Spider Mites

When you have been gardening for a while, you start to notice that the weather affects all kinds of things in the garden.  It is easy to see the effects of the heat and drought on our plants as they wilt in the sun, but the weather also affects disease and pest problems.

During a rainy season, I know that we will get a lot of calls about fungus like take-all-root-rot and large patch in lawns.  Lots of rain and high humidity makes the perfect conditions for those fungal pathogens to multiply.

The heat and drought might make you think that all the pests are dying or in hiding until it cools off, but one pest really makes itself known during hot weather.  Spider mites increase in hot weather, and that is certainly true this year!

Spider mites, Tetranynchus urticae Koch, are very small and live most of their life on the underside of plant leaves.  The adult mites are 1/32 inch or less, so you have to look closely or use a hand lens to spot them.  I usually spot the damage to the leaf before I see the spider mites.

The adults use tiny toothpick shaped mouthparts to pierce plant cells and suck out the contents.  The damaged cells will look like yellow to bronze colored dots on the leaves, usually centered around the leaf midrib and larger veins.  Leaves with a heavy infestation of spider mites will turn yellow, then bronze and fall off.  When a heavily infested plant starts to decline, the mites will spin silk threads and use them to “fly” in the wind and disperse to other plants.

Female mites lay eggs on the underside of plant leaves, and a six-legged nymph will hatch from the egg.  The nymphs develop two more legs, then molt two more times to become an adult mite.  One generation from egg to adult can occur in five to 20 days, and we can see many generations per year.

Spider mites can affect more than 180 agronomic and horticultural crops including cotton, small grains, vegetables, and ornamental plants.  I usually see them on tomatoes, strawberries, rosemary, marigolds, roses, junipers, and fruit trees.

Spider mites can be difficult to treat, especially if you have a large population.  Insecticidal soap, neem oil, and Spinosad are some good options that are less toxic to beneficial insects.  Multiple applications might be required for severe infestations.  Be sure to follow the label instructions when you apply these products, so you do not damage tender plant growth.  If the infestation is very severe, you can use stronger products that contain pyrethrin, imidacloprid, or tebuconazole.

For more information about lawn and garden topics, contact Kate Whitney, Williamson County Horticulture Extension Agent, at 512-943-3300.  If you are interested in gardening classes, you can register for the Lawn and Garden 201 class starting on August 30.  The four-part series will cover fall vegetable gardening, fruit trees, composting, and tree care.  Register at https://williamson.agrilife.org/.



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