Root Weevils

Root weevils are a type of insect pest belonging to the family Curculionidae. These beetles are mainly known for their root-feeding larvae and foliage-feeding adults. They affect a wide variety of host plants and are a common nuisance in both agricultural and ornamental settings. Root weevils are also occasional nuisance pests indoors.

Host Plants

Root weevils have a broad host range, including shrubs, trees, and various crops. Some common host plants are:

• Astilbe
• Azalea and Rhododendron
• Coral Bells (Heuchera ssp.)
• Cotoneaster
• Dahlia
• Euonymus (Euonymus spp.)
• Grape (Vitis)
• Hosta
• Lilac (Syringa spp.)
• Mint (Mentha spp.)
• Peony (Paeonia spp.) 
• Privet (Ligustrum spp.)
• Raspberry (Rubus idaeus)
• Rose (Rosa spp.)
• Strawberry (Fragaria)
• Viburnum (Viburnum spp.)
• Wisteria

Regions impacted

Root weevils are prevalent pests across North America, but interestingly, all 17 species found on the continent are non-native. They were introduced from different parts of Europe, Northern Africa, and Northern Asia.

Description

Adult root weevils are small, ranging from 1/8 to 3/8 inches long, and are typically dark brown or black. They have a distinct snout and are wingless or have underdeveloped wings, making them unable to fly.

The larvae are legless, grub-like, and cream-colored with pale orange-brown heads, usually feeding underground on plant roots.

Root weevils are flightless insects that are active at night. During daylight hours, they hide around the base of their host plants, often under some form of cover. They become active roughly an hour after sunset, climbing onto plants to feed on leaves and creating their distinctive angular notches. If they feel threatened, they’ll drop from the plant and feign death as a defense mechanism.

Life Cycle

Root weevils have a one-year life cycle. In most cases, larvae overwinter in soil, feeding on roots whenever temperatures permit.

From late winter to early spring, the larvae finish their development and enter the pupal stage, which also occurs in the soil. The majority of adult root weevils appear from mid-spring to late spring. Some species, like the black vine weevil and strawberry root weevil, consist only of females and reproduce asexually. In contrast, others, such as the lilac root weevil, have both male and female populations.

Adult root weevils usually have a lifespan of a couple of months, although some can persist into the fall season. Females typically lay their eggs in late spring and early summer, depositing them into cracks in the soil. Within a few days of being laid, the eggs hatch, and the emerging larvae make their way to plant roots to feed.

Damage and Detection

Damage

Root weevils can cause significant harm to plants, affecting both the roots and the foliage. The damage can be categorized as follows:

• Root Damage: The larvae are primarily responsible for damaging the roots of host plants. This can lead to stunted growth, wilting, yellowing of leaves, and, in severe cases, the death of the plant.
• Foliage Damage: Adult root weevils feed on leaves, usually leaving distinct, angular notches along the edges. While the cosmetic damage may not be severe, it indicates a root weevil presence.
• Overall Plant Health: The combined effect of larval root feeding and adult foliage feeding can severely weaken a plant, making it more susceptible to diseases and less robust.

Detection

Detecting root weevils involves both visual inspection and understanding their behavioral patterns:

• Leaf Inspection: Look for the characteristic notches along the leaf edges. This is usually the first sign of a root weevil infestation.
• Nocturnal Activity: Adult root weevils are primarily active at night. You can go out with a flashlight about an hour after sunset to spot them on leaves.
• Soil Examination: To check for larvae, gently dig the soil around the base of affected plants. Larvae are legless grubs with a cream-colored body and a pale orange-brown head.
• Drop and Feign: If you disturb adults while feeding, they’ll drop from the plant and play dead, another behavioral indicator of their presence.
• Check for Overall Plant Health: Root weevil damage often leads to general plant decline. Monitor for stunted growth, yellow leaves, and wilting, especially in plants susceptible to root weevil damage.
• Frass: Sometimes, you might notice a powdery substance near the base of the plant or on the soil, which is the excrement from the larvae. This can also indicate their presence.

By staying vigilant and regularly inspecting your plants for these signs, you can detect root weevils early, which is crucial for effective management and control.

Prevention and Control

Mechanical Control

• Hand-picking: As root weevils are active at night, you can pick them off plants manually with the aid of a flashlight. Drop the caught weevils into a bucket of soapy water.
• Sticky Traps: Adhesive barriers like Tanglefoot, Tangletrap, Tack Trap, or StickEm can effectively deter adult weevils from ascending the trunk to reach the foliage. These barriers should be uniformly applied around the trunk to ensure there are no unguarded paths for the weevils to climb.
• Water Traps: Placing shallow water pans around the plants can also trap weevils.

Biological Control

• Predators: Encourage natural predators like ground beetles, spiders, frogs, toads, rodents, and birds by creating a garden habitat that is friendly to these animals. To attract ground-feeding birds that may dig up and eat grubs, spread a layer of shredded oak leaves around the base of your plants.

Cultural Control

• Crop Rotation: In agricultural settings, rotating crops can break the weevil life cycle.
• Sanitation: Regularly remove plant debris and leaf litter where adult weevils may hide.
• Irrigation Timing: Avoid evening watering, as wet foliage can attract weevils.

Chemical Control

• Insecticides: Chemical insecticides can be effective but should be a last resort due to their impact on non-target species. Applying pyrethroids such as beta-cyfluthrin, bifenthrin, cyfluthrin, permethrin, etc., to the foliage where adults feed, the stem, or the ground around the stem where weevils conceal themselves during the day can successfully manage adult populations. Always read and follow the manufacturer’s instructions.
• Systemic Insecticides: These are taken up by the plant and can control the larvae when they feed on the roots. Apply in spring to kill overwintering larvae before they pupate. Use these as a last resort and follow all safety guidelines.
• Nematodes: Some beneficial nematodes (Heterorhabditis spp. or Steinernema spp.) are effective against the larval stage of root weevils and are a more eco-friendly option. Target weevil larvae (and pupae) in the spring when soil temperatures are at least 60ºF (15ºC) or when root weevil eggs begin to hatch.

Monitoring

• Regular Checks: Continue to monitor plants for signs of infestation, particularly during peak weevil activity periods, to determine the success of your control measures.

By integrating several of these methods, you can create a more comprehensive and effective root weevil management plan.