The Q&A Archives: Hornet Problem Within Boxwoods, Rhododendrons, Azaleas And Hollies.

Question: Since the summer of 2001, I've noticed an increase in the hornet population in the above mentioned shrubberies surrounding our home. Most of the activity is on the south side of the house where it's very warm in the mid to late afternoon. Through the peak summer months, several specimens of each shrub have had 10-20 yellow-bodied insects swarming around the perimeter of the foliage, they land and then make their way to the interior of the shrub. Some of the "infected" shrubs are showing serious browing of the foliage. Two rhododendron are dropping leaves like crazy, 2 boxwoods have broad, brown spots dotted across their surface and a large 9 ft. holly, while not exhibiting browning, does have similar insects buzzing in and around it's surface. I would especially not want to lose that. We've had a drought-inflicted watering ban in effect here most of the summer so it hasn't helped the condidtion of the plants much. I can't seem to determine where those insects are going inside the shrubs.

Is their a variety of hornet that nests in the interior or even below ground level, that could do physical damage to a weak specimen?

Answer: There is an insect called the giant hornet (Vespa crabro germana) that will occasionally drill into the branches of lilacs, rhododendrons, and boxwoods, however I did not find it listed as a holly pest. The drought would also cause the symptoms you are describing, so there may be a combination of forces at work here since plants will die back and/or drop foliage as a protective measure in drought.

The hornets can be deterred by applying carbaryl (for example, Sevin) to the actual branches deep in the shrub where they are drilling (it is not particularly helpful to spray the foliage since that is not where they are going). This is best done in July, however since they are still active it is probably worth a try now to try to limit their presence next year.

If they have caused serious damage the branches may eventually die off and need to be removed neatly. On a more encouraging note, these shrubs are able to regrow remarkably quickly due to their large established root systems and any dieback will help offset the effects of drought.

I hope it rains soon.

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