Walnut Twig Beetle

This sneaky pest and its fungus friend pack a double punch to black walnut trees.  They have jumped hosts and now pose a big threat to black walnut trees, which are valuable sources of lumber for furniture and edible nuts. Be sure to check out the info below the image slider to learn how you can help stop the spread this deadly pest!

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  • Highly magnified side view of the Walnut Twig Beetle (Pityophthorus juglandis).  Photo Credit: Steven Valley, Oregon Department of Agriculture, Bugwood.org

  • Highly magnified top view of the Walnut Twig Beetle (Pityophthorus juglandis).  Photo Credit: Steven Valley, Oregon Department of Agriculture, Bugwood.org

  • Walnut Twig Beetles are so incredibly tiny and reproduce so fast that over 23,000 adults (found in just two logs of firewood) can fit in this small vial! Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Walnut Twig Beetles are TINY! Adults are usually about 0.07 inches (1.7 millimeters) long.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Adult beetles spend the winter within cavities in the bark of the trunk. They resume activity by late-April and most fly to branches to mate and initiate new tunnels for egg galleries. During this tunneling the Geosmithia fungus is introduced and begins growing in the tree's wood.  Photo Credit: Steven Valley, Oregon Department of Agriculture, Bugwood.org

  • After the eggs hatch, the larvae feed for 4-6 weeks (May-July) under the bark, creating tunnels (galleries) that run perpendicular to the egg gallery. The larvae pupate at the end of the tunnel.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Close-up showing both larval (milky white) and adult (redish-brown) stages of the Walnut Twig Beetle. The second generation of adults emerge from mid-July through late August, mate again and, by early fall re-enter trees, as hibernation sites.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Close-up showing both Walnut Twig Beetle larva and the white powdery spores of the Geosmithia fungus that causes the tree to develop cankers and die.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Colony of the fungus, Geosmithia morbida, that causes Thousand Cankers Disease.  Photo Credit: Ned Tisserat, Colorado State University, Bugwood.org

  • Asexual spores of the fungus, Geosmithia morbida, that causes Thousand Cankers Disease.  Photo Credit: Alan Windham, University of Tennessee, Bugwood.org

  • Grove of young black walnut trees.  Photo Credit: Robert Vidéki, Doronicum Kft., Bugwood.org

  • Black walnuts grow to be medium to large trees (up to 100 feet in height) and usually have a straight trunk and narrow crown under competition in the forest.  Photo Credit: Vern Wilkins, Indiana University, Bugwood.org

  • Branch end of black walnut showing the alternate arrangement of its large compound leaves.  Photo Credit: Paul Wray, Iowa State University, Bugwood.org

  • The bark of 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.  Photo Credit: Jason Sharman, Vitalitree, Bugwood.org

  • 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.  Photo Credit: Paul Wray, Iowa State University, Bugwood.org

  • The young fruit of black walnut is light green, round and 2 - 2 1/2 inches across.  Photo Credit: Rob Routledge, Sault College, Bugwood.org

  • 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.  Photo Credit: Lyndon Photography, Bugwood.org

  • Cross section of black walnut twig showing the chambered sections of pith.  Photo Credit: Paul Wray, Iowa State University, Bugwood.org

  • To identify black walnut in winter look for tan buds that are alternately arranged on the stem. Leaf scars are 3-lobed, resembling a "monkey face."  Photo Credit: Rob Routledge, Sault College, Bugwood.org

  • Close-up of galleries created by Walnut Twig Beetle tunneling under the bark.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Close-up of walnut branch showing the early stages of canker development around beetle tunnels.  Photo Credit: Ned Tisserat, Colorado State University, Bugwood.org

  • Close-up of walnut branch showing the early stages of canker development around beetle tunnels.  Photo Credit: Ned Tisserat, Colorado State University, Bugwood.org

  • Dark staining caused by Geosmithia morbida cankers in black walnut. As these cankers grow together they stop the flow of water and nutrients in the branch and dieback occurs.  Photo Credit: Ned Tisserat, Colorado State University, Bugwood.org

  • Example of a large trunk canker caused by the fungus Fusarium solani that can also occur on trees in advanced stages of decline.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Yellowing leaves at branch ends can be an early symptom of dieback from Thousand Cankers Disease.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Black walnut tree in decline from Thousand Cankers Disease and showing dieback in the upper canopy.  Photo Credit: Curtis Utley, CSUE, Bugwood.org

  • Black walnut tree in decline from Thousand Cankers Disease and showing dieback in the upper canopy.  Photo Credit: Karen Snover-Clift, Cornell University, Bugwood.org

  • Close-up of bark showing small piles of sawdust created by beetle tunneling.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • Tiny exit holes created by adult Walnut Twig Beetles as they leave the tree.  Photo Credit: Karen Snover-Clift, Cornell University, Bugwood.org

  • Aerial traps like the ones pictured here use smelly plant compounds (oils) to attract Walnut Twig Beetles.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

  • There are no known controls for Thousand Cankers Disease so early detection and removal of infected trees is critical to stopping it from spreading into new areas.  Photo Credit: Whitney Cranshaw, Colorado State University, Bugwood.org

Scientific Name: Pityophthorous juglandis

Description:  The adult beetles are extremely tiny, about the size of a grain of sand. They are reddish-brown in color and can fly.  They create tiny burrows underneath the bark of walnut trees to feed and lay eggs.

 

Native Range: Mexico, Arizona, New Mexico, and possibly parts of California

 

Introduced Range: First discovered in Colorado (2009), WTB is also found in Idaho, Nevada, Oregon, Utah and Washington.  More recently it has been found in North Carolina, Ohio, Pennsylvania, Tennessee, Virginia and northeastern Italy.

 

Habitat: So far, WTB has been found mostly in urban areas and suburban woodlands. 

 

Host Trees: Black walnut is most susceptible, but Persian (English) walnut and Northern California walnut can also be affected.

 

The Facts:  The walnut twig beetle (WTB) carries a nasty fungus, Geosmithia morbida, which causes cankers in the trunks of walnut trees, cutting off the tree’s flow of nutrients.  Many WTBs attack a walnut tree at once, making burrows in its trunk and branches and bringing their fungus friend into the tree as well.  The combination of the beetle’s feeding in its burrows and the growth of the fungus cuts off the tree’s food supply.  It can take ten years or more for these two bad guys to kill a healthy black walnut tree, but they have already collaborated to destroy millions of street trees.  WTB is very bad news for the eastern US states where it has recently been found, where walnut trees are grown for their tasty nuts and beautiful, dark-colored, highly valuable wood.  Black walnuts are a very important food source for wildlife in forests, and if WTB were to spread, it could cost millions of dollars in timber losses.