Root Weevil on Rhododendrons

Topic editor

This integrated pest management (IPM) fact sheet provides information useful for identifying and combating a particular pest while minimizing environmental impact. It was originally written by Philip Dickey of the Washington Toxics Coalition and designed by Cath Carine of CC Design for the Green Gardening Program which is funded by Seattle Public Utilities. It has been modified for use on Toxipedia.org._

Species


Root Weevil
Ken Gray Collection, Entomology Department, Oregon State University.

About a dozen kinds of weevils attack these plants, but the following are the most common:

  • Obscure root weevil (Sciopithes obscures)
  • Black vine weevil (Otiorhynchus sulcatus)
  • Woods weevil (Nemocestes incomptus)

Host/Site


Rhododendrons and azaleas (some varieties are resistant; see last paragraph of #Cultural and Physical Controls). Root weevils feed and develop on a variety of plants, including salal or huckleberry thickets and wax myrtle.

Typical Root Weevil Damage on Rhododendron


Identification


Adults
Adults are 5 to 12 mm long, black or brownish in color, with wavy brown line across the back or with flecks of yellow, white, or gray. Prominent snout-like feature has antennae attached.

Larvae
Larvae are legless, white grubs with brown heads.

Life Cycle
Most species are less active in winter. Larvae feed on roots (especially damaging to shrubs in containers) and generally pupate in spring. Adults emerge from early April to August, depending on species. Leaf damage is from feeding by adults and occurs as new foliage expands and continues from late spring into autumn. The adults chew leaves, buds, and flowers. Adult root weevils do not fly, so spreading from adjacent areas is not rapid. The weevil must climb up from the soil level to reach leaves.

Natural Enemies


  • Ground beetle larvae
  • Rove beetle larvae
  • Parasitoid flies
  • Parasitoid wasps

Monitoring


Leaf damage occurs at night. To confirm that observed damage is from root weevils, monitor at night to retrieve specimens for identification. Monitoring can be combined with control by knocking them off onto cardboard placed carefully under plants. Fallen weevils can be collected in a jar and drowned with soapy water or placed in the freezer to kill. If leaf holes are large, scooped out bites, damage is probably not from weevils, which cut out small notches.

Action Threshold


Mild to severe notching of new leaves by adults causes mainly aesthetic damage. Plant health is usually not in jeopardy. (Root damage occurs but is usually only serious in potted nursery stock or very sandy soil or dry shade.) Action threshold varies depending upon the shrub placement and aesthetic standard required.

Cultural and Physical Controls


Always apply a compost mulch 2 to 3 inches deep. Water and fertilize appropriately. Most rhododendrons suffer heavy weevil damage because they are already stressed from poor care.

Sticky or slippery barriers are most useful on isolated, high-value plants with few main stems. Another barrier method that avoids trapping birds or beneficials is teflon barrier spray, marked under the brand name SureFire. This product makes the trunk slippery so that weevils fall off and cannot cross the sprayed area. With any barrier, trim off any plant parts that touch the ground, structures, or other plants that will act as a bridge around the barrier. Don't allow barriers to constrict plant growth.

Replace severely damaged species with resistant types. Extensive research by WSU entomologists has identified species and hybrid rhododendrons that are resistant to adult weevil feeding. Some recommended hybrids (color/rating) include:

Biological Controls


Beneficial nematodes (e.g. BioSafe) are available for control of root weevil larvae. Apply as a soil drench as directed on the label. Soil temperatures must reach at least 55 degrees before using in the spring. Don't apply in direct sunlight. Water in thoroughly and keep soil damp. Summer/fall applications are most effective. Meerkirk Rhododendron Garden is now using nematodes exclusively in potted plant stock for weevil control. Product has short shelf-life and must be kept out of high heat. Nematodes do not survive the winter in the Pacific Northwest and must be reapplied, if needed, each season.

Chemical Controls


Chemical control should be avoided if possible. Acephate (Orthene) is registered for use against adult root weevils, but is extremely toxic to bees and other beneficial insects such as predacious ground beetles and should never be used on blossoming plants. If chemicals are used, spray only foliage that is being eaten.

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