Solving Cotoneaster Webbing and Dieback: Webworms vs. Spider Mites

Brown, dead branches covered in thick webbing on established cotoneaster suggest two likely culprits: cotoneaster webworm or spider mites. Accurate identification is crucial because treatments differ.

Cotoneaster Webworm

Signs: Dense silken webs enveloping branch tips, filled with dead leaves and dark brown caterpillars up to 1 inch long. Larvae feed in late July–August, skeletonizing foliage.

Control: At first sign of feeding, apply a labeled insecticide (carbaryl/Sevin, malathion, or cyfluthrin/Bayer Advanced). Thoroughly wet the webbing to penetrate. Remove and destroy webbed branches if feasible.

Spider Mites

Signs: Fine webbing on leaf undersides; leaves turn silvery, yellow, or bronzed, then curl and drop. Mites are microscopic (1/50 inch), reddish-brown or pale. Favored by hot, dry conditions and south-facing walls.

Control:

• Blast plants with a strong jet of cold water early morning, daily for 3 days.
• If needed, apply insecticidal soap every 3–5 days for 2 weeks.
• Dormant oil spray in late winter kills overwintering eggs.
• For severe cases: 1 part flowable sulfur + 10 parts superior dormant oil before spring growth.

Diagnosis Tips

• Webworms: thick, obvious webs with visible caterpillars and frass.
• Spider mites: fine, gossamer webbing; stippled foliage; mites visible with hand lens.

Since a similar issue killed another cotoneaster two years ago, monitor closely in spring. Prune out dead wood, improve air circulation, and avoid drought stress. A proper ID now will guide the right treatment.