Agriculture: Apple Pest Management Guidelines

Western Tussock Moth

  • Orgyia vetusta
  • Description of the Pest

    Western tussock moth has one generation a year. Wingless females lay 125 to 300 eggs in a single mass on the empty pupal case. Overwintering eggs hatch when spring growth is expanding. Young larvae are black caterpillars with long bristles; mature larvae have numerous red and yellow spots and four median dorsal tufts.


    A heavy infestation will destroy all spring growth. Larvae take shallow bites out of newly set, young fruit; these injured areas eventually scab over and russet. Infestations are spotty and may be isolated in certain areas of an orchard.


    Western tussock moth is common throughout California, especially in unsprayed orchards. Monitor for egg masses in winter and larvae in spring to determine need for treatment.

    Organically Acceptable Methods

    Sprays of Bacillus thuringiensis and the Entrust formulation of spinosad are organically acceptable.

    Monitoring and Treatment Decisions

    During winter look for western tussock moth egg masses on props or tree trunks. Monitor during bloom by examining 100 fruit clusters for larvae. Spot treat infested areas in the orchard. Applications made at night during bloom will minimize damage to bees. This insect is usually controlled by sprays for other pests.

    Common name Amount per acre** REI‡ PHI‡
    (Example trade name) (conc.) (dilute) (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.
      (various products) Label rates 4 0
      COMMENTS: Least harmful to beneficials. Bacillus thuringiensis is a stomach poison and must be consumed by the caterpillar; therefore, it is most effective when applied during warm, dry weather when larvae are actively feeding. Spray during bloom or when monitoring indicates a need. Most effective against young larvae. Requires more than 1 treatment; apply second application 7–10 days after first.
      (Entrust)# 2–3 oz 0.67–1 oz 4 7
      (Success) 6–10 oz 2–3.3 oz 4 7
      COMMENTS: Apply at petal fall. To prevent the development of resistance to this product rotate to a material with a different mode of action after treating two consecutive generations. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
    ** For dilute application, rate is per 100 gal water to be applied in 300 to 500 gal water/acre, according to label; for concentrate applications, use 80 to 100 gal water/acre or lower if the label allows.
    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 two intervals is the minimum time that must elapse before harvest.
    * Permit required from county agricultural commissioner for purchase or use.
    # Acceptable for use on organically grown produce.
    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).
    Text Updated: 10/15
    Treatment Table Updated: 10/15