How to Manage Pests

UC Pest Management Guidelines



Scientific Name: Loxostege spp.

(Reviewed 11/06, updated 8/15)

In this Guideline:


Webworms are dark green caterpillars with two white stripes and black spots on their backs. They are often found within webbed leaves. They vary in size up to almost 1.5 inches in length.

Webworms overwinter as larvae in the ground adjacent to their fall food host. Moths emerge in early spring and lay eggs on leaves of host plants. Larvae will feed for 3 to 5 weeks.


The larval stage feeds inside of webbed leaves on the upper parts of the plant in summer and fall. If numbers are abundant, this webbing will be clearly visible and will cover extensive areas of foliage.


Early cutting may give satisfactory control. Treatment is rarely justified in California.

Common name Amount per acre** REI‡ PHI‡
(Example trade name)   (hours) (days)

Calculate impact of pesticide on air quality
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. Not all registered pesticides are listed. Always read the label of the product being used.
  (Intrepid 2F) 4–8 fl oz 4 0
  COMMENTS: Make no more than one application per cutting. Not for use in alfalfa grown for seed or for sprouts for human consumption.
  (Belt SC) 2–4 fl oz 12 0
  (Steward EC) 6.7–11.3 fl oz 12 7
  COMMENTS: Highly toxic to bees; do not apply when bees are actively foraging.
** See label for dilution rates.
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.
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



[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430

Insects and Mites

L. D. Godfrey, Entomology, UC Davis
E. T. Natwick, UC Cooperative Extension, Imperial County
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
R. F. Long, UC Cooperative Extension, Yolo County

Acknowledgment for contributions to Insects and Mites:
C. G. Summers, Entomology, Kearney Agricultural Center
M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
D. R. Haviland, UC Cooperative Extension, Kern County

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