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.