Mites destroy arborvitae

Q. We are seeing problems on arborvitae this summer – some sap on the branches and bronzing on the leaves. Is this an insect issue? A result of drought conditions?

Tony from Austintown

A. Tony brought in samples of arborvitae that were not thriving, had damage, and looked brown. Although we are in a drought, we looked for clues as to what could cause this kind of damage on just one plant in a line of plants. Upon inspection (shaking the branches onto white paper) we found spider mites.

Spider mites are tiny eight-legged animals closely related to spiders, and can be a real threat to many species of plants. In this case, his arborvitae were in decline. Spider mites can build rapid populations in dry weather. They are hard to see, but the evidence is clear under a simple microscope or by using a hand lens. They feed on leaves, piercing and sucking the green out of leaves, white to brown areas left behind.

Inspection is a must to identify and treat for these deadly pests. A white sheet of paper (index card) is held under a suspected branch. Vigorously tap the branch and the crawling insects will show amid the debris.

There are warm-season and cool-season mites. Warmth (90-75-degree day/night temperatures) can build up large numbers, especially plants with a southern exposure that are sheltered. When found in spring or fall, controls are vital, as these overwinter and explode in numbers the following year.

Two-spotted spider mite – attacks over 150 species. They are white/green with dark areas on each side and feed on young tender leaves. They overwinter in protected areas.

The European red mite attacks flowering fruit trees, deciduous trees and shrubs of the rose family. They are brownish red in fall and overwinter as adults in bud scales and bark.

Oak red mites attack oak, birch, beech, chestnut, maple and hickory. They are small and red, usually on the upper part of the leaf. They deposit red eggs on top leaves. Nymphs are oval and grey/green.

Inspect every two weeks for damage and webbing, taking your index card to tap suspected branches. Most deciduous plants have the hot summer mites, with conifers affected by the cool seasons of spring and fall. Recognize the early feeding damage. These mites thrive on stressed plants. Make sure water and light are adequate. Do not over- or under fertilize. If you must, spraying vigorously every two weeks dislodges adults.

Use of biorational pesticides is recommended. Microbial insects for caterpillars (Bacillus thurnigiensis), Dipel, thuricide, and spinosad, as well as insecticidal soaps and sprays go a long way to gaining control. With the insecticidal soaps and sprays, be sure to thoroughly cover your plants.

Stephanie Hughes is an OSU Extension master gardener volunteer. Call the office hot line at 330-533-5538 to submit your questions. Clinic hours are Mondays and Thursdays from 9 a.m. to noon through October.