Left: A severe infestation of two-spotted mites on a calendula growing in Bonnefont garden. Right: A detail of the damage done by this common hot-weather garden pest, which sucks the chlorophyll from the leaves of the host plant. Photographs by Corey Eilhardt
The hot, dry weather that has us struggling to keep the gardens watered is all too welcome to the two-spotted mite, Tetranychus urticae, a worldwide pest of crop plants, ornamentals, and houseplants that is as much at home in greenhouses and apartments as it is outdoors. Two-spotted mites, along with other members of the Tetranychus family, are commonly known as spider mites. They are arachnids but are more closely related to ticks than to spiders.
The most common mite pest in the United States, the two-spotted mite has always been one of the most prevalent pests at The Cloisters; the heat and light that reflect off the brick and stonework in our enclosed gardens create just the sort of baking heat these destructive creatures thrive on. The depredations they’ve made in the last several weeks are visible on a wide variety of plants from catmint (Nepeta x faassenii) to lovage (Levisticum officinale), hops (Humulus lupulus) to pot marigold (Calendula officinalis), and meadow rue (Thalictrum aquilegifolium) to lady’s mantle (Alchemilla vulgaris). The mites themselves are small, but not microscopic, and can easily be seen with a hand lens (see example). Their tiny, translucent eggs are deposited in the webbing they spin on the underside of the host plant’s leaves. This webbing may be the first sign of an infestation.
There are several species of predatory mites that will take on the two-spotted variety, but not all of them tolerate the weather conditions in which their prey is most active. After cutting back some of the most severely infested plants, we’ve released a voracious and aggressive warm-weather predator, Phytoseiulus persimilis, on the leaf surfaces of the plants attacked by Tetranychus urticae. It’s a mite-eat-mite world, and we hope that P. persimilis will make short work of T. urticae. (See an image of the two species side by side in the Natural Enemies Gallery on the University of California’s Integrated Pest Management website.)
For more information on two-spotted mites and their biological control, see “Two-Spotted Mites and More” on the Green Methods website, and the reports posted by the Penn State College of Agricultural Sciences Cooperative Extension.
Tags: Bonnefont Garden, Calendula officinalis, catmint, hops, humulus lupulus, lady’s mantle, Levisticum officinale, lovage, meadow rue, Nepeta x faassenii, pest, Phytoseiulus persimilis, pot marigold, spider mites, Tetranychus urticae, Thalictrum aquilegifolium, two-spotted mite