Root weevils: troublesome pests

Root weevils are one of the top pests that damage plants in the Pacific Northwest—and around the world. They are difficult to find because they are most active at night, and they are not easy to control.

There are, however, a number of things you can do to prevent and limit damage from root weevils.

What are root weevils?

Adult weevils are small dark beetles with a short broad snout and elbowed antennae. Root weevils cannot fly. They spread through infested plants, soil or debris. The adults are usually all females and capable of laying eggs.

There are several species of root weevils. They vary in color from slate gray to blackish brown and generally have some small yellow or white flecks on their backs. They range in length from about ¼ inch to ½ inch long.

Larvae, found around roots, are C-shaped and legless. They may be white or slightly reddish, with tan heads, up to ½ inch in size.

Ground beetles

Do not confuse root weevils with ground beetles, which are beneficial. Ground beetles can be brown, black, or metallic green or blue in color.

Ground beetles live on the surface of the soil, where they capture and consume a wide assortment of soil-dwelling insects, including caterpillars, wireworms, maggots, ants, aphids and slugs. They can consume up to their body weight daily in food.

Ground beetles play a major role in reducing weeds and insect pests in agricultural fields. One study found that without seed predators, weed emergence on farmland can increase up to 30%.

Read more about ground beetles from eOrganic and Penn State Extension.

What root weevils eat

Root weevils primarily chew on broadleaf evergreen shrubs. In particular, they seem to love rhododendrons, azaleas, pieris, viburnums and laurels. Because these plants take a while to lose their leaves, the notching will stay visible for many years.

Adult weevils prefer to feed on young new growth. Root weevil larvae feed on roots and stems. They may girdle (remove a ring of bark) around twigs or even the plant stem. If they girdle the stem, the plant may die.

How to find root weevils

If you see ragged notches on the edges of leaves, or flower petals, you likely have root weevils. Mild to severe notching of new leaves is mainly an aesthetic nuisance and will usually not damage the plant’s overall health.

Look at plant leaves for fresh notching in early spring and continue to look throughout the season. If you see damaged plants during the day, go out with a flashlight just after dark. Put a piece of paper, light-colored cloth or a pizza box under the plant and shake or tap on the plant to see what falls.

How to manage root weevils

Take good care of your plants

Plants that are stressed are more vulnerable to insect damage, so keeping your plants healthy will limit the damage.

Water: Make sure you are using the right amount of water. Young rhododendrons need frequent watering for the first few years. As they mature, they still need regular watering to stay healthy.

Mulch: Use organic mulch, such as compost or arborist chips, to feed the soil and help keep your shrubs healthy.

Fertilizer: Proper fertilization is also important. Fertilize in early spring with a complete fertilizer. Use a light hand with rhododendrons and azaleas. They need less fertilizer than other shrubs, and too much fertilizer can harm them.

Pruning: Prune up branches that touch the ground to reduce access. Root weevils need to travel along branches to get to the leaves.

In Harmony services: We can help keep your landscape healthy. Contact us for more information about our natural tree and shrub care service.

Choose plants that are resistant to root weevils

Whether buying new plants or replacing plants that have been damaged by root weevils, choose something that root weevils don’t like. There are many varieties of rhododendrons and other susceptible shrubs that are naturally resistant to root weevils. Plants vary in resistance, but some can be 100% resistant.

Leaf shape appears to affect a rhododendron’s resistance. Leaves that have a slight roll on the edges tend to be avoided by weevils because the roll creates an edge too wide for the root weevil’s mouth. Thinner foliage is more susceptible than thick foliage. WSU research also found that rhododendrons with dark red flowers are likely to be susceptible to root weevil damage.

The American Rhododendron Society has a long list of rhododendrons and azaleas that are good performers in our area. The lists give plant varieties with nice form, foliage and flowers that are hardy and resistant to pests and diseases. They have consistently performed well in members’ gardens and are recommended to others living in the nearby area.

These publications from the City of Seattle and Thurston County lists rhododendrons that are resistant to root weevils. You could also ask a WSU Master Gardener, extension agent or your local nursery for recommendations.

The American Rhodendron Society noted that some plants that are particularly susceptible include ‘Blue Peter’, ‘Elizabeth’, ‘Ruby Hart’, ‘Creeping Jenny’, and ‘Scarlet Wonder’. Less susceptible varieties include ‘Sappho’, ‘Sir Charles Lemon’, ‘Pink Petticoats’, ‘Golden Torch’ and Nobleanum Album Group.

Generally, hybrids are more susceptible than species. However, some species have been seen with weevil damage, including R. piercei, R. campanulatum, R. chaetomallum, R. degronianumvar. heptamerum and R. celebicum.

Limiting root weevil damage

Use beneficial nematodes 

The least toxic method to control weevils is to treat them in the larval stage, using beneficial nematodes. Nematodes are microscopic round worms. These tiny organisms attack the root weevil larvae as they grow in the soil below the shrubs. This reduces populations before they become adults.

Beneficial nematodes are also known as insect parasitic nematodes and predator nematodes. They do not appear to have any significant harmful effects on other beneficial organisms, plants, people or other animals.

We use beneficial nematodes in our ongoing tree and shrub care service. Our service technicians look for recent root weevil damage to the leaves and apply nematodes if they see damage.

Fall is a great time to apply nematodes. The soil temperatures are still warm, and rain will keep the soil wet enough for nematodes survive and feed on young root weevil larvae. If it’s dry and hot out, the nematodes won’t survive in the soil very long.

You can read more about beneficial nematodes in these articles by OSU Extension and CSU Extension.

Apply sticky or slippery barriers

Barriers are applied around the trunks or stems near the ground. These products discourage or trap the adults as they travel up trying to reach leaves. Barriers work best when the shrubs are not touching other plants.

You can tightly wrap the trunk with a strip of polyethylene, waterproof tape or thin plastic and apply a sticky product, such as Tanglefoot, Tangletrap, Tack Trap or StickEm, onto the tape. Or you can use slippery teflon barrier tape. The weevils either will choose not to travel up the stem or will become stuck in the barrier and thus cannot move into the canopy to feed.

The barrier must be applied to the trunk so there are no unprotected avenues of travel up the stalk. Make sure the barrier is tight enough that the insects cannot make a path under it. It must be removed before it restricts stem growth.

We do not recommend chemical controls

Some of these chemicals will kill beneficial insects such as predacious ground beetles and bees.

Resources 

Root Weevils: Troublesome Rhododendron Pests, American Rhododendron Society. Information about physical, chemical and biological control methods.

Common Sense Gardening: Root Weevils, Thurston County. How to use nontoxic control methods, along with a list of resistant hybrid rhododendrons.

Root Weevils, Colorado State University.