Strawberry root weevils destroy more than strawberries; mint, raspberries, and rhododendrons, among other plants, suffer their invasions throughout most of the United States and Canada. In late spring, and doing most of their work at night, the brownish-black 1/5-inch insects chew ragged edges in young leaves. Even though the leaf damage is mostly cosmetic, it's wise to take control of the situation before midsummer, when the adult weevils lay eggs in the soil around plants. The resulting larvae—white, legless, and C-shaped—feed on plant roots and crowns, stunting growth, depressing yield, and potentially killing the plants.
At the first sign of leaf damage, an after-dark stroll through the garden with flashlight in hand will help gauge the extent of the invasion.
"There can be just a few insects, or seething hordes," says Sharon Collman, an extension agent at Washington State University who has studied root weevils for decades. She places open pizza boxes under infested plants early in the evening. An hour later, she shakes the plants, causing weevils to fall into the boxes, after which she scoops them into a disposable, lidded cup. A night in the freezer is a sure way to kill insect pests, she says—but be sure to label containers. Collman favors "Do Not Open. You Don't Want to Know."
For infested strawberry beds, autumn is a good time to strike back. Collman advises mowing the plants down like grass, exposing the crowns to sun.
A better solution for serious infestations is to remove and destroy strawberry plants in the affected area, till the soil to disturb overwintering larvae, and plant a cover crop. Locate new strawberry beds as far away as possible, and replant the old bed with a crop that is not susceptible to strawberry root weevils, such as sweet corn or pumpkins. Crop rotation, as usual, prevents a multitude of sins.
Two hopeful, if less proven, strategies for coping with root weevils:
- Apply parasitic nematodes in spring.
Erect 2-foot fabric barriers around new beds. Adult root weevils can't fly, and as the theory goes, they can't climb that high either.
Adults are brownish black ¼ inch long weevils, larvae are ¼ inch long, fat, white, and legless with light brown heads.
Where they live
They are found throughout the United States and Canada.
Their life cycle
The adults overwinter in leaf litter, emerge in late spring and feed on the leaves. They lay eggs on the crowns of plants. The larvae feed on roots.
Plants they attack
Strawberries, raspberries, tree fruit, and ornamentals.
Why they're a problem
The larvae feed on roots, which cause plants to be small and stunted and makes your plants vulnerable to disease. The adults feed on leaves and flower stems, which reduces berry yields.
Organic damage control
Don't replant strawberries in beds where they have grown before. These beetles can't fly, so go out at night and shake the adults off the plants and onto a white sheet; then destroy them. Cover a new planting with row covers until the plants flower; then remove the cover so blossoms can be pollinated. Parasitic nematodes (Hetero-rhabditis bacteriophora) will control larvae if applied in spring or late summer. Rove and ground beetles are the strawberry root weevil's natural predators.
Photo: Whitney Cranshaw, Colorado State University, Bugwood.org