Extension Master GardenerSM Volunteers of Buncombe County

rhododendron

Act Now to Protect Rhododendrons from Stem Borers

Look for signs of stem borers on rhododendrons, azaleas, mountain laurel, and blueberries
Adult borers—slender long-horned beetles—emerge from hibernation in the soil at the base of these shrubs, climb up stems, and lay eggs under the bark. On hatching, the young larvae bore into the stem and then turn downward, headed for the soil where they’ll spend the winter. Everything above the entry point on the stem will gradually wilt away.

In addition to rhododendrons, the stem borer, Oberea myops Haldeman, also attacks azaleas, mountain laurel, and blueberries. It is alternately called the rhododendron stem borer, azalea stem borer, or blueberry stem borer, depending on host plant affected.

Rhododendron stem cut away to chow tunnel of stem borer, Oberea myops Haldeman.
Rhododendron stem cut away to show tunnel of stem borer, Oberea myops Haldeman.

Symptoms
Symptoms to look for include a small hole in a stem and frass—fine sawdust-like pellets—on stems, leaves, or underneath the shrub. Look above where you see the frass and you’ll find the hole where the borer pushed these deposits out of the stem. Or, worst case, you’ll see a stem with drooping leaves. These are all signals that the borer is somewhere inside the stem below that hole. 

Solutions
From the hole, feel your way down the stem to the next joint, and with sharp pruners, cut off the section of the branch above that point. Make good, clean pruning cuts because in addition to searching for the insect, you are shaping the shrub.

Look at the cut stem. If there’s no hole in the center of the stem, you’ll know that the borer is still inside the stem above your cut. However, if there is a hole in the stem, the borer is deeper than your cut and you’ll need to repeat your pruning lower down. Dispose of the cut-off stem portions in a plant disposal or burn pile. 

Control and prevention
If you’ve had a history of borer problems on rhododendrons, azaleas, and mountain laurels, check your garden center for an insecticide labeled for borer control with a pyrethroid insecticide, such as permethrin or cyfluthrin. Apply after the new growth has hardened off about mid-May; apply again in early June. Be sure to read and follow the insecticide label for specific application instructions. For blueberries, pruning your plants every year in late winter is usually sufficient to control stem borers.

For more information, view the North Carolina State University Azalea Pest Management Calendar at http://www.ces.ncsu.edu/depts/ent/notes/O&T/calendars/note052.html and Pruning Blueberries at https://growingsmallfarms.ces.ncsu.edu/wp-content/uploads/2016/02/Pruning-Blueberrries-25JAN14.pdf?fwd=no. 

Article written by Glenn Palmer, Extension Master Gardener Volunteer.

Two Insects to Watch for on your Rhododendrons

Probably the first insect of the season to show up on your Rhodos will be the Black Vine Weevil…so-called because grape vines are among the various plants on which it’s found.   The adult is a small, 3/8″, black beetle that emerges in late spring.  They feed at night, chewing on the edges of the leaves and dropping to hide in the litter below during the day. 

courtesy of the University of Maine
courtesy of the University of Maine
the CULPRIT...courtesy of the University of Maine
the CULPRIT…courtesy of the University of Maine

You want to distinguish between damage that occurred last year and that caused by the current generation.  If you’re into nocturnal adventure, in late May start scouting for them with a flashlight after dark. Look for those notches  particularly,  on the lower leaves. Or, you could lay sheets of cardboard beneath the shrubs and check under the sheets for the beetles’ presence during the day. 

Unfortunately the vine weevil larva, small white grubs with brown heads, will soon be feeding on the plants’ roots, limiting their ability to take up moisture or nutrients. Your rhodos may be getting a double whammy and that would show up as chlorotic, yellowing, leaves.

If you do identify the black vine beetle as the culprit, Beauveria bassiana, a naturally-occurring fungus that causes a disease in the insect, is registered for its control. Read the label to see how to apply.

The presence of the Rhododendron borer will be signaled by the wilting, curling and eventual dying of the leaves, so by that time you see it, it’s too late to mount an effective attack. The objective is to catch the culprit before they can do significant damage and that means checking below the branches looking for frass or sawdust expelled by the insect as it bores his, or hers, entry hole.  The frass signals that the Borer is inside the stem above that point working its way downward. Identify the branch by locating the small, quarter-inch entry hole.

courtesy of University of Maryland
courtesy of University of Maryland

Now how far down has the critter gotten?  We can determine by cutting off successively lower parts of that branch until we find a section with no hole bored in the center.  The borer hasn’t gotten that far so now remove all of that branch down to where it joins the next stem.  Make a clean cut so that it will heal quickly. Dispose of the pieces removed.  Bury them, trash them, somehow get them out of the garden.

Rhodo Borer2

Article written by Glenn Palmer, Extension Master Gardener Volunteer.

Contact Us

Buncombe County Extension Office
49 Mount Carmel Road
Asheville, NC 28806
Helpline 828-255-5522

Events

Keep up with our events by subscribing to the blog or checking our Events Calendar.

Explore the Archives

Back to Top