The main advantage of diatomaceous earth (DE) over many other forms of pest control is that it is an entirely natural, easy-to-use product. It has no impact on the environment and is harmless to most higher life forms.
Furthermore, as DE is a natural substance, insects cannot build up resistance to it when it is used over an extended period.
Ancient animals to the rescue
Despite its name, ‘diatomaceous earth’ is not soil, but the fossilised remains of single-celled algae-like plants called diatoms. These still exist and are one of the most common forms of phytoplankton. But those from which DE is manufactured existed millions of years ago.
As the great lakes dried up, the skeletons of these diatoms, which consist of 33% silicon, 19% calcium, 5% sodium, 3% magnesium, 2% iron, and other trace elements, left behind chalk-like deposits called diatomite. This is mined and ground up to produce a powder that resembles talcum powder, with a similar texture and consistency.
Safe for us, deadly for pests
Although it has a soft texture, DE is made up of tiny particles with sharp, jagged edges like shards of glass, which can be clearly seen under a microscope. These sharp edges cut into the exoskeleton of the pest, causing it to haemorrhage body fluid and ultimately die. If ingested, it shreds the innards of the pest. DE is effective against small, soft-bodied pests such as ants, aphids, earwigs, mites, slugs, snails, thrips and vine borers. Small caterpillars can also be controlled to a limited extent. DE is not harmful to earthworms or other soil micro-organisms.
Food grade only – and take care
DE is manufactured for a number of applications. For use against pests, it’s important to purchase ‘food grade’ quality DE, and not the type for swimming pool filters. The latter has a higher content of free silica, which can irritate the mucous linings of the nose and mouth of the person working with it. Even when using food-grade quality, it is advisable to wear a mask to avoid inhaling the dust.
How to apply it
Unlike synthetic insecticides, which yield almost immediate results, DE may take a day or two to eradicate the problem. During this time, some plant loss should be expected. DE can be applied as a powder or in wet form. Depending on the extent of the infestation and the size of the pest(s), DE can be sprinkled on or applied with a shaker (simply punch a few holes in the bottom of a tin).
Do not apply the dry powder if rain is forecast, as it will be washed off the plant. Rather use the wet form. Coat the plant just after watering or when it has rained, or else early in the morning when there is dew on the leaves. These create ideal conditions for the DE to adhere to the foliage, and maximise its effectiveness. Alternatively, mix a mug of DE in 2l of water, shake well and lightly spray the foliage.
Sap oozing from holes in trees, or frass (the solid excreta of insects) at the hole’s entrance can indicate a boring insect. Squirt DE into the holes and then seal them up.
Spread a layer of DE around the base of the plant. This will deter slugs, snails and other soft-bodied insects. Sprinkle DE around anthills to act as a barrier.
Paul Donovan is a biologist who advises farmers on the best ways to control crop pests.