How to Manage Pests

UC Pest Management Guidelines


Fall Webworm

Scientific Name: Hyphantria cunea

(Reviewed 12/07, updated 3/11)

In this Guideline:


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.


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. If insecticide treatments are required, generally all that is necessary are localized treatments on individual trees applied when evidence of caterpillars is first observed. Spray must penetrate silken tents for effective control.

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

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

When choosing a pesticide, also consider information relating to impact on natural enemies and honey bees as well as environmental impact.
  (Altacor) 3–4.5 oz 4 10
  COMMENTS: Larvicide. The best timing is to apply before egg hatch. Do not apply more than 0.2 lb a.i. (9 oz)/acre/year. Do not 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 destroy natural enemies.
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
** For concentrate application, use the amount given in 80–100 gal water/acre, or lower if the label allows; for dilute application, amount is per 100 gal water to be applied in 300–500 gal water/acre, according to label.
+ 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 these two intervals is the minimum time that must elapse before harvest may occur.
# Acceptable for use on organically grown crops.



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471

Insects and Mites

  • C. Pickel, UC IPM Program/UC Cooperative Extension, Sutter/Yuba counties
  • J. A. Grant, UC Cooperative Extension, San Joaquin County
  • W. J. Bentley, UC IPM Program/Kearney Agricultural Center, Parlier
  • J. K. Hasey, UC Cooperative Extension, Sutter/Yuba counties
  • W. W. Coates, UC Cooperative Extension, San Benito County
  • R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to Insects and Mites:
  • W. H. Olson, UC Cooperative Extension, Butte County
  • L. C. Hendricks, UC Cooperative Extension, Merced County
  • G. S. Sibbett, UC Cooperative Extension, Tulare County

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