Raspberries are fairly care-free plants to grow in your garden. For most of us, the main challenges of growing these brambles are keeping them under control (some varieties spread like crazy) and protecting the berry harvest from birds and other animals. But if you notice that the tips of some of the branches have died back or look wilted, or if the foliage is turning red well before fall and entire canes are wilting, you very likely have a borer problem.
There are three very common raspberry pests that cause damage to canes through boring: raspberry cane borers, red-necked cane borers, and raspberry crown borers. Identifying the precise insect you're dealing with will help you target the most effective control methods.
Identifying the Borer
Identifying the borer insect that's causing the damage to your raspberries is a matter of observing the type of damage, as well as studying the insect itself, if you can spot it.
Raspberry Cane Borer
If you're seeing just the tips of some of your raspberry canes wilting, you're probably dealing with the raspberry cane borer. Raspberry cane borer is a small (1/2-inch), black long-horned beetle. It has yellow stripes on its wing covers and a yellow thorax with two black dots on it.
The female beetles create a double row of holes a few inches below the leaf tips in spring, and they lay their eggs in those holes. In late spring to early summer, the larvae hatch and start burrowing their way down to the roots, where they will overwinter and start the whole cycle again the following spring. In the first year, you'll see the wilting cane tips from the punctures the adults made, as well as the damage the borer is causing as it makes its way down the cane. The following year, if the borer has made its way all the way to the roots, the cane will likely die back completely.
If you're noticing random swollen areas along the canes of your raspberry plants, you're dealing with red-necked borers. The red-necked borer is a bluish-black wood-boring beetle. The coppery-red area just behind its head gives this pest its name. Adult females lay their eggs in the bark of raspberry canes in late May through early June. Upon hatching, the whitish larvae bore their way deeper into the cane, where they overwinter. This causes the branch to swell about a half-inch or so in diameter for a few inches along the length of the cane. The cane might die or just break off at this weakened, swollen point.
Raspberry Crown Borer
If the leaves on your raspberry bushes are turning red prematurely, or entire canes are wilting, you're dealing with raspberry crown borers. The adult raspberry crown borer, a clear-winged moth that resembles a black and yellow wasp, lays its eggs on the underside of raspberry leaves in late summer. In early fall, the eggs hatch, and the larvae make their way to the soil, where they begin to feed on the roots. The pupae in the crown of the plants hatch the following spring.
4 Ways to Get Rid of Borers on Raspberries
Cut Back Wilted Canes
To get rid of the two types of cane borers, keep an eye out for wilted tips. Once you see them, prune back the damaged canes to a point at least 6 inches below where the wilting stops. Likely, the section you've removed contains the troublesome larvae as well. Destroy (don't compost) any tips you've pruned out. Keep an eye out the following spring for any additional wilting tips, and prune them out as well.
For crown borers, remove the affected canes all the way down to crown level and destroy them. However, if upon inspection you see that the whitish larvae are already in the crown, the only thing you can do is dig up and destroy (don't compost) the entire plant.
Destroy Nearby Wild Brambles
Wild raspberries and other native brambles located in nearby properties may be the source of the borers that are affecting domestic plants in the area. Where it's feasible, examine nearby areas for wild brambles and eliminate them. Make sure you have permission for this kind of search and destroy effort; most landowners will cooperate if they understand your goal—especially if they grow their own domestic raspberries.
Introduce Helpful Nematodes
While studies are still ongoing, there is some evidence that certain soil nematodes, including Steinernema feltiae, Steinernema carpocapsae, and Heterorhabditis bacteriophora, are effective against raspberry crown borers (the evidence is less convincing regarding their effectiveness against the two cane borers). Nematodes are microscopic roundworms that dwell in the soil. While some species are damaging pests that attack plant roots, other species are known to attack insect pests and are often introduced as an organic pest control measure. Helpful nematodes are often sold at larger garden centers, or from specialty online retailers.
A variety of chemical pesticides, both organic and synthetic, are available that will work to control borer pests on raspberries. These are best applied before the plants bloom, when the beetles or worms are the most active. Spraying or painting affected canes with a pyrethrin-based pesticide (a natural pesticide made from an extract of certain chrysanthemum flowers) is a good choice if you must use an insecticide. But be aware that pyrethrins are toxic to fish and other aquatic life, so be careful in your application to avoid runoff into streams and rivers.
Using pesticides, especially synthetic chemical products, should be approached with great caution when you are dealing with edible plants. Wherever possible, use organic products that are deemed safe for use on edibles.
What Causes Borers on Raspberries?
Borers are more likely to occur in large raspberry patches, or in areas where many gardeners are growing bramble species or where wild bramble patches are common. The two cane borers are common across North America; the crown borer is more prevalent in the eastern U.S. and along the Pacific coast.
Borers are more common in gardens where no routine control is practiced, but they are rather easily controlled by gardeners who regularly inspect the plants.
How to Prevent Borers on Raspberries
Preventing borers is best approached through yearly inspection and removal of affected canes. This approach usually prevents widespread infestations from setting in. It also helps if other nearby gardeners also practice good borer control, and if nearby wild bramble patches are eliminated.
Do Raspberry Borers Affect Other Plants?
Raspberry cane borers and crown borers can be a problem on other fruit-bearing cane plants, including blackberries or boysenberries. They sometimes also appear on roses, evidenced by individual canes that wilt back or develop swollen galls.
Are There Any Natural Predators of Borers?
Other than soil nematodes, the only effective natural predators that feast on cane borer beetles are birds, especially finches and woodpeckers. The finches are normally eating the beetles, while the woodpeckers bore into the canes to feast on the larvae. But because most gardeners make efforts to keep birds away from their raspberry fruit, it's hard to be selective about what birds are allowed into your raspberry patch. Further, woodpeckers can do substantial damage to canes, so they usually aren't encouraged. However, if you spot these birds in your patch, it may be a strong indication that you have a borer infestation.
Will Borers Go Away on Their Own?
Cane borers tend to be sporadic insects that appear in cycles, often lasting for two years or so. It's possible that a bad year for borers may be followed by several pest-free years. And it's also likely that strong treatment for borers may relieve you of the problem for several subsequent years, with no follow-up treatment needed until the borers appear again some years down the line.
Crown borers do not generally follow this cyclical pattern. Unless treated, infestations typically increase year over year. But systematic inspection and removal of affected plants usually keep crown borers from being a severe problem.