Blackberry borers can mean big problems

Weakened blackberry plants, Guilford County, NC. Photo: Gina Fernandez

NC State caneberry specialist, Gina Fernandez has just returned from sabbatical and has wasted no time getting out for field visits. Yesterday, she brought me several samples from blackberry fields in Guilford County that were in severe decline. Their problems were almost all insect related. There were more cane boring pests from these few sites than I have seen my entire time at NC State! I'd like to use these samples as an overview of the key cane boring insects in NC, what symptoms to look for, and what the management strategies for these pests should entail.

Evidence of borer damage is often visible from a distance. Plants will appear weakened, and in the case of raspberry crown borer, floricanes will be loose and easily removed. There are 3 key cane boring insects in North Carolina, and these locations had all of them!

Rednecked cane borer
The rednecked cane borer (Agrilus ruficollis) is part of a family of beetles known as metallic wood boring beetles. The adults lay their eggs on the surface of primocanes and the larvae bore into them. When used, insecticide treatments target the adults, since the larvae do not spend time outside the plant. Larval rednecked cane borer feeding produces galls on the canes. The larvae are flat and found within the cane. As they grow, larvae will tunnel above the gall and reduce plant vigor and yield.

Rednecked cane borer galls (top), larva in gall (middle) and outside of gall (bottom). Photos: Gina Fernandez (top), HJB (middle, bottom).

Low levels of rednecked cane borers can be managed with cultural control, specifically by removing and destroying all infested canes during the fall. The larvae overwinter inside the cane, so pruning will remove next year's generation of adults. For large infestations, chemical control of the adults may be necessary (see the Southern Region IPM Guide for chemical information). Dr. Donn Johnson, University of Arkansas fruit entomologist, has also had success removing primocanes in the early summer after the adult beetles laid their eggs (a June removal date worked the best in Arkansas and did not impact fruiting). This strategy may work well in North Carolina, because our long growing season allows plenty of time for primocane regrowth.

Raspberry cane borer

Raspberry cane borers (Oberea bimaculata), members of a family known as long horned beetles due their prominent antennae, were also present in the Guidford County plantings. The adult beetles create paired girdles at primocane tips in summer. These tips wilt and will eventually fall off, and the entire cane may die. Larvae tunnel downward from the tip and have a 1-2 year life cycle in the southeast.

Raspberry cane borer girdling. Photo: Gina Fernandez

Cultural control, via pruning, is also an effective means of managing raspberry caneborers. Insecticide treatments may be targeted to the emerging and ovipositing (egg laying) adults just after bloom in cases where large infestation exist.

Raspberry crown borer

Raspberry crown borer (Pennisetia marginata) is perhaps the most severe pest of caneberries in the southeast. The larvae of this clearwing moth feed on the roots and crowns of caneberry plants and can kill an entire plant. Because they spend most of their 1-2 year larval stage underground, they are extremely hard to manage. Work conducted in Arkansas demonstrated that late fall or early spring soil drench pesticide treatments are most effective at reducing raspberry crown borers. These timed treatments target the early instar larvae before they are ensconced in the crown. Insecticides are, at this time, the most effective means of raspberry crown borer management. Infested plants should be completely removed, since mature larvae will not be impacted by pesticide treatments.

Raspberry crown borer damage (t0p) and larvae in a blackberry cane (bottom). Photos: Gina Fernandez (top), HJB (bottom)

In addition to the damage caused by these 3 wood boring insects, there were also several canes with damage at the tip that were dying back.

Unidentified damage at cane tips and associated tunnels. Photos: Gina Fernandez

It is possible that this damage was cause by raspberry cane borers, but no larvae were found in the canes, and what tunnels there were stopped several inches from the top. If this was raspberry cane borer injury, we would expect to see tunnels continuing to the soil level or larvae present. These canes may have been injured during tipping and then feed on be opportunistic secondary pests. I dissected one of these canes, and there was fungal growth in the gallery, although this may also be secondary.

Fungal growth in damaged cane. Photo: HJB

Wood boring insects are a fact of life for caneberry growers and can easily get out of control if a planting is not managed carefully. There are abundant feral and wild brambles in the landscape, which serve as hosts for all three of these insects. Carefully scouting, good cultural management, and insecticide treatment when needed will keep borers from destroying caneberry plantings.

More information
Rednecked cane borer - Virginia Tech
Rednecked and raspberry cane borers - University of Kentucky
Blackberry Insect Pest Management - Presentation from Donn Johnson, University of Arkansas