It’s October, and soon it will be time to bring potted plants indoors to protect them from freezing weather. Over the spring and summer, harmful bugs may have gone unnoticed because they were kept in check by natural predators, but if the pests hitchhike with the plant into the house, they may spread to other pots. Spider mites can be deadly to susceptible ornamental plants.
Webs are sometimes seen on mite-infested plants. Technically speaking, mites are not true insects, but are arachnids and in the same class as spiders. Spider mites are smaller than the head of a pin and may be brown, red, green, or cream in color. If shaken out of a plant onto a piece of paper, they will look like tiny moving dots. Mites can ride the wind to colonize other plants.
Leaves of infested plants have tiny white spots where mites have sucked out the chlorophyll, and the leaves may turn yellow or bronze, shrivel up, and drop off. Some plants, like potted fuchsia, are particularly susceptible to damage from mites, while other plants, such as peace lilies, are more resistant.
Mites live in colonies on the underside of leaves and can reproduce every week to ten days. They like hot, dry weather, but also live through the winter. Because they multiply so fast, mites may not be discovered on a plant until the plant already has suffered significant damage.
Take steps to combat spider mites:
- Keep plants well-watered and fertilized to reduce plant stress. A healthy plant is more resistant to spider-mite infestations.
- Wash the mites away. Spray plants forcefully with a garden hose or a shower nozzle, especially on the underside of leaves, to physically remove the mites and their webs. Repeat as necessary.
- Apply insecticidal oil. Oils that are directly sprayed onto a mite will either suffocate it or penetrate its body to destroy it. Oil formulations that are recommended for use during times (generally the winter) when a plant is dormant are referred to as dormant oils, while those used in the spring or summer, when a plant is leafing out or blooming, are referred to as summer oils. Oils are helpful against most mites and scales, are less harmful to beneficial insects and mites than other insecticides, and are relatively safe to birds, humans and other mammals. Unless the spray comes into direct contact with the mite, however, it will not be effective. Any dead leaves and debris should be removed from beneath the plant to eliminate their hiding places.
- Apply insecticidal soap. Insecticidal soaps are potassium fatty-acid soaps sprayed on plants that control pests by penetrating their cell membranes so that the contents leak out and the cells dry up. Soaps are effective only when directly sprayed on the insect or mite. Unfortunately, soaps can also produce yellow or brown spotting on the leaves or cause the leaf tips to wither, especially if they are applied too frequently, or if the plant is especially sensitive, so it is best to test the soap on a small portion of the plant before using it.
Commercially available oils are usually made from mineral oils. A gardener may easily make a homemade version from vegetable oils, and cotton seed oil, canola oil, or soybean oil are best for the job. Garlic extract, clove oil, mint oils, rosemary oil, and cinnamon oil may be added, as they have been found useful in controlling mites. A commonly used oil mixture is made of about four parts oil and one part dishwashing liquid, which is vigorously shaken to thoroughly blend the contents, and then diluted with water for spraying: one part mixture to 50 parts water. Aim for the underside of the leaves where mites congregate.
The type and amount of dishwashing liquid used in a homemade recipe for insecticidal soap is very important. It should not contain a degreasing agent. Formulas generally call for a mixture of 2% soap in in distilled or soft water (five tablespoons of the soap to one gallon of water), although a 1% solution (two tablespoons of soap to one gallon of water) may be sufficient against mites and gentler on the plant. The use of hard water in the mixture can render it ineffective. Baking soda may be used as a water-softening agent. Spray on the undersides of the leaves.
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