Spider mites in raspberries

A local gardener came to me today with some sad looking raspberry leaves. They were wilted, had yellow spots and had curled and shriveled up. My first thoughts were that some pesticides had inadvertently drifted into the raspberries, but upon further investigation another culprit was identified: spider mites.

The twospotted spider mite (Tetranychus urticae Koch) is the most common mite that attacks raspberries. Spider mites are microscopic arthropods, only 1/60-inch, 0.5 mm, long. Populations can build quickly during the heat of the summer. They typically live on the underside of the leaves, where they can build up their colonies under protection from the environment and predators. They suck the sap from the leaves which causes a fine, white to gray stippling (fine dots).

They can reproduce a new generation in as little as 10-14 days during the summer.

Early in the spring, spider mites begin feeding on ground vegetation and then climb into the raspberry plant as the cover dries out and the population increases. Leaves are extremely sensitive to spider mites feeding on them. Reduced photosynthesis decreases the vigor of the cane, decreases yield, and weakens primocanes (this year’s growth, next year’s fruiting canes) making them susceptible to winter injury.

Monitor and Control

Monitoring is a key element of control and prevention of spider mites. Monitoring should begin when you first notice feeding injury. Place a light-colored sheet of paper under the leaves and shake the lower canes and leaves. Look for ‘moving specks’ with a hand lens (10-30x magnification) to observe mites and natural enemies. Look for early leaf bronzing on lower canes.

Management and control of spider mites requires a three headed approach. Cultural, biological, and chemical control methods should all be used to effectively control spider mites.

Cultural Control

Spider mites prefer to feed on common broadleaf weeds like field bindweed, common mallow, and knotweed. Plant a hardy grass or vegetation mix between rows. Avoid disturbing ground cover or spreading dust onto foliage. These disruptions promote mite dispersal into raspberry plants.

Ensure adequate watering and fertilizer applications are made to minimize stress by raspberries. During hot periods use overhead sprinklers to cool and moisten foliage and berries. Utilize shade to cool plants. Select cultivars with hairy leaves to deter mite feeding.

Biological Control

Promote natural predators to spider mites in your garden. The western predatory mite, (Galendromus occidentalis) is the most important predatory insect. Other predatory insects include western flower thrips, minute pirate bug, big-eyed bug, the spider mite destroyer ladybeetle and lacewings.

Avoid the use of toxic insecticides and miticides to protect these natural enemies. Unless you are using a closed production system like a high tunnel or greenhouse, the release of supplemental insectary-reared predators is not useful.

Chemical Control

Spider mites develop resistance to miticides, so it is important to rotate chemicals. Because the same chemicals that kill spider mites will also kill beneficial predator insects, always use the least disruptive chemical formulations such as neem oil, horticultural oil, or insecticidal soap.

A variety of miticides are registered and available for use including etoxazole (Zeal), hexythiazox (Savey), acequinocyl (Kanemite), bifenazate (Acramite), and fenbutantin-oxide (Vendex). Savey and Zeal are mite growth inhibitors and are only effective against mite eggs and nymphs. They should be used when adults first appear and are ineffective if the population has grown too large. Acramite, Kanemite, and Vendex are active against adults and nymphs. Vendex is toxic to predators; avoid it when predators are active. Always read and follow chemical labels.