Diatomaceous earth has increase effectiveness when applied dry. Dry diatoms can quickly get through the targeted areas, which consequently dehydrate the source of moisture for tiny living things like microbes, fungi, and aphids. But this doesn’t mean, you can’t apply it wet. It’s a matter of creativity and technique. You can mix diatomaceous earth with water and spray it on the stem of your plant. Just because it’s wet doesn’t mean it’s not effective. Applying it wet can help it stick to the plants so when it dries up, it dehydrates whatever inset is invading your plants. It’s a very efficient solution for aphids.