Diatomaceous earth is a fine white powder composed of the fossilized skeletons of microscopic algae called diatoms. They are extremely sharp, like glass, and that’s precisely the utility of diatomaceous earth in the gardening world: you apply it where harmful crawling insects are likely to be found. When they come into contact with the product, it causes small abrasions in their body and the pest gradually loses its body fluids, dehydrates and dies. Despite claims to the contrary, diatomaceous earth does not kill mollusks (slugs and snails), but they find the particles irritating and will hesitate to cross a barrier of diatomaceous earth.
Note too that diatomaceous earth is considered biological pesticide, so safe for the home vegetable bed. Even so, use it judiciously, only when you have a pest problem you absolutely need to solve, as it can harm beneficial insects. For example, avoid getting it on flowers to avoid harming pollinating insects. And when applied to soil, it can harm earthworms.
Diatomaceous earth is most often sprinkled directly onto the foliage of infested plants to treat leaf-eating critters like potato beetles and inchworms.
It is also applied to the soil around the base of plants as a barrier to prevent crawling insects and mollusks from reaching the plant. Don’t mix diatomaceous earth into the ground, however: it must remain on the surface to be effective.
It is important to understand that this pesticide is most effective in dry weather: it loses effectiveness once it is moistened. So water first, let the plants and soil dry, then apply it.
And rain and even dew will also render diatomaceous earth ineffective: it will then just dissolve away into the soil below. It is therefore best to apply it when no rain is announced for several days.
Keeping Insects Outdoors
Where diatomaceous earth is most useful is in and around buildings. In such spots, it is dry all the time, so the product can remain effective for months. Apply it to cracks and fissures in your house, around windows, and other places insects (ants, earwigs, crickets, cockroaches, etc.) might enter the house. Obviously, it would be more effective to properly caulk those entries to prevent insects from entering once and for all, but diatomaceous earth still makes a good stopgap measure.
If you’re wondering if such a simple organic product could possibly be effective, it is worth noting that is widely used as a barrier pesticide by professional exterminators.
Diatomaceous earth is not toxic to humans, but can be irritating, so avoid inhaling it or getting it in your eyes. That’s way it’s best to wear a mask and goggles when you apply it. Outdoors, only apply it on windless days to keep it from drifting. And store it out of reach of children.
Not the Pool Product!
Many pool owners are familiar with diatomaceous earth, because it is used as a filtering product for home pools. But be aware that diatomaceous earth sold for pool use has been calcined (heat treated) and will not be effective against insects.
Where to Find It?
Almost all garden centers sell diatomaceous earth in one form or the other, but often the name is not in evidence. It is often sold under names like “insect dust”, “ant killer”, “crawling insect killer”, etc. Alternatively, the label may call it silicon dioxide (just another name for diatomaceous earth). So if you want to get the right product, either read the label carefully or ask a clerk to help.
No, diatomaceous earth will not solve all your garden pest problems, but it is an interesting and fairly safe product to add to your pesticide arsenal.