus female may introduce a Fusarium fungus into the tree as she excavates her tunnel into wood. This fungus causes a cankered area in the wood, usually causing top dieback and resprouting from the base of the tree. Cankering, however, is not always apparent. In some plantations, dieback in 1 year due to ambrosia beetle/Fusarium canker attack has been reported on 30 to 40 percent of the trees.

    Ambrosia beetle attack is usually not detected until there is profuse sprouting from the base of the trees or until the trees are dead. Close examination is necessary to locate the tiny pinholes in the lower stem area or in small, lowhanging branches.

    Cut and remove dead or Fusarium-cankerinfected tree tops and branches and burn, if possible.

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