Crossville Chronicle, Crossville, TN

May 24, 2013

PLATEAU GARDENING: Watch for rhododendron borers and carpenter bees

By C. Rae Hozer
Chronicle contributor

CROSSVILLE — Inspect rhododendron shrubs showing wilted or dead branches. In early February I noticed one of the main stems on a rhododendron by my driveway had broken off. Thinking a photo might make a good illustration for an article on pruning to remove damaged limbs, I took a picture with my digital camera. However, when enhancing the image, two oval holes with the longer part aligned lengthwise on the stem caught my eye. One was right at the break, the other further down. Those holes looked like borer damage.

The chances of killing a borer with pesticides after it is inside the host plant are between slim and none. However, once the pest’s identity is known, learning about its life cycle may pinpoint one development-stage during which it would be vulnerable to controls and also indicate which life-form causes the most damage.

My research suggested that the stem broke as a result of injury by rhododendron borer (Synanthedon rhododendri) larvae.

Life Cycle: Adult rhododendron borers are moths which look like wasps. They have clear wings and black bodies with three yellow bands on the abdomen. They fly in daytime. Adults typically appear in late-May and early June. Adult females lay eggs on twigs and trunks of rhododendrons which hatch into small grub-like larvae with white bodies and dark heads. The larvae bore through the bark into the sapwood where they overwinter. There is one generation per year.

Controls: Verify rhododendron borers are present. Remove wilting or dead branches then look for larvae inside. Monitor for adult borers using pheromone traps. If evidence of rhododendron borers is found treat rhododendron trunk and branch surfaces with an insecticide spray when the adult moths are active (late May through June) at intervals recommended for that pest on the product label. Look for a pesticide containing the chemical Permethrin which targets borers and is safe for use on rhododendrons. 

Carpenter bees: When what looked like large bumble bees began flying around our entrance ramp a few weeks ago, I suspected they were carpenter bees (Xylocopa species). Then two little piles of sawdust appeared on the ramp, each below a round hole in the underside of the handrail above, which confirmed that diagnosis.

Damage: Carpenter bees buzzing around buildings seem more threatening and scary than they are in reality. The male adults act aggressive but do not have stingers. Females have stingers but rarely attack. The real damage is to wooden structures. Female carpenter bees chew holes in wood then create tunnels inside where they lay eggs. These nests can weaken the wood. Replacing handrails on the deck at the back of our house last year where woodpeckers riddled the boards trying to get into carpenter bee nests was costly.

Life cycle: Carpenter bees go through the complete cycle (egg, larva, pupa, adult) in from seven to 12 weeks. The adults mate in April or May. The female may bore new entry holes and galleries or use old ones. They overwinter as adults in old nests. There is one generation per year.

Controls vs. Management: Though there are pesticides and dusts labeled for use against carpenter bees available, most university publications recommend prevention. See the online fact sheet by the University of California online at www.ipm.ucdavis.edu/PMG/PESTNOTES/pn7417.html. These creatures pollinate crops and other plants making them of value. Since honey bee populations are shrinking and more pollinators are needed, preserving rather than eliminating carpenter bees may be the better course of action.

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Plateau Gardening is written by Master Gardeners for gardeners in Tennessee’s Upper Cumberland Region.  Contact UT Extension Cumberland County at P.O. Box 483, Crossville, TN 38557 (484-6743) for answers to horticulture questions, free publications and how to become a Master Gardener. Send email comments or yard and garden inquiries to Master Gardener Rae, MGardenerRae@frontiernet.net.