Raspberry Crown Borers

As your raspberry and blackberry seasons wind down and you begin your winter preparations, watch for signs of raspberry crown borers, also known as blackberry crown borers (Pennisetia marginata).

Delicious fresh raspberries! (Kate Russell)

​The first signs of infestation include withering and wilting. Close inspection of the crown at soil level may yield a small hole surrounded by frass that looks like sawdust. There are no biological or organic treatments against this pest. Commercial growers often use Diazinon to combat these pests, but that chemical is highly toxic to bees (and it probably isn’t good for us, either).

The more you know about raspberry crown borers, the better equipped you will be to prevent infestation.

Is it a moth or a wasp?

It all starts with what looks like a wasp. This type of clearwing moth has a one-inch wingspan. It is black with four or more stripes around the abdomen. The antennae are feathery and curled at the tips, the wings have longitudinal bars, and the legs are yellow. First-year larvae are one-half to one-inch long, while second-year larvae may reach an inch-and-a-half long.

Adult raspberry crown borer with egg (University of Georgia Plant Pathology Archive) CC BY 3.0

Raspberry crown borer larva (University of Georgia Plant Pathology Archive) CC BY 3.0

Raspberry crown borer lifecycle

These fascinating (and destructive) parasites go through a two-year cycle that starts with females laying up to 140 rust-colored eggs on the underside of leaflets in late summer. Take a peek and extermigate any eggs you see. Larvae create tiny blistered hibernaculum (“winter tents”) in the base of the crown, where they are safe from winter weather and predators.

As temperatures rise, larvae burrow galleries throughout the crown, feeding as they go. In some cases, these larvae emerge as adults at the end of the summer. They often remain in these protected galleries through a second winter, causing even more damage.

Keeping plants healthy and monitoring for eggs and entry holes are the best preventative tools. Any plants infested with raspberry crown borers should be removed and destroyed.