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How to Manage Pests

UC Pest Management Guidelines

Western tussock moth larva.


Western Tussock Moth

Scientific name: Orgyia vetusta

(Reviewed 8/06, updated 3/09)

In this Guideline:


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 to use** R.E.I.+ P.H.I.+
(trade name) (conc.) (dilute) (hours) (days)

Calculate impact of pesticide on air quality
When choosing a pesticide, consider information relating to the impact on natural enemies and honey bees and environmental impact. Not all registered pesticides are listed. Always read label of product being used.
  (various products) Label rates 4 0
  COMMENTS: Least harmful to beneficials. Bt 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 0.5–0.75 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.
** For dilute application, rate is per 100 gal water to be applied in 300–500 gal water/acre, according to label; for concentrate applications, use 80–100 gal water/acre or lower if the label allows.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) 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.
Not recommended or not on label.
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). For additional information, see their Web site at http://www.irac-online.org/.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Apple
UC ANR Publication 3432
Insects and Mites
J. L. Caprile, UC Cooperative Extension, Contra Costa County
L. R. Wunderlich, UC Cooperative Extension, El Dorado County
P. M. Vossen, UC Cooperative Extension, Sonoma and Marin counties
W. W. Coates, UC Cooperative Extension, San Benito County
H. L. Andris, UC Cooperative Extension, Fresno County
L. G. Varela, UC IPM Program, Sonoma County
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
Acknowledgment for contributions to Insects and Mites:
C. Pickel, UC IPM Program, Sutter and Yuba counties

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Statewide IPM Program, Agriculture and Natural Resources, University of California
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