AURORA - Do you have notches in your rhododendron and azalea leaves? Root weevils, a type of beetle, are the likely culprits. These beetles especially love to feast on new growth of shrubs, including rhodies and azaleas.
Root weevils are the most serious insect pests of woody ornamentals in the Pacific Northwest. All are beetles with root-feeding larvae and leaf-eating adults. The most common type, black vine weevils, emerge as adults from the end of May until June. Other root weevils commonly found in Oregon landscapes include the obscure root weevil, rough strawberry root weevil and strawberry root weevil.
Adult weevils emerge from the soil from late May through the end of June each year. They vary from 1/4 - to 1/2-inch in length, depending on species. Adults are reddish brown to black. Larvae of all species are quite similar in appearance and habits. They are C-shaped, legless, and white with tan heads, up to a half an inch in length.
The adult root weevils cannot fly, but walk or are carried from one location to another on nursery stock. All adult beetles are females capable of laying eggs. These are slow moving and should not be confused with swifter predacious ground beetles.
Eggs are laid in clusters in or on the soil from June to September. The eggs hatch, and the larvae immediately wriggle through the soil to begin feeding. There is one generation per year.
The adult beetles are hard to detect because these they move onto a plant after sundown and feed on leaf margins sometimes throughout the night. During daylight hours they seek a moist, shady spot to rest, usually in the duff at the base of the plant.
The best way to check for root weevils is to examine woody plants for fresh notches on leaf edges, evidence of adult feeding damage, especially in tender new leaves. The notching usually begins at the base of the plant and moves upward during the summer, explained Robin Rosetta, nursery crops pest control agent at Oregon State University's North Willamette Research and Extension Center in Aurora.
"This year's damage can be distinguished from previous damage by looking at the new growth," said Rosetta. "Last season's growth often has brown edges along the notching."
Late spring and early summer is the time to start to think about control, especially if you saw lots of damage last year. Root weevil larvae live in the soil between August and the following May. These caterpillar-like creatures then pupate, to emerge as egg-laying, flightless female adults, active between June and October. Some may survive the winter months. The adults feed on leaves of woody ornamentals for four to six weeks, and then lay eggs in the soil near woody plants through September.
After hatching, the larvae enter the soil where they develop, feeding on plant roots. They spend winter months in the soil around roots and continue to develop in the spring as the soil warms. Pupation (change from larvae to adults) occurs in late spring. There is only one generation per year.
Although the notched leaves that the adult root weevils create are most visible, it is the root-feeding larvae that do most of the damage, explained Jack DeAngelis, entomologist with the OSU Extension Service. Living in the soil around plant roots on which they feed, root weevil larvae feed on roots from late summer's hatching through the following late spring. They are somewhat dormant during winter months. The older larvae do the most damage. Sometimes older larvae and overwintering adults girdle plants at the bottom of the trunk.
Start your control program as soon as you detect new notching.
Root weevil pests can be controlled in two ways. One is by applying a chemical insecticide to the leaves when adults are feeding in late May through June. For more information on chemical control in home gardens, consult OSU Extension's Insect Management Handbook or your local OSU Extension Master Gardener help desk at your local county Extension office.
A second method is applying beneficial nematodes to the soil to control the larvae in August through early October. The least toxic method to control weevils is to treat them in the larval stage, using beneficial nematodes.
"This special strain of nematode, or microscopic worm, attacks only soil insect larvae," he said. "These nematodes are non-toxic to plants and other animals."
The nematodes, which can be purchased at farm and garden suppliers, are added to water, then applied according to package instructions. They are also available via the WWW by checking websites such as California's Department of Pesticide Regulation's Suppliers of Beneficial Organisms in North America website.
Timing is of utmost importance - apply beneficial nematodes when the soil temperature is above 55 degrees F and the soil is moist.
"Releasing them on a cloudy day or early in the morning or evening is useful as they are sensitive to ultraviolet (UV) light from the sun," said Rosetta.
The nematodes move through the soil and enter a weevil larva through natural body openings. They enter the larva's blood system and release bacteria that multiply rapidly. Within 24 to 48 hours, the weevil larvae are dead. The nematodes continue to live in the dead larval tissue for several generations until they are ready to leave, attack new larvae and begin the cycle again.
"Nematodes are very effective against root weevil larvae and white grubs," said DeAngelis.
The nematodes do not usually last through the winter.
"But the good news is that if you do a thorough job of control, you probably won't have your weevil pests back, at least for a few years," he said.
"The one advantage we have with root weevils is that they do not fly as adults," she said. "We can clean up an area and then diligently guard our borders for intruders.
"The best way to keep weevils out of one's landscape is to not bring them in, in the first place. Check all new plant material thoroughly for these and other pests before planting them."
For more information, including photos of larvae and adult root weevils, visit this OSU website.