How Do Organic Pesticides Work?

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It’s always best when you know the how and why of what makes pesticides work, so I thought I would compile a list of the most popular organic options. Here are a few of the names you’ll commonly see referenced in the battle of the garden pests.


This is derived from a tree (Azadirachta indica) in India. Even though it had been used regionally for years for its antiseptic properties in the 1920s research demonstrated it was effective in repelling insects. When used against insects, the chemicals in the Neem oil are taken into the body and interfere with the hormones. The insect forgets to eat, and it reduces reproduction.


Made from a type of chrysanthemum, pyrethrin is a fairly powerful organic pesticide. It also has a good knockdown quality when you use it against wasps or other stinging insects. There’s also a synthetic version called pyrethroids that tend to last longer than the natural option. Both are used are a wide range of insects, including mosquitoes. As with any insecticide, be very specific when you use it because it repels (and kills) beneficial bugs as well as the ones eating your plants.

Diatomaceous earth

Using fossils in your garden might seem like the wrong thing to do, but that’s what makes diatomaceous earth so effective. The microscopic edges of the fossilized algae-like plants cut the devil out many insects. It’ll affect soft-bodied insects, as well as the innards of others that ingest it. It works well against beetles, aphids, flies, slugs and many others. Just be sure you use the food grade DE and not the stuff they use in swimming pools because the swimming pool DE is chemically treated and can cause respiratory issues.

Insecticidal soap

The insecticidal soap frequently used to battle pests is made of potassium salts and fatty acids. When it’s sprayed directly on soft-bodied pests (such as caterpillars, aphids, whiteflies, etc.) the fatty acids permeate the pests’ skin and the potassium salts dry it out. It is only effective when it’s in contact with the insect, plus once it dries out the pest can crawl over and through it without an issue. It does work well when you can see the little nasties on your plants and is satisfying to spritz them.

Bacillus thuringiensis (Bt)

Unlike the insecticidal soap, Bt has to be ingested to work. Once it’s in the gut, it becomes active and attacks the cells in the gut wall. This results in much slower growth and eventual death. When the insect dies, the contents of the gut (including Bt spores) spill out and are consumed by the next insect.


This is a microbe (Saccharopolyspora spinosad) found in the soil. It affects the insects in the gut, like Bt, but also the nervous system. This means the insects will start to feel the effects within a few hours, and will die in a couple of days. It also lasts a week, which is considerably longer than Bt, so it requires fewer applications. Spinosad is a relatively new insecticide, but it works well against some pests, such as the Colorado potato beetle and European corn borer, that are resistant to other treatments.

Hopefully this gives you an idea on how and why a handful of the organic treatments work to help you better understand which one to choose. Just remember to use them judiciously to prevent killing the beneficial insects and pollinators that are so important to your garden.

(The photo is copyright of bstarz01 from istock because, thankfully, we don’t have these awful things on ours this year.)

Meet Amy Grisak

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