TCD is a very interesting disease because it is caused by a fungus, but is carried from tree to tree by a beetle.
Without the walnut twig beetle, the fungus would have a very hard time infecting more trees. The walnut twig beetle’s life cycle includes four stages.
The adult lays eggs on a black walnut tree, and larvae hatch from the eggs.
The larvae burrow into the tree, and eat the phloem and cambium under the bark, leaving behind tunnels or “galleries”
where ever they go. The larvae will eventually pupate and turn into adults, who chew their way out of the tree and fly on to lay eggs somewhere else.
The fungus, Geosmithia morbida
, grows in the galleries created by the larvae.
The fungus also feeds on the tree, killing the plant tissues as it grows.
All of this happens on a really small scale, but eventually enough galleries are formed and become infected with Geosmithia morbida
that they kill the entire tree.
You probably won’t see the insect itself, (the beetle is about the size of a grain of rice!), but you can see the symptoms of yellowing leaves and top down dieback in black walnut.
Infestation Update (Fall 2020): Since PWC was quarantined in 2012, the Mosquito and Forest Pest Management Branch has been investigating the extent of the problem. Monitoring efforts were initiated in Summer 2013 by establishing trap sites throughout the county. Our monitoring efforts involve setting up black funnel traps with a pheromone that attracts walnut twig beetles. Technicians check them weekly and send suspect insects to Virginia Tech for official identification. Visual surveillance is also performed to track tree health and symptoms of TCD.
Four new positive sites were found in addition to the initial infestation found in Occoquan in 2012. In 2017 Walnut Twig Beetle was confirmed at six sites on the eastern end of the county including sites along the Route 1 corridor, Old Bridge Road, and eastern Dumfries Road. Monitoring efforts will continue to track the spread of the infestation.