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Non-Chemical Control of Plant Pests: Spider Mites


Non-Chemical Control of Plant Pests: Spider Mites
Photo by Jarmo Holopainen

One of the most common mite pests of plants in the U.S. is the Spider Mite. These mites appear as tiny dots moving around plant surfaces. As the name of this species implies, mites are arachnids, as are ticks and spiders. Because the tiny mites will spin webs that become evident with high populations, they are sometimes called web-spinning mites.

The most common of these is the two-spotted spider mites, but the Pacific spider mite and strawberry spider mite also occur in the U.S. However because the biology, habits and damage of all these species are similar, so too are the recommendations for their management.

Identification: At only 1/60 to 1/20 inch long, the tiny mite is very difficult to see. At less than 1/20 inch, the female is the largest of its species. When magnified, it is shown to have eight legs (except the immature which have only six legs); they are red, brown, or green with translucent and cream-colored eggs, depending on their age. Populations can grow quickly as the females deposit their eggs on leaves or bark, or in the mite webbing, and these can mature to reproductive age within 10 days.

Plant hosts: Spider mites live in colonies on plant foliage and they will attack and damage a wide variety of indoor as well as outdoor plants, as well as trees, shrubs and weeds. They also feed on fruit trees, vines, berries and vegetables.

Damage: With its piercing/sucking mouth, the spider mite sucks the fluids out of plants in order to feed. With colonies containing hundreds of mites, a single colony can be of significant damage to a plant. The first sign of damage will be numerous small dots on the leaves that look like pepper. If not controlled at this point, the plant will begin to lose chlorophyll, causing it to yellow or brown and leaves to fall off.

Indoors and in areas that remain temperate throughout the year, spite mites can be a year-round pests. Outdoors in cooler regions, the mites overwinter, then reemerge to feed and lay eggs in the spring. They thrive best in hot weather, thus are most common and most reproductive during the hot summer months.

Signs: Red dots of active mites, large numbers could cause a silk-like webbing on the plant. One way to determine if a plant is infested is to hold a strip of paper or fabric beneath the plant foliage while gently shaking or tapping the plant. If small spots fall from the plant and begin moving around the paper/fabric - the plant has mites.

Additionally, the spider mite's webbing is the best way of distinguishing it from other plant pests, such as aphids or thrips.

Non-Chemical Control

Wash entire plant with soapy water (approx. 2 tsp mild detergent per gallon of water).
  • Plants that can withstand high water pressure can be hosed with a forceful spray to knock off the mites and destroy webs. Pay particular attention to the underside of the leaves. This can be done as often as can be withstood by the plant.
  • Mites thrive in dry, hot conditions, thus locating the plants in cooler areas, keeping them well-watered, and regularly misting the plant and its foliage can decrease mites and activity.
  • Keeping dust and dry ground in the area to a minimum - outdoors and in - can also reduce the mites potential to thrive and move into the area.
  • Inspect new plants to ensure they are mite-free before placing them with established plants. It can be helpful to quarantine them first to ensure no less-visible eggs were brought in with the plant.
  • The mite's natural enemies are lacewing larvae, pirate bugs and some species of ladybugs, thrips and other mites.

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