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Fall Webworm (red headed/black headed)

Characteristics:

  • Eggs: Females deposit a mass of small white or yellow eggs on the underside of leaves.
  • Larvae: Seen mostly in July and August. When young, larvae have a tawny appearance. They group together to build a large silk nest around a branch and eat the leaves from inside this nest. When mature, larvae may have either a red or black head, grey in colour, and covered with thick yellow-orange hairs on its back. Larvae can grow up to 35 millimetres long.
  • Pupae: After feeding over the summer, the webworm finds sheltered areas and pupates to overwinter.
  • Adults: Adults appear in the late spring and early summer. They are moth-like, with white wings sometimes covered with spots. Body is white with two rows of dark spots on back.

Identifying infestations:

  • Branches entirely covered with webs: This indicates the silk nest where the larvae are feeding.
  • Defoliated trees: Larvae can eat the leaves off an entire tree.

IPM Toolkit:

  • Preventive tools:
    • Avoid planting trees the webworm eats, including fruit trees, alder, birch, willow, western cokecherry, and cottonwood.
  • Physical tools:
    • Remove and destroy infected branches where the larvae’s silk nest is located
  • Biological tools:
    • Beetles and stink bugs eat eggs
  • Least toxic chemical tools:
    • Apply Bacillus thuringensis var. kurstaki (Bt.k.) according to label directions on infested branches.