While you can kill a lot of garden and backyard pests with diatomaceous earth, bees are not usually one of the creatures you'd associate with the term "pest." That said, if you're allergic to bee stings or have some other reason for wanting to evict them from your living space, diatomaceous earth (DE) can indeed harm them. Whether it's the most effective way to tackle the problem is a different question.
How Diatomaceous Earth Kills Bugs
To the naked eye, or to the human fingertip, diatomaceous earth appears to be just a chalky powder. It's actually made up of the fossilized skeletons of ancient, single-celled creatures called diatoms. If you look at it under a strong microscope you'll understand at a glance why it works so well: it's like microscopic razor blades, or shards of glass.
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For a slug crawling across a protective barrier of diatomaceous earth, or a voracious insect pest (at either the adult or larval stage) encountering it on your plants, this is the proverbial "death by a thousand cuts." The microscopic shards slash the hard exoskeletons of insects and the tender underbellies of snails and slugs, causing them to die by dehydration.
Diatomaceous Earth, Bees and Pollination
Diatomaceous earth is usually applied as a ring around plants to protect against slugs and snails, or dusted directly onto the plants to kill insects. It washes off with every rain, and will need to be reapplied once the plant dries. To target bees you'd dust the blossoms of plants the bees visit frequently, whether that be your garden plants or any wild flowers (ie, weeds) in your yard. Manufacturers typically assume you want to avoid hurting bees and other pollinators, so you can guide yourself by reading any bee-related warnings and then doing the opposite.
There's some question as to how effective DE actually is against bees. The fuzziness of many bee species gives them some protection, as their hairs keep the tiny shards away from their bodies. They do groom themselves though, much as cats do, and may fatally injure themselves in the process. There's also some evidence that if blossoms are treated with diatomaceous earth, bees will simply avoid them. If a deterrent is all you're looking for, that might be enough in itself.
A larger issue is that any time you target bees, other pollinators (and beneficial insects in general) can become "collateral damage." Killing these pollinators will sabotage your own garden, and can cause wider damage throughout your local ecosystem. There are alternatives that may be better options for you.
Alternative Treatment Options
Part of the appeal of DE is that it's relatively low-impact on desirable species, so its impact on pollinators in general (when used against bees) might come as an unpleasant surprise. One option is to change tactics and opt for a more conventional, but more targeted, pesticide. Spinosad, for example, is lethal to bees but generally innocuous for other beneficial insects and other pollinators.
Another option is planting a border of bee-friendly plants around the perimeter of your property as a "trap crop." Bees are pragmatic; they'll seldom fly past a premium food source to browse the leftovers in the rest of your yard. If you're allergic to bee stings, or just concerned about them, you could also consider screening your back deck, or setting up a screened-in gazebo where you can enjoy your outdoor spaces without fear. That way the bees can fulfill their niche in the environment without being a threat to you.
Are Your Bees Even Bees?
One final point to consider, before you tackle the bees in your yard, is to make sure that they really are the pest you are concerned about. There are plenty of colorfully striped flying insects: many are bees, but others are various wasps, hornets or even sneaky flies that have adopted bee-like coloration to deter predators. Only a few are aggressive, and those aren't bees. In most states, your local extension service has resources to help you identify the specific critter you're dealing with.
Learning the true identity of your invader can give you customized options for keeping them away. Knowledge is power, and knowing which insect you're really trying to deter can be the difference between success and failure.