If you’ve had difficulty with rose canes dying and are unsure what caused their demise, cane borers could be the mysterious pest. Cane borers are larvae that bore into your rose canes and then consume the center of your canes leaving them hollow. There are a variety of rose cane boring larvae and they leave different clues to show their presence.
Cane borer larvae are laid on your roses by a variety of different insects, hybrid teas and floribundas are the most commonly effected. Since the insect doing the laying is only there for a short period of time and then it departs, there isn’t an effective chemical control. Once the insect has laid its larvae on the rose cane, the larvae can then enter the cane in several ways. Some types of larvae chew through the side of the cane, girdling the cane or eventually causing the cane to snap off. Most commonly the larvae enter through a pruning cut made at the top of the cane. However, once the larvae have entered the cane the result is the same. The larva survives by eating the inside of the cane and then leaving you with a hollow tube.
Cane borers are often noticed in the summer even though the infestation usually becomes active in the spring months. Rose growers typically notice canes and leaves that are turning brown and eventually dying. Often it will appear your canes are simply rotting and dying. It can be difficult to locate either the original larvae laying insect or the larvae inside the cane that’s doing the damage. Most cane borers won’t kill the entire rose. However, there is a type of borer called the Flat Head borer that will burrow into the cane and then eat until it reaches the bud union. This can and will cause death to your rose.
The only method of control involves cutting the cane down past the borer, until you are left with only a full healthy cane that is filled with white inside. If the inside of the cane is brown or hollow you’ll need to trim down further. Once you’ve made your cut and are left with only a healthy cane you’ll need to close off your fresh pruning cut so a new larva doesn’t reenter. The best way to accomplish this is by covering your pruning cut with a dot of Elmer’s glue. If you’re so inclined you can even add food coloring to the glue bottle so it will be easier to identify the canes you’ve sealed from the ones you haven’t.
Nothing else will have an effect on cane borers other than your vigilance with pruning off the damaged canes and then sealing the pruning cuts. The insects and larvae will often over winter on your plants only to reemerge and start the same annoying cycle next spring. By keeping your roses pruned and sealed you can send a message that they’re not a tasty snack, which will eventually lessen the cane borer population in your rose garden.
Parasitic wasps do eat some cane boring larvae, but only when they’re on the outside of the rose cane. It’s still a good thing to encourage the establishment of all beneficial insects in your rose garden as they’ll also help with your other pest situations. Often the spraying of a pesticide only worsens the issue in the long run by killing off all your beneficial insects.
Stay vigilant by keeping your roses pruned and healthy. Apply Elmer’s glue to new pruning cuts especially on your hybrid teas and floribundas. Now, the next time you see a dying rose cane you’ll know what to look for and how to stop the problem.