Beware the root weevils — they’re coming for your plants

By Bonnie Orr
WSU Chelan/Douglas County Master Gardener
May 18, 2022

Root weevil larvae on a tarp with some soil and leaf.
Root weevil larvae are foun a few inches under the soil and are easy to eradicate by squishing. – Provided photo/Bonnie Orr.

Bonnie Orr
Bonnie Orr – WSU Extension Chelan/Douglas County Master Gardener – photo by Don Seabrook, Wenatchee World

Snout-nosed weevils are fairly common. There are, believe it or not, hundreds of types. The gardeners of Europe were complaining about this pest in the 18th century. The most common ones in our gardens are the Rhododendron, strawberry and vine weevil. They gnaw on the young leaves of rhubarb, liliac, hosta, peony and many other ornamental leaves from late May through September. The little c-shaped notches are a sign that weevils are present.

Now is the time to dig lightly among the roots of target plants to find and destroy the larvae. These grubs are white with a brown “head” and are c-shaped. They emerge as adults in late May or early June. There is only one generation a year.

The adults cause the leaf damage — usually it is cosmetic — and the chewing will not kill the plant. The insects start feeding on the lower leaves and work their way up the plant. So being observant serves you well since it is easier to trap the insects on the lower leaves.

The larvae feed on roots; unless the infestation is enormous, they will not eat so many roots that the plant will die.

The way to control the weevil is first of all not to have it transported into your yard. The good news is these insects are flightless; the bad news is how did they arrive in your garden? One of the most common means of introduction is receiving gifts of plants with soil attached from fellow gardeners. The eggs, larvae or adults could be hidden in the soil. Always wash all the soil from the roots of gifted plants before placing them in your garden.

If you select to use a pesticide on the insects, you must go out at night with a flashlight to find them. Pesticides only work if you directly spray the product on the adult insect. As long as you are out there in your jammies, you might as well just collect them by placing a ground cloth under the bush and shaking the insects into the cloth. Then drop the weevils into a container of soapy water.

We cannot suggest that you use systemic insecticides because most of the plants affected by the weevil produce flowers. The systemic insecticide not only kills the few bad guys but also kills the dozens of types of pollinators visiting the flowers in your garden. There are nematodes that can be purchased to stir into the soil of affected plants. These nematodes feed on the weevil’s eggs and larvae. The soil must be at least 55 degrees before you stir in the nematodes.

If you leave a light mulch of leaves on the soil, scratching birds that love to kick around mulch, such as song sparrows, towhees, robins, quail or white-crowned sparrows, will unearth both the hidden adults and larvae.

More from Bonnie