Agriculture: Walnut Pest Management Guidelines

Fall Webworm

  • Hyphantria cunea
  • Description of the Pest

    Larvae of the fall webworm are pale brown or gray caterpillars. Their bodies are covered with long white hairs arising from black and orange spots. Fall webworms spend the winter as pupae. Moths emerge in late spring and lay eggs that hatch into caterpillars in late summer. There is one generation each year.


    From July to September, fall webworm caterpillars can be found forming silken tents and skeletonizing leaves, leaving behind only leaf veins. The silken tents are often filled with fecal pellets and cast larval skins.


    On small trees, infested twigs may be cut out and destroyed. Insecticide sprays applied for other pests often keep these leaf-eating caterpillars in check. Monitor by checking for hatched larvae. If insecticide sprays are required (high webworm numbers are found), generally all that is necessary are localized sprays on individual trees applied when evidence of caterpillars is first observed. Spray must penetrate silken tents for effective control. Most insecticides that are effective against codling moth and navel orangeworm are also effective on fall webworm.

    Organically Acceptable Methods

    Sprays of Bacillus thuringiensis are acceptable for use in an organically certified crop.

    Common name Amount to use REI‡ PHI‡
    (Example trade name) (hours) (days)
    Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
    (Altacor) 3–4.5 oz 4 10
    COMMENTS: Larvicide. Webworm damage is typically limited; only apply insecticide if webworm numbers are high. Do not apply more than 0.2 lb a.i. (9 oz)/acre per year or make more than four applications per year. To reduce the development of resistance do not make more than three consecutive applications of any group 28 insecticides (anthranilic diamide) per generation per season.
    (various products) Label rates 4 0
    COMMENTS: Most effective on small caterpillars. Does not kill natural enemies. Webworm damage is typically limited; only apply insecticide if webworm numbers are high.
    1 Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).
    Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.
    # Acceptable for use on organically grown crops.
    Text Updated: 07/17
    Treatment Table Updated: 07/17