Walnut Twig Beetle
Walnut Twig Beetle
This tiny beetle and the fungus it carries can greatly affect black walnut trees, a valuable source of wood and delicious nuts!
Black walnuts grow to be a medium to large tree up to 100 feet in height and usually have a straight trunk and narrow crown under competition in the forest.
Branch end of black walnut showing the alternate arrangement of its large compound leaves.
Grove of young black walnut trees
The bark of the black walnut (Juglans nigra) is usually light brown, ridged and furrowed with a rough diamond pattern. Walnut has large compound leaves (12-24 inches long) each of which has 10 to 24 leaflets.
To identify the black walnut in winter, look for tan buds that are alternately arranged on the stem. Leaf scars are 3-lobed, resembling a “monkey face.”
Cross section of a black walnut twig showing the unique chambered sections inside the twig.
Close-up of flower spikes on a black walnut tree. These appear in late spring, usually near the end of twigs and are 2.5-5.5 inches long (6-14 centimeters) long.
The young fruit of the black walnut is light green, round, and 2- 2 1/2 inches (5-6 centimeters) across.
The husk of the walnut fruit turns black as it ripens in late summer to fall. Inside the husk you can find an irregularly furrowed, hard nut that contains sweet, oily and edible meat.
Yellowing leaves at branch ends can be an early symptom of dieback from Thousand Cankers Disease.
Black walnut tree in decline from Thousand Cankers Disease and showing dieback in the upper canopy.
Tiny exit holes created by adult Walnut Twig Beetles as they leave the tree.
Close-up of bark showing small piles of sawdust created by beetle tunneling.
Example of a large trunk canker caused by the fungus Fusarium solani that can also occur on trees in advanced stages of decline.
Close-up of galleries created by Walnut Twig Beetle tunneling under the bark.
Dark staining caused by Geosmithia cankers in black walnut. As these cankers grow together they stop the flow of water and nutrients in the branch and dieback occurs.
Close-up of walnut branch showing the early stages of canker development around beetle tunnels.
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Walnut twig galleries and cankers: Ned Tisserat, Colorado State University, Bugwood.org; Size comparison to penny: Eric R. Day, Virginia Polytechnic Institute and State University, Bugwood.org.; Top view, side view: Steven Valley, Oregon Department of Agriculture, Bugwood.org.; Adults on penny, vial: Whitney Cranshaw, Colorado State University, Bugwood.org.; Larva, adult, beetle galleries: Whitney Cranshaw, Colorado State University, Bugwood.org.; Thousand Cankers Disease spores: Alan Windham, University of Tennessee, Bugwood.org.; Cultured colony of thousand cankers disease: Ned Tisserat, Colorado State University, Bugwood.org.; Black walnut tree: Vern Wilkins, Indiana University, Bugwood.org.; Black walnut leaves: Paul Wray, Iowa State University, Bugwood.org.; Grove: Robert Vidéki, Doronicum Kft., Bugwood.org.; Bark: Jason Sharman, Vitalitree, Bugwood.org.; Twig lobes,black walnut fruit: Rob Routledge, Sault College, Bugwood.org.; Chambered pith, flower spikes: Paul Wray, Iowa State University, Bugwood.org.; Husk: Lyndon Photography, Dried Botanical ID, USDA APHIS PPQ, Bugwood.org; Yellowing leaves, canopy dieback: Whitney Cranshaw, Colorado State University, Bugwood.org. Crown thinning and dieback: Curtis Utley, CSUE, Bugwood.org. Exit holes: Karen Snover-Clift, Cornell University, Bugwood.org. Sawdust, trunk canker, galleries: Whitney Cranshaw, Colorado State University, Bugwood.org. Wood staining: Ned Tisserat, Colorado State University, Bugwood.org.