In recent visits to the garden, you may have noticed plants that looked like they were being redecorated. Notched leaves are prevalent on many shrubs, flowers, vegetables and even trees. Notches around the edge of the leaf are the work of elusive root weevils. Root weevils include strawberry root weevil and several other species of small beetles that occur in Utah. All have similar life cycles and cause similar injury. Control, if it can be accomplished, is also similar.
The adults of each species are hard-shelled, dark-reddish brown to black snout beetles. They are 1/4 to 3/8 of an inch long and all have the long snout common to all weevils. The larvae have no legs and are generally a white, wormlike grub up to 1/2-inch long with a tan head.These insects have a tremendous rate of reproduction. Each female deposits up to 200 eggs in the soil at the base of the host plant. After hatching, the young larvae move down to the root zone where they feed on the fibrous root system. As the larvae continue to grow, they feed on larger roots and may kill the plants. The full-grown larvae then pupae in the soil, transform to the adult stage and eventually emerge to feed on the foliage.
When adults start feeding on the foliage, many gardeners become concerned. The foliage looks as though someone has taken pinking shears or some other tool and notched around the edges of the leaf. The feeding on the foliage by the adults does not damage the plant significantly, but it affects the cosmetic appearance. Adult feeding damage usually appears in midsummer when it is often too late to do much about the problem. The underground feeding is far more serious because it destroys the root system, but it is seldom noticed by gardeners. Even the adult insects are seldom noticed as they feed only at night. They are difficult to locate unless you arm yourself with a flashlight and make a midnight foray into the garden on your hands and knees.
Few plants are safe from this pest. Strawberries, of course, are a favorite target, while cane berries, squashes, cucumbers, cabbage, peppermint and many other edible plants are damaged. Roses, lilacs, arborvitae, yews, junipers, euonymous, privet and many other ornamentals are also damaged. Peonies and many herbaceous perennials regularly show the characteristic notched leaf edges.
Attempts to control the root weevil larvae are generally not successful. Two products (Orthene and bendiocarb) are labeled to control the adults on ornamental crops. Orthene is sold as Orthene Systematic Insect Control, while bendiocarb is available in a dust formulation as Lilly Miller Rose and Rhododendron Dust. Start controls as soon as feeding is observed on the leaves. Diazinon or Malathion as directed on the label for other pests may reduce weevil populations on berries. Morgro also markets a pest pellet that is an effective bait in edible crops.
A new ally in controlling these pests includes beneficial nematode. Beneficial nematodes seek out and destroy root weevil larvae in the soil. They release fatal bacteria, reproduce in the insect's body, then reattack additional larvae. They are available under several different brand names. BioSafe, marketed by Ortho, is one that is available in our area. These nematodes don't damage people or plants. Unfortunately, they are not cold-tolerant and need to be reapplied yearly.
Trap weevils by burying straight-sided glass tumblers so the top lip of the glass is even with the soil. These must be emptied each morning. The process is slow, but it does reduce the population over time.
Root weevils may become a problem in the fall when they migrate into homes. They do not infest stored foods and do not damage people, pets or structures. Keep them out by making the structure as insect-proof as possible. Seal cracks in the foundations, repair torn screens, and weatherstrip around doors and windows. Once they are inside the home, utilize the vacuum cleaner for control.
Root weevils may sculpture and redecorate your plants, but they do not improve the health of your garden. Carefully monitor pest populations and do your best to control the adults in order to prevent them from starting over the following year. Few pests are so exasperating to deal with and do so much unseen damage. Hopefully one of the suggested solutions will help reduce the problem in your garden.
- TIME TO SPRAY FOR PEACH TREE BORERS. Dursban is the preferred product. Spray only the trunk and let it puddle at the base. Spray all stone fruits, including ornamental forms.