Mint-Root weevil

Black vine weevil (Otiorhynchus sulcatus)
Strawberry root weevil (O. ovatus)

Pest description and crop damage Larvae are legless white grubs with tan heads. They overwinter 2 to 8 inches deep in the soil. Adults generally are black but may be brown or chocolate brown. Larvae feed on mint roots, and adults feed on foliage.

Scouting and thresholds Sweep fields for adults on a calm, warm night (above 70°F) beginning in late May, June. A few weevils (five or more) in the net after 10 sweeps at various sites within the field may need control. An infestation of from five to eight larvae per 1 sq ft soil sample the depth of the mint roots usually signals need for control. Sample for larvae in April/May.

Management-biological control

  • parasitic nematodes (BioNem-C, Becker Underwood) at 3 billion per acre. Apply post-harvest in SEP to pre-moistened soil in irrigation water. Application with irrigation water is essential for nematodes to be effective. OMRI-listed for organic use.

Management-chemical control

  • acephate (Acephate 90WDG) at 1 lb ai/a. For control of adult weevils. PHI 14 days. Apply late May through early June after adults emerge but before egg laying. Apply when weevils are feeding actively on foliage. Apply on warm, still evenings, usually between 10 p.m and 2 a.m. Two applications 10 to 14 days apart may be necessary to reduce large infestations, particularly in central Oregon. Do not use Orthene more than twice per season.
  • Beauveria bassiana (Mycotrol-O, Emerald Bioagriculture, Laverlam Int.)-OMRI-listed for organic use.