Cane Borers

We’ve already talked about several different borer species, including the European corn borer, squash vine borer, and the raspberry crown borer. Today, we will look at more borer threats to your blackberry, boysenberry, dewberry, and raspberry plants.

Raspberry cane borer (Judy Gallagher) CC BY 2.0

Cane borers
 
Borers lay eggs in woody tissue. When the eggs hatch, larvae begin feeding and tunneling. This weakens the plant and increases the chance of other pests and diseases taking hold. Adult borers are most active during late spring through summer.
 
There are three different species of cane borers: the raspberry cane borer, red-necked cane borer, and bronze cane borer. Bronze and red-necked borers are nearly identical in life cycle and behavior, while raspberry cane borers behave somewhat differently. All three are narrow-bodied beetles.

Raspberry cane borers
 
Raspberry cane borers (Oberea bimaculata) are also known as the raspberry root borers. Adults are approximately half an inch long (12mm), black with an orange thorax, and have two or three black dots (as seen above). They have long antennae.
 
Adults feed on tender new cane tips, leaving behind scarred brown patches. They have a two-year life cycle. After feeding all summer, female raspberry cane beetles drill a double ring, about half an inch apart, around a cane or lateral shoot, girdling the stem, usually in the uppermost portion of the cane. Here, she deposits her eggs, where her offspring can develop in relative safety. This egg-laying causes the tips to wilt and turn black, so be on the lookout for that symptom.
 
Once the eggs hatch, the larvae begin boring downward, in a slow march to the base of the plant, which they usually reach by fall. By the following summer, they have usually reached the crown, feeding and causing damage as they go. This can result in the death of the cane.
 
Flat-headed borers
 
Bronze and red-necked cane borers are also known as flat-headed borers, since their larval stages have flattened heads, similar to Pacific flathead borers.

Two flathead borers

Bronze cane borers (Agrilus rubicola), also known as rose stem girdlers, are slightly smaller than other cane borers. Adults may only be one-quarter of an inch long (6mm) and are iridescent copper or bronze.

Bronze cane borer (AfroBrazilian) CC BY-SA 4.0

Red-necked cane borers (Agrilus ruficollis) are often described as one of many jewel beetles due to their metallic exoskeleton. The wing covers (elytra) are black, and the area just behind the head (pronotum) is reddish orange. If you look closely, you can see a divot at the base of the head. These beetles are one-quarter to one-half an inch (6–12mm) long.
 
You can often see adults feeding on the edges (margins) of leaves on warm, sunny days. If you are quick, you can knock them into a container of soapy water to eliminate them.
 
Adult females lay whitish, scaly eggs on and in the bark of new cane growth, usually in late spring, near the lower portion of the cane. Newly hatched larvae frequently enter the bark at the axil of a leaf stem, spiral the sapwood a few times before heading deeper, into the hardwood, and then the pith. At this stage, the larvae are full-grown and single-minded, heading straight down to the crown where they overwinter and pupate. All those early twists and turns create symmetrical galls. These galls can be one-half to three inches long and often exhibit slits.

Red-necked cane borer (Katja Schulz) CC BY 2.0

How to manage cane borers
 
First, monitor canes for signs of blackened or brown cane tips, dieback, galls, or wilting. If symptoms occur, you can try to save the cane by making a cut two or three inches below the affected area, removing and destroying the infected portion. Continue to monitor those trimmed canes for signs of speedier-than-normal larvae who may have gotten below where you made your cut. It is often easier to completely remove affected canes.
 
Keeping your garden and landscape free of wild brambles can help reduce the chance of cane borers finding your berries.
 
The insecticides used by commercial growers against adult borers are rather nasty and not recommended for the home gardener (or anywhere, for that matter).