Q: I have toothpicks of sawdust coming out of the trunk of one of my crapemyrtles. The other one looks OK but this one can’t put out much new growth.
A: I feel sure you have an infestation of Asian ambrosia beetle.
The tiny female beetle bores into the trunk of susceptible trees (crape myrtle, redbud, dogwood, cherry, etc.) and deposits her eggs. Lovingly she leaves a deposit of ambrosia fungus in the hole, to provide a first meal for the larvae when they hatch. Unfortunately, the beetle fungus is usually contaminated with other, more virulent, fungi – which often lead to a tree’s demise.
There is no treatment for an Asian ambrosia beetle attack. The fungus inside the tree can not be eliminated.
Asian ambrosia beetle is serious pest. It is possible you’ll have to cut the tree down. It all depends on how much fungus was introduced. Crepemyrtles might resprout and grow another trunk to replace what you remove but most other trees do not respond so nicely.
Your best bet is to spray susceptible trees each spring with cypermethrin, permethrin or deltamethrin in order to protect them.
To know when to spray, you can build a trap to monitor their emergence in early spring.