Agriculture: Alfalfa Pest Management Guidelines


  • Loxostege spp.
  • Description of the Pest

    Webworms are greenish black caterpillars over an inch in length, greenish or occasionally brownish orange, with six black spots (tubercles) on each segment, and a stiff hair extending from each tubercle. They are often found within webbed leaves that may be connected to the ground by a silken tunnel through which it will retreat rapidly when disturbed. They vary in size, reaching up to nearly 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.


    Management of webworms in alfalfa can be achieved by early cutting of the hay if close enough to harvest, because the larvae cannot survive on dried alfalfa. Monitor alfalfa stubble and the new regrowth, especially under the windrows, for 2 to 3 days after cutting. If the webworms are defoliating the stubble after the hay is harvested, an insecticide may be warranted, although a pesticide application is rarely justified in California.

    If found in combination with other worms:

    • count all larvae from sweep net samples and
    • apply a pesticide when there are 10 or more nonparasitized alfalfa caterpillars, armyworms, and webworms combined per sweep.
    Common name Amount per acre** REI‡ PHI‡
    (Example trade name) (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.
      (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.
      (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).
    Text Updated: 01/17
    Treatment Table Updated: 01/17