Crown Borer in Blackberries
I have noticed holes at the base of some of my blackberries, with some kind of sawdust around them. What is this and what should I do about it?
The holes and sawdust-like material you see at the base of your blackberries is most likely made by larvae of the raspberry crown borer. It is actually quite common in blackberries grown on the Central Coast of California.
If you uproot the plant and cut open the cane, you will see a rather large one-inch long, white to grey larva nestled within. The growth and burrowing of the larva destroy the inside of the blackberry or raspberry cane. Since water conducting elements of the cane are destroyed by this activity, infested plants will exhibit wilted, withered and dead leaves.
The adult raspberry cane borer is a moth, but resembles a yellowjacket wasp, with similar markings and size. Adults lay eggs individually on the underside of leaves in the late summer, and larvae move downwards on the cane after hatching. Larvae will form an overwintering structure at the base of the plant, and bore into the crown the following spring. They pupate that summer and emerge in mid-summer.
It can be helpful to remove surrounding growths of wild blackberries which serve as alternate hosts. Currently available insecticides are generally not efficacious because of the inaccessibility of the larvae buried inside the cane. The best strategy for controlling raspberry cane borer is to remove plants containing the larvae and destroy them.
Please contact Mark Bolda at UCCE Santa Cruz if you have more questions on this topic or any other topics concerning blackberry, raspberry or strawberry production.