5/29/2023 0 Comments Walnut twig beetleThe beetle is completely dependent on walnut, butternut, or wingnut trees as hosts. Beetles do not bore through bark below the soil line to start their galleries. It colonizes standing trees, fallen branches, and prostrate trees. The insect is present throughout California walnut orchards and in black walnut trees growing in windbreaks and surrounding riparian areas. Males produce an aggregation pheromone, which attracts more males and females to the colonization site and results in a mass attack. Several females then join the male and create the gallery system. Male beetles bore into the bark first, creating a push-pin-sized hole in the tiny cracks and corky furrows of the otherwise smooth bark surface of English walnut. Egg galleries created by adults are generally horizontal (across the grain) and the larval galleries tend to be vertical (along the grain). Walnut twig beetle larvae are white, C-shaped, and found in the phloem. saxeseni bores into the xylem of walnut trees and colonizes trees that are typically in a later stage of decline than those favored by walnut twig beetle. ![]() ![]() saxeseni resembles walnut twig beetle in size and shape, there are clear morphological differences visible under a dissecting microscope or hand lens (see Figure 21 in Detecting and Identifying Walnut Twig Beetle: Monitoring Guidelines for the Invasive Vector of Thousand Cankers Disease of Walnut for more details). In California walnut trees, the only other related insect that might be confused with walnut twig beetle is the fruit tree pinhole borer, Xyleborinus saxeseni, which is an ambrosia beetle. This species is characterized by four to six concentric ridges on the upper surface of the pronotum (the shield-like cover behind and over the head). ![]() Walnut twig beetles are very small, about 1/16 inch (1.5 mm), reddish-brown bark beetles that bore through the outer bark and into the phloem of the branches and main stems of walnut trees.
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