I recently received a leaf sample from a homeowner whose tree had been rapidly declining for a few months. The leaves showed symptoms of wilting, but there was no distinct pattern as there would be for a fungal leaf spot. I went out to look at the tree a few days later, and by the time I arrived almost all of the leaves were dead and crispy.
As I observed the tree, I immediately noticed a sign that was unmistakable - ‘toothpick'-like projections sprouting forth from the trunk. I knew the culprit was the Asian ambrosia beetle.
The Asian ambrosia beetle, also called the granulate ambrosia beetle, is an invasive pest first introduced in Carolina peach orchards in the 1970s.
They have since spread all over the United States and caused many millions of dollars in damages. Many tree species are susceptible to attack, including oak, dogwood, maple, cherry, peach, crape myrtle, hickory and many more.
Ambrosia beetles are small wood-boring insects that spend most of their lives inside trees. This makes them difficult to control. Females emerge for a short time in spring to find suitable nest sites. They usually fly to young trees (1-3 years old) and bore into branches or trunks.
The ambrosia beetles do not eat the wood, but they carve out tunnels and galleries in which to lay their eggs. Their sawdust tends to collect on the outside of the bark and form ‘toothpicks.' The females carry a fungus on their backs that they use to feed the newly-hatched larvae. The fungus will then spread to the tree and clog up its vascular system. This fungal infestation usually results in tree death.
The best time to scout for Asian ambrosia beetles will be next February and March. This is when the females emerge and look for new homes. Look for the toothpick sawdust projections, especially on young trees. If you have trees that are only showing a few toothpicks on small upper branches, there is a possibility of saving the tree. Cut out the infested branch and burn or dispose of it in the trash.
There are also ethyl alcohol traps that can be made and set out to monitor for early season beetle activity. If any Asian ambrosia beetles are trapped, you may choose to begin a spray program for your most vulnerable trees. Pyrethroid-based insecticides, such as bifenthrin, can be used to protect the trunks and branches of susceptible trees. Most of these sprays can be applied every 10-14 days until the infestation threat lessens. Always follow label directions when using pesticides.
If you observe a tree that is already showing signs of wilted leaves, along with the toothpick sawdust projections, chances are that tree will die. Trees killed from the ambrosia beetle fungus should be taken down and burned or otherwise removed from the area. Only vigilant monitoring and protective sprays can help prevent your trees from falling victim to this invasive pest.
Clark MacAllister is the Dawson County extension agent. For more information, call (706)265-2442.