Diatomaceous Earth is consistently misused. But despite the negativity in this article’s title, we’ll begin by explaining the positives, before focusing on the mistakes.
Diatomaceous Earth (DE) actually works. It actually kills insects. It does so through one and maybe two mechanisms. First, DE is made up of (usually) ground up diatoms, which are essentially the hard shells left behind by certain algae (this is explained in detail here). They are primarily made of silica. When in contact with some insect species, the DE absorbs lipids from their exoskeletons causing them to desiccate (die by drying-out). That mechanism is fairly well researched. The second mechanism is believed to work, though the data is less thorough. And that’s not because it might not be true, it’s because it’s a hard trial to set up: in certain forms (shapes) DE can be sharp, and thus can kill invertebrates by actually penetrating the exoskeletons or soft tissue (slugs and snails). Your typical off-the-shelf DE will likely not be formulated for this secondary mechanism.
So – it works. DE applied to some invertebrates will kill them. But here is where it gets complicated.
SLUGS: It can kill slugs. BUT Diatomaceous Earth must be dry for it to work. Since slugs and snails will be where it is wet, DE has never been an effective control for slugs and snails. This has been proven time and time again. Instead, do this.
OUTDOORS: Therefore, outdoor applications are of limited use. You must apply them during dry weather, and where it will not get wet from dew, irrigation, or rain. So, sprinkling it around a plant is useless if it gets damp. Indoors in potted plants it might have a much better success rate.
CHECK THE LABELS: Some formulations are combinations. One product is Diatomaceous Earth with Orthene. So, while you might think you are applying DE for the health of your garden, you might be killing everything with a harsh insecticide.
CONTACT: I see people regularly recommend DE for aphids. Not only do trials show minimal to no effect on aphids, but the problem is also how you get it on them. If you sprinkle it on the tops of leaves you will miss the aphids under the leaves. Aphids don’t tend to move much, so the application is a total waste. For aphids, try this.
NON-TARGET: Just like the chemicals that are being banned, DE has non-target implications. This means while you are using it to kill cutworms (for example), you will be killing the beetles that eat cutworms. Just like chemical applications if you kill the good bugs as well you end up in the pesticide cycle. That means, you strip the plant of pests and predators, but the first thing to come back is the pest, but without the presence of predators. That means you are stuck reapplying the chemical, DE, or having to buy beneficial insects to finish the job. That might make sense you to, but studies looking at the pesticide cycle show that just starting with beneficial insects (whether you buy them or encourage them naturally) saves your plants better than engaging with a non-target pest control.
In a petri-dish Diatomaceous Earth performs well; You put an insect in the dish, cover it in DE and it dies. But the real-word applications are limited. It cannot get wet, or even damp; it cannot get fogged to reach under leaves; and it will kill good bugs as well. Fortunately, it has no toxicity and is generally safe to work with. So while it is limited now, and we are faced with greater needs to pest control, companies will be working on different, more successful formulations to try to take advantage of this relatively inexpensive means of pest control. In the meantime, do what nature intended and employ beneficial insects.
Here is an interesting source, that looks at how DE can be successful: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8706096/
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