Teague: Less-toxic ways to fight pests

FresnoJune 14, 2013 

Many home gardeners are becoming increasingly interested in using the least toxic methods to control for pest insects. Concerns about the long- term residual effects of broad spectrum chemical insecticides on the environment might have prompted gardeners to try less toxic and organic insecticides sooner, but they weren't always easily available, were often expensive, and efficacy was doubtful.

Less toxic pesticides are now available at most garden centers and nurseries, and their safety and efficacy are time tested. The four insecticides listed below have short term or no residual effect and, when applied properly, kill only the targeted insects, not pollinators or beneficial insects.

Bt or Bacillus thuringiensis is a naturally occurring bacillus that destroys the digestive system of all caterpillars, including tomato hornworms, cornworms and budworms (that eat out the unopened buds on geraniums and petunias).

The concentrate of Bt is mixed with water and, because it breaks down quickly in sunlight, should be sprayed at nightfall. It kills only the caterpillars that ingest the treated foliage and buds and is not toxic to humans, pets or birds that eat the dead caterpillars.

Neem oil does triple duty as an insecticide, fungicide and miticide. Made from the oil and the extracts of the neem nut, it kills a wide range of insects. The oil smothers soft-bodied insects (aphids, mealybugs, cotton cushiony scale), as well as the crawler stage of hard scales. It also kills the red spider mites that weave webs on the undersides of dusty leaves. As a fungicide, it kills the fungal spores of powdery mildew and black spot.

Neem oil concentrate is also mixed with water to be sprayed directly onto the pest insects or to drench fungus-infested green growth.

Spinosad is a naturally occurring soil-borne bacteria, a byproduct of fermentation. It overstimulates the nervous system of insects that ingest it, and they die of exhaustion within a day or two. Only insects that ingest plant material that is treated with spinosad are affected.

Spinosad is considered a broad spectrum insecticide since it kills a wide variety of insects — including grasshoppers, leaf hoppers, borers, aphids, leaf miners. The spray is highly toxic to bees, so treat with spinosad in the evening when bees are not active.

Diatomaceous earth — not the carcinogenic type used in pool filters, but food grade silicon dioxide — kills crawling insects such as ants, earwigs, cockroaches, silverfish and bedbugs. DE is an desiccant and an abrasive that scratches through the exoskeleton of pest insects so that they dry out and die within 48 hours.

Food grade diatomaceous earth is sprinkled into crevices and cracks and onto areas where the pest insects cross. Some formulations have a bait mixed into the product.

Always read labels and follow directions carefully for any pesticide. Some of the newer, less toxic insecticides can affect aquatic vertabrates (frogs, fish, etc.), some cannot be applied when temps are above 100 degrees, and some lose effectiveness over time or when stored in hot conditions.


Elinor Teague is a Fresno County Master Gardener. Send her plant questions at etgrow@comcast.net or features@fresnobee.com ("plants" in the subject line).

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