Bramble Borers - Or Solitary Bees?

Raspberry and blackberry borers make tunnels in canes, but so can solitary bee pollinators. This can lead growers to think they have borer problems when they might not. 

There are 3 common borers affecting brambles in PA. The first is raspberry cane borer, which also attacks blackberries. The first symptom of this pest is primocane tips that wilt in early summer. If raspberry cane borer caused the wilting, you will see two rows of punctures encircling the primocane right below the wilted tip. These punctures are made by the female who will lay an egg between the two rows of punctures. Once the egg hatches, the larva tunnels downwards.  Infested cane tips should be cut below the lower row of punctures (Figure 1), removed from the planting, and destroyed as soon as possible. The tip will die eventually and break off, usually at the top line of punctures.

If you purposely break the tip off at the lower row of punctures, you may find the cane is solid, in which case the borer would have been removed (Figure 2), but if more time elapses, you may see a hole in the center of the stem indicating the larva has already tunneled downwards (Figure 3).  In order to ensure that you have removed the borer, you will need to continue to cut the cane at lower points until the cane is solid, and destroy the cut portions.

Solitary bee pollinators can and do make holes that look similar, usually in black raspberry or blackberry canes that had been tipped or broken off (Figure 4). Once the soft pith has been exposed, it is easy for the female to tunnel in and lay her eggs.  A key difference from tunnels made by borers is that the tunnel made by a solitary bee will only be within the top few inches of cane, and never in the lower portions of the cane. This is where the bees will develop, waiting for the right time to emerge and pollinate your crops. If you happen to cut these tips off while pruning, you can place them in a protected location where the bees will be safe until they hatch.

Two other types of borers are common in brambles. One is red-necked cane borer, which usually causes a swelling about a foot above ground level, though it can appear anywhere on the cane. This is caused by the young borer first feeding just under the cane’s surface and encircling it several times (Figure 5) before tunneling either upwards or downwards. Once you notice damage, remove the cane from the planting, cutting as low as is needed to make sure you have removed the borer.

Another common borer is raspberry crown borer.  Infested plants become weaker over time, and new canes become progressively spindlier. You may find holes in the base of canes while pruning in the spring (Figure 6), and if you cut the crown open, you may find the larvae and/or pupa (Figure 7).

There are also other types of borers like the stalk borer that are generalists and occasionally end up in brambles, but these have minimal effects.

Because borers are protected inside of the plant for most of their life cycle, removing infested canes or cane sections is the most straightforward method of management. If a source of pests can be identified like a nearby wild bramble patch, numbers may be reduced by eliminating the source.

If borer infestations do become severe enough to warrant an insecticide application despite the use of cultural controls, timing can be tricky and effectiveness will be only partial. Raspberry cane borer and raspberry crown borer both have a two-year life cycle, so not all of the population is vulnerable at the same time meaning that multiple applications will be needed. Raspberry cane borer adults are black beetles with an orange neck and emerge in early summer. If they are seen on the foliage, a broad-spectrum insecticide may be used prior to bloom whereas with red-necked cane borers, similar sprays directed to the ground before and after bloom may help. For raspberry crown borer, new larvae at the base of the canes in early spring are the target, but as mentioned above, only a portion of the larvae will be exposed each year. As always, the label is the law, not all materials are labeled in all states, and be sure to follow the regulations that apply in your state.

Photos: Kathy Demchak