Roses are one of the most beloved of all flowering plants in our landscapes. Unfortunately, they are also one of the most pest-prone. Earlier this year a sample arrived in the Bug of the Week mailbox. The sample consisted of a badly swollen stem of a sturdy hybrid of Rosa rugosa. Accompanying the sample was a note describing an unfortunate bed of hybrid roses with splitting canes, lodging stems, and untimely death of plants.
Swollen stems indicate trouble beneath the bark.
Deformities such as swelling and cracking on stems and branches of woody plants usually indicate the presence of some unseen evil attacking the delicate meristematic tissue lying just beneath the bark. A few deft cuts with a scalpel revealed a gorgeous but dastardly larva of a flatheaded borer tunneling within the pithy center of the stem. Flatheaded borers are larvae of a remarkable group of beetles known as buprestids or metallic wood boring beetles, so named for their luminous, metallic exoskeletons. While the adults levy no particular offense other than to nibble a few leaves, their youngsters are real trouble makers and some of the most devastating pests of woody plants. We visited another deadly metallic wood boring beetle, the emerald ash borer, in a previous episode of Bug of the Week.
As the borer feeds beneath the bark, it destroys vital tissues of the rose.
Last spring adult rose stem girdlers emerged from the bark of infested rose stems, mated, dined on leaves, and deposited eggs on the bark of small rose stems. Eggs hatched and tiny larvae, called flatheaded borers, entered the tree and fed on nutritious plant tissues creating spiraling galleries just beneath the bark. In response to this assault, the stem of the rose swelled in an apparent attempt to defeat the attacking borer.
Not much to look at, but the flatheaded borer larva can really damage a stem.
As larvae feed and grow, accumulating damage weakens the stem making it susceptible to bending or breaking. Disruption of the normal movement of nutrients up and down the stem may cause the cane to die above the telltale swelling that marks the site of the beetle’s attack. With the return of warm weather in a few months, borers will complete their development, pupate in chambers in the stem, and emerge as adults to repeat this circle of life. The rose stem borer probably arrived in our country from Holland back in the day when Warren Harding was our president. This scoundrel is now found in many states in the eastern US and also in Utah where it is a major pest of raspberries. What should you do if you discover the nefarious borer in your favorite roses? Do not despair. Winter is an excellent time to deal with this villain. Simply grab your pruners, cut the damaged stem a few inches beneath the swelling, and destroy the cutting along with the borer in a safe and environmentally acceptable way, no thermonuclear devices please.
References:
Bug of the Week thanks Roger for providing specimens and inspiration for this episode. The awesome “Guide to Insect Borers in North American Broadleaf Trees and Shrubs” by J.D. Solomon was used as a reference for this episode. To learn a bit more about this insect, please visit the following web site.