The Ten Most Un-Wanted Pests

Introduction to IPM

Winter Moth
Root Weevil

Himalayan Blackberry

Black Spot

Powdery Mildew

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Root Weevil Root Weevil

Aliases Midnight Bandit; Snout Beetle, Black Vine Weevil, Clay Colored, Root Weevil, Otiorhynchus spp.; Obscrue Root Weevil, Sciopithes spp.

Hangout: Berry plants and ornamental plants; especially rhododendrons
Physical Features: Young bend their bodies into c-shapes, are plump and usually legless with white bodies and brown heads; adults are 1/4" - 1/2" beetles with long protruding noses and pear-shaped bodies with gray, black or brown coloring

Life Cycle
A root weevil's ruthless assault is from top to bottom. Larvae hatch from eggs laid in the soil or, in the case of the obscure root weevil, from eggs laid on leaves. These murderous munchers spend the winter eating plant roots, weakening them. In mid-spring, the young deviants mature into adult offenders, continuing the assault by climbing up the trunk and chomping on leaves, usually at night. They make a characteristic notching pattern, vandalizing the edges of the leaves. Most root weevil species are all female so they reproduce by cloning in the summer.

Uproot, Weevil!

1. Patrol the area

  • Kick dirt away from the plant base. Scout the crown of the roots for bite marks.
  • Stake out suspicious areas at night, using a flashlight to look for adults eating leaves. Check leaves close to the ground. Different species begin feeding at different times in the growing season. The earliest weevils begin in early spring.
  • Look for notched edges of leaves. Notches may be from a previous invasion, so record where you see notches and check for new bites, or bites on new leaves.

2. Make a positive I.D.

  • Check out adult beetles. Soil-dwelling larvae are harder to identify. Don't confuse them with predaceous ground beetles, which are good guys.
  • In general, adult root weevils are pretty easy to identify, but figuring out the exact species is difficult. It's essential to research the correct species because information you find about one type of weevil is not necessarily true for other types. Let a trained eye help you out at the Cooperative Extension office.

3. Do a thorough background check

  • Get to know the habits of each life stage. Adults are mostly active at night, so this would be a good time for S.W.A.T. missions.

4. Determine the danger level

  • Plants can handle some damage from weevils. Bites out of leaves are ugly, but bites out of roots are more harmful. Check the root crown for damage.

5. Make a plan.

  • Do-Nothing Method
    Inspect new leaves closely for notches-good clues for determining population size. If no newnotches appear, you don't need to do anything.
  • Manipulative Measures
    -There's nothing better than S.W.A.T.! Handpick weevils off plants at night or whenever you see them. They are hardened criminals, so be prepared to expend some energy cracking 'em. Or if you prefer, place a white sheet under the plant, shake vigorously, and dump apprehended weevils into a bucket of soapy water.
    -Don't mulch over the root crown. Young weevils that eat underground roots can hide under the mulch and could nibble the plant to death.
    -Trap rascally root weevils in sticky roadblocks around tree trunks. You can buy kits at a garden store. Just wrap tree gauze or tree bands around the trunk, apply a 3-4" wide band of the sticky stuff, and replace it after it's filled with weevils or not sticky anymore. It's not expensive, less toxic than insecticides, and effective!
    -Purchase plants that can hold their own against weevils. See Passive-Aggressive Plants for a list of resistant or tolerant rhododendrons.
  • Secret Agents
    -Ground beetles and other predators will hungrily devour weevils.
    -Send in squads of parasitic nematodes, micro-worms that'll destroy weevils. Follow directions for use, storage, and disposal carefully. Not doing so could mean death for your helpers. Locate suppliers through Cooperative Extension, garden stores, or the Internet.
  • Armed and Dangerous
    -Chemical weapons are not effective against all types of root weevils. Be sure you know species you have before you arm yourself.
    -If you must use chemicals, spray leaves in the evening since weevils get the midnight munchies. Spray before adults lay eggs, which is usually by June, but it depends on the species. Only spray leaves of assaulted plants.
    -When using chemicals, follow directions for use, storage, and disposal.

6. Evaluate the results

  • Root weevils may leave the scene of the crime quickly, but the bite marks they leave behind are essential evidence. Document bites on roots, notches in leaves, and the size of their mob. Also note if you did anything that may have made them scram.

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For more information, contact Scarlet Tang or Todd Murray
WSU Cooperative Extension (360) 676-6736
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