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UC Pest Management Guidelines

Fall webworm larvae feed inside a silken tent that they enlarge as they consume foliage.


Fall Webworm

Scientific name: Hyphantria cunea

(Reviewed 5/06, updated 4/09)

In this Guideline:


Larvae of the fall webworm are pale brown or gray caterpillars with long white hairs arising from black and orange spots. Fall webworms spend the winter as pupae on the tree trunk or ground litter. Moths emerge in late spring and lay eggs on undersides of leaves; eggs hatch in late summer. Larvae feed in webbed colonies on ends of branches. In the lower Sacramento Valley there are two complete generations per year, but in most other sections of the state only one generation occurs.


From July to September, fall webworm caterpillars are found eating leaves (not veins) and forming silken tents on host trees.


Insecticide sprays applied for other pests often keep these leaf-eating caterpillars in check.

Cultural Control
On small trees, cut out and destroy infested twigs.

Organically Acceptable Methods
Cultural control and Bacillus thuringiensis (Bt) sprays are organically acceptable management methods.

Treatment Decisions
If insecticide treatments are required, localized treatments on individual trees applied when evidence of caterpillars is first observed are generally all that is necessary. The addition of a wetting agent to increase penetration of the webbing by the insecticide enhances control.

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

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy, impact on natural enemies and honey bees, and impact of timing on beneficials. When choosing a pesticide, also consider information relating to environmental impact. Not all registered pesticides are listed. Always read label of product being used.
  (various products) Label rates 4 0
  COMMENTS: Most effective on small caterpillars. Does not destroy natural enemies.
B. DIAZINON* 50WP 3 lb 1 lb 24 21
  4EC 3 pt 1 pt 24 21
  COMMENTS: Avoid drift and runoff into surface waters. Where plums are grown near waterways, do not use diazinon.
** For dilute applications, 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: Plum
UC ANR Publication 3462
Insects and Mites
W. J. Bentley, UC IPM Program, Kearney Agricultural Center, Parlier
K. R. Day, UC Cooperative Extension, Tulare County
Acknowledgment for contributions to Insects and Mites:
R. E. Rice, Kearney Agricultural Center, Parlier

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