How to Use Diatomaceous Earth for Tomatoes

By Caryn Anderson

Apply food-grade diatomaceous earth to your tomato plants (Solanum lycopersicum) as a form of organic pest control. This natural, nontoxic substance won't hurt children or pets, but it will kill the blight of pests that can quickly destroy this beloved warm-season annual plant. Because diatomaceous earth also kills beneficial insects, only apply diatomaceous earth to clear infestations of harmful pests. Store any unused portion in an airtight container placed in a dry area.

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Diatomaceous earth allows you to maintain pest-free plants without pesticides.

How Diatomaceous Earth Works

Diatomaceous earth is an abrasive dust made from the crushed silica-rich shells of diatoms, a type of prehistoric crustacean. The sharp edges of each particle puncture insect shells and absorb the protective waxy coating on the outside of insects' bodies. As a result, insects become dehydrated and die. Additionally, insects that ingest diatomaceous earth suffer reproductive disruption, along with digestive and breathing problems that kill the pests. Diatomaceous earth works best in dry conditions when pests are most likely to become thoroughly covered in the dust.

Applying Diatomaecous Earth

Don't confuse food-grade diatomaceous earth with the stuff used to filter pools. That form of diatomaceous earth is chemically treated and poses a risk to your respiratory system. Even though the food-grade material is safe, wear a dust mask while applying diatomaceous earth to tomato plants. Although diatomaceous earth is nontoxic to humans, it can irritate your mucous membrane if you inhale the particles.

Apply a thin layer of dust around the base of each tomato plant to help control pests, such as slugs, that dwell in the soil. Add the dust to a bulb puffer to control foliage-dwelling pests such as aphids by coating the tomato plant's foliage -- leaf tops and undersides. Although you can apply diatomaceous earth at any time, dusting the plants right after it rains or while the tomato plant is still dewy helps the dust stay in place. Reapply the diatomaceous earth after it rains or if you no longer see it on top of the soil or foliage. This organic pest control product is generally most effective when you can see that it's present.

Controlling Tomato Pests

Before you treat the tomato plant, be sure that the insect you've seen is actually a harmful pest. Although diatomaceous earth doesn't affect earthworms, it can kill beneficial insects that help naturally control the harmful pests that destroy tomato plants.

Tomatoes are commonly affected by aphids, cutworms, flea beetles, hornworms and nematodes. Diatomaceous earth can also help provide control against soft-bodied pests such as caterpillars, leaf hoppers and thrips. Other garden pests that can be controlled using diatomaceous earth includes slugs, snails, flies, mildew, gypsy moths, codling moths, pink boll weevils, mosquitoes, earwigs, twig borers, mites, cockroaches and lygus bugs.

Symptoms of Pest Problems

Watch for signs of a pest infestation by inspecting the plant's foliage every few days. For example, although the tomato hornworm can be tough to spot thanks to its green body, the holes that it leaves in the foliage are unmistakable. Nematodes are another destructive problem. These microscopic, soil-dwelling worms cause bumps that prevent the plant from being able to perform photosynthesis or absorb nutrients.

Aphids are another common tomato pest. They cause distortion, discoloration and curling of the tomato plant's leaves. Holes chewed into the tomato fruits may be a sign of slugs, while small holes chewed into tomatoes that collapse in your hand is a sign of tomato fruitworms, which destroy the fruit if left untreated.