Thursday, July 21, 2011

Mite versus Mite

Severe mite damage on calendulaClose-up of mite damage on calendula leaf

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.

—Deirdre Larkin

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Comments (2)

  1. Don Statham Says:

    I Love it- Mite eat Mite World! And chance of a Japanese beetle eat Japanese Beetle? I would take a Godzilla eating beetle? Thanks for you wonderful posts- I will keep my eyes peeled for these mites.

    Best Don

  2. Deirdre Larkin Says:

    Hi, Don—I think Delaware County is too cool and moist for two-spotted mites to prosper. The roses in my Catskills garden, like yours, have been hard hit by Japanese beetles this summer. Unfortunately, the best way to control them seems to be picking them off by hand . . . and I’m only there on weekends. You can go out picking after breakfast every morning! We haven’t had a Japanese beetle problem here at The Cloisters, but we did have a brief but intense invasion of Colorado potato beetles, who ate huge holes in the leaves of the many solanaceous species collected in our bed of plants used in medieval magic. The magic plants have been grow in the same bed for years, and I intend to move them to another quarter of the garden next year to help control potato beetles, which overwinter in the soil in their adult form and emerge to feed in spring.

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