Borer-resistant rhododendron?

acer(6b western NC)February 19, 2008


We live in western North Carolina and have lots of rhodos in our landscape. Unfortunately we've had a serious infestation of borers lately and are loosing a lot of nice plants. I don't know why they're so bad suddenly, but I know of other folks with the same problem. Anyway, I'm wondering if some cultivars may be more resistant than others.

Also, what can we do to save what's left other than heavy pruning? It's hard to tell when a shrub is infested before it starts falling apart, and sometimes it hardly seems worth saving...



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rhodyman(SE PA, USDA Z6)

Hi Shannon,

The Rhododendron Borer is Synanthedon rhododendri. This clearwing moth lays its eggs on the bark of the shrub, frequently in a branch crotch. The larvae tunnel into the branch, severely weakening it structurally. Pale, spotted and wilted leaves appear in the branch above the mined area.

Branches attacked by borer will often break off easily in the hand, and the tunnelings of the insect can then be examined. Exit holes and sawdust provide additional evidence. The weakened branches should be pruned from the rhododendron, ideally before the larvae has matured and flown away. If rhododendron borer is more than a sporadic problem, you may want to use insecticide sprays in May when the adult is active, to prevent new infestations.

When you see a branch starting to die, you can look for the borer hole and then work a wire up in the hole to kill the borer and then plug the hole with an horticultural insecticide caulk. I found one a few years ago that is in a tube and is like bathtub caulk.

Damaged rhododendrons will try to come back. They have adventitious buds under the bark and on the roots that can form new branches after severe damage. Prune out the dead part back to where the borers entered the stem. Then treat the plant with care for about 12 months to see what will come back.

Good Luck!

Here is a link that might be useful: Rhododendron Borer Control - Penn State

    Bookmark   February 20, 2008 at 4:51PM
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Birdsong72(7/Northshore NJ)

While I don't have the problem with any of my rhodies, thatks rhodyman. Your posts are always written in an informed clear coversational style that makes it easy to read & understand.


    Bookmark   February 24, 2008 at 3:14PM
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None of my native rhodies have been affected, only cultivars and/or hybrids. These have almost all been killed.


    Bookmark   May 15, 2009 at 4:21PM
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