What Do They Look Like?
Weevils are small, oval beetles with very hard shells. Their heads are elongated into a pronounced "snout". Common species locally are:
Black vine weevils
The adults are charcoal or black and 8-12 mm long. Their wing covers have fine, parallel grooves flecked with gold patches. Larvae are plump, pearly white grubs, up to 1 centimetres long, with tan heads. They attack rhododendrons, azaleas, yews, laurels and sometimes grapes, blueberries and other plants.
Strawberry root weevils
Adults are dark brown and 6 mm long. A similar weevil species is smaller and light brown. The larvae are small, plump, whitish grubs with tan heads. They attack strawberries, raspberries and other small fruit and ornamentals.
What Does Weevil Damage Look Like?
Adult weevils chew small, half-circle notches in the margins of leaves of host plants (see photograph). They feed at night and hide under leaf litter during the day. Root weevil larvae feed on roots and burrow in the crowns of their host plants. This feeding can stunt plants or kill them if enough larvae are present. Some plants die from root rot organisms that enter the damaged roots.
Root Weevil Life Cycle
Larvae over-winter in the soil, feeding on roots until about mid-May. When full-grown, they change into the immobile stage called a pupa. Inside the pupa, the larvae transforms into an adult weevil. Adult weevils begin to emerge in June. They feed on leaves for a couple of weeks, then start laying eggs in the soil around their host plants. When the eggs hatch, the next generation of larvae feed on roots for the rest of the season. They overwinter in the soil and resume feeding again in the early spring. Some adult weevils also over-winter under debris and other sheltered sites.
When Are Root Weevils A Problem?
Leaf damage from weevil feeding does little harm to the plants (beyond being unsightly). If leaves have many notches, however, it is likely there are large numbers of larvae in the roots. A few larvae in roots are of little concern, but large numbers of larvae feeding in the crowns of rhododendrons and other plants can seriously weaken or even kill plants.
How Can I Prevent Damage?
Plant weevil resistant rhododendrons and azaleas. Such plants have leaves with rolled under edges that weevils cannot chew. Ask at the garden centre for advice on which cultivars resist feeding. Check plants before you buy them to make sure they are not infested. If possible, slide the root ball out of the pot to look for larvae.
What Can I Do to Control Root Weevils?
Knock adults from plants at night. Tap or shake the branches of the plant over a ground sheet or large tray to catch the weevils. Dump the collected weevils into soapy water to kill them. Lure weevils to artificial hiding places where you can find them during the day. These can be boards laid flat under plants, loosely draped burlap around the base of plants, or overturned pots stuffed with crumpled newspaper.You can also make more elaborate traps by winding strips of corrugated cardboard several times around stakes; drive these into the ground near plants. In the morning, kill any weevils hiding in or under the traps.
Sticky painted bands can be used to catch weevils on shrubs with a few main trunks. Tree banding with sticky gum works as an insect and pest barrier painted on the trunks (Treekote®, available from garden centres). Remove the bands in mid-July. Tiny worms, called nematodes, are sold at some garden centres to control root weevil larvae. Nematodes are expensive, but when used correctly, they are effective on root weevils. Apply them in the fall while the soil is still warm and water them in well. Check with suppliers and follow package directions carefully.
Sprays containing pyrethrins control adult weevils if timed correctly. Start checking plants for new feeding damage in late May. As soon as you see new damage, spray the plant in the evening. If done promptly, you can control the weevils before they start laying eggs.
Tips For A Healthy Garden
- Enrich the soil once or twice a year with compost or other organic fertilizers.
- Choose plants adapted to the conditions of sun or shade, moisture and soil acidity. If necessary, correct the drainage and acidity to suit the plants.
- Plant native plants, which are adapted to the local climate. Most are easy to care for and have few pest problems.
- Before buying plants, make sure they are healthy and free of diseases and insect pests.
- Water deeply, but infrequently, to encourage deep rooting.
- Cover the soil between plants and under shrubs with organic mulches. This insulates the soil, keeps in moisture and suppresses weeds.
- Protect and attract native beneficial insect, birds and other animals.
© Image courtesy of E. Cronin