Rose Stem Boring Insects

(Question)

Hi,
I live in Woodstock Ontario. I have been having trouble with boring insects doing significant damage to roses and lilacs as well as my other woody plants.
I am now debating weather I should even prune my roses because of the subsequent invasion. I have tried sealing cut ends but this has not stopped the problem. Internet searches have left me solutionless. I have not wanted to use insecticides against what I cannot even find.
Is there a solution or maintenance to help. Is not pruning a viable solution.
Thank you ,
Laura Jeffery

(Answer)

I sense your frustration with these pests.  Borers are the larvae of various insects – beetles, flies, wasps and moths.

The most important consideration is to keep your roses as healthy as possible.  So certainly do your regular spring pruning as well as keeping up with irrigation and fertilization.  You are already sealing the cut ends which is a recommended practice where cane borers are a problem.  A healthy plant can withstand minor pest damage.  I think you probably already know to remove and destroy any infested canes by cutting just below the damaged area.

I know you’ve done your own Internet research, but I’m including below links to two articles that I found helpful:

https://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/insects/sawflies/borers-and-miners-sawflies.aspx

http://www.marinrose.org/caneborers.html

As for your lilac, there is a borer specific to lilacs and other members of the olive family – Podosesia syringae.  I’ve copied a link below to an article from the Missouri Botanical Garden which describes the symptoms of this borer as well as some control strategies.  One of the strategies is the use of a biological control, the beneficial nematode Steinernema carpocapsae.  Nematodes are microscopic worms which feed on larvae by entering their body and feeding from the inside, usually killing the host insect within a day or two.  I’ve had good success with beneficial nematodes in dealing with lawn grubs.  I was not familiar with this species so checked that it is available in Canada.  However, the vendors I located did not specifically list it as a target for their product.  If you wish to pursue this strategy I would recommend discussing with the vendor the appropriateness of the product for your situation.

http://www.missouribotanicalgarden.org/gardens-gardening/your-garden/help-for-the-home-gardener/advice-tips-resources/pests-and-problems/insects/caterpillars/lilac-borer-and-ash-borer.aspx

Note, that under the Ontario pesticide legislation, your strategy of using chemical pesticide control, would not be applicable.

I hope this information offers some encouragement and maybe some new insights / strategies for dealing with these pests.