The botanical equivalents of a preschooler's under-the-bed monsters, root weevils strike under the cover of darkness or soil. Often named for the plants they attack, these destructive pests feast on a range of trees, shrubs, annuals, perennials and edibles. Adult weevils consume leaves from spring to fall, but the real damage comes when their root-munching larvae get to work. Stop them in their tiny tracks with all-natural, nontoxic diatomaceous earth.
1. The Pests
The most common of these pests include the black vine weevil, the strawberry weevil, the sweet potato weevil and the woods weevil. Like all of their kind, the six-legged adult insects have hard-shelled, 1/4- to 1/2-inch bodies with curving snouts and elbowed antennae. Robotic movements distinguish root weevils from other nocturnal leaf-chewing beetles. After feeding, the all-female weevils descend to the soil and lay their eggs. Their legless, creamy-white C-shaped larvae hatch in about 10 days.
2. The Remedy
Battling root weevils with diatomaceous earth is akin to throwing dirt on a fire. The organic pesticide takes its name from diatoms, the fossilized remains of prehistoric phytoplankton. When pulverized into razor-edged dust, it lacerates larvae and dehydrates adult weevils by absorbing the oils in their protective shells. Surprisingly, food-grade diatomaceous earth is safe enough to use as a topical flea treatment on pets. Even so, it may temporarily irritate your lungs, eyes or skin unless you apply it correctly. Don't confuse the food-grade product with the diatomaceous earth used in swimming-pool filters; that product has carcinogenic properties resulting from exposure to high temperatures during processing.
3. Using Diatomaceous Earth
If you don't have a pesticide duster, apply the diatomaceous earth with a manual shaker. An empty baby powder container works well. To protect multiple plants, make a larger shaker with a coffee can. Punch holes in its base, cover them with a lid, fill the can and seal it with another lid. Remove the lower lid to apply the dust. When dry weather is in the forecast, put on protective clothing and eyewear, gloves and a dust mask. To kill the hatching larvae, sprinkle the diatomaceous earth around the bases of the plants. Hit the lower stems and branches to eliminate the climbing adults. A spray solution of 1/4-cup of the dust in 1 gallon of water also works. It covers the plants more effectively than the powder but doesn't kill the weevils until it dries. For ongoing coverage, reapply the dust following rain.
Diatomaceous earth also kills beneficial insects, including plant-pollinating bees and butterflies and the wasps, lady bugs and lacewings that prey on plant-eating pests. To minimize the risk to the friendly bugs, sprinkle or spray the diatomaceous earth at night. In addition to sparing the beneficials, you'll kill the adult weevils before they leave your plants to lay their eggs.
- Missouri Botanical Garden: Root-Feeding Weevil Larvae
- North Carolina State University Extension: Common Sense Gardening: A Guide to Root Weevil Control for the Earth-Friendly Gardener
- Arizona Cooperative Extension: Pest Press: Diatomaceous Earth
- Kemper Center for Home Gardening: Diatomaceous Earth
- The Dirt Doctor: Diatomaceous Earth
- Green Harvest Organic Gardening Supplies: Diatomaceous Earth