Agriculture: Alfalfa Pest Management Guidelines

Alfalfa Caterpillar

  • Colias eurytheme
  • Description of the Pest

    The yellow and white alfalfa caterpillar butterflies lay eggs on new alfalfa growth. Eggs hatch into green caterpillars in 3 to 7 days. Full-grown caterpillars are about 1.5 inches long and are distinguished from other caterpillars on alfalfa by their velvety green bodies with white lines along their sides.

    Alfalfa caterpillars overwinter as larvae almost entirely in annual vetch at low altitudes and colonize alfalfa only as the vetch senesces in May and June. Aside from alfalfa and annual vetches, the butterfly also breeds on a variety of clovers and sweet clovers and occasionally on lupines. Caterpillar outbreaks usually result from a flight of butterflies into the field when the alfalfa is less than 6 inches tall.

    Alfalfa caterpillar swarms appear to be triggered when growers cut and bale alfalfa when adult numbers are high, with outflow in all directions. Extremely large numbers of adults migrating between fields are often present from June to September in the Central Valley and from May to October in the southern desert. Factors contributing to economically significant caterpillar numbers are

    • Slow and uneven crop growth
    • Lack of natural enemies
    • Hyperparasites (other parasitoid wasps attacking the natural enemy wasps reducing their numbers)
    • Hot, dry weather.

    There are four to seven generations per year of alfalfa caterpillar, and each generation is closely synchronized with the hay-cutting cycle so that the caterpillar pupates before cutting occurs.


    Alfalfa caterpillars can consume entire leaves. The larger larvae are most destructive. In contrast to armyworms, alfalfa caterpillars do not skeletonize leaves and will also consume the midrib.


    The most important way to control the alfalfa caterpillar is to preserve natural enemies that parasitize and prey upon this pest. Use selective insecticides against caterpillar pests in the summer to maintain natural enemies and minimize subsequent build-up of caterpillars. Preserve and encourage natural enemies by avoiding unnecessary insecticide applications for aphids or weevils in the spring.

    Biological Control

    An important parasite of the alfalfa caterpillar is Cotesia medicaginis, a dark brown to black wasp about 0.25 inch long. This wasp stings very small alfalfa caterpillars and lays an egg inside. The egg hatches and the wasp larva eats the inside of the caterpillar. A parasitized caterpillar dies before it reaches 0.5 inch in length. A parasitized caterpillar is

    • lighter in color than normal,
    • somewhat shiny rather than velvety on the surface, and
    • swollen toward the rear.

    Grasping the caterpillar at each end of the swelling and pulling it apart will expose the shiny, white parasite. It is important to determine the amount of parasitism because the economic threshold takes parasitism into account. For more information on identifying parasitized caterpillars, see the video in the ALFALFA CATERPILLAR AND ARMYWORM MONITORING section.

    Cultural Control

    Border-strip harvesting is a useful method for preserving the natural enemies of both the alfalfa caterpillar and aphids because it helps retain parasite larvae in the field. (For more details, see BORDER-STRIP HARVESTING.) Early harvesting of fields infested with economically significant levels of alfalfa caterpillars kills a large number of caterpillars, preserves crop yields, and avoids reducing natural enemy numbers. Time this cutting to avoid serious damage, yet obtain satisfactory yield.

    Organically Acceptable Methods

    Biological and cultural controls, as well as sprays of Bacillus thuringiensis, are acceptable for use on an organically certified crop.

    Monitoring and Treatment Decisions

    (View photos of caterpillars)

    In early summer start sweeping fields with adequate plant height 2 to 3 times per week to monitor for caterpillars.

    Combine monitoring of alfalfa caterpillars with armyworm monitoring as described in ALFALFA CATERPILLAR AND ARMYWORM MONITORING. Count and record the number of healthy and parasitized caterpillars caught in your sweep net on a monitoring form. (PDF)

    If cutting is not practical or not scheduled soon after monitoring, apply an insecticide if there is an average of:

    • 10 or more nonparasitized alfalfa caterpillars per sweep,
    • 15 or more nonparasitized armyworms per sweep, or
    • 10 or more nonparasitized alfalfa caterpillars and armyworms 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.
      ( Javelin WG)# 0.25–1.5 4 0
      COMMENTS: Bacillus thuringiensis will give satisfactory control of the alfalfa caterpillar, does not affect natural enemies, and leaves no undesirable residue on the hay. Upon ingesting Bacillus the caterpillars cease feeding but may remain on plants 3 to 4 days before dying.
      (Vantacor) 1.2–2.5 fl oz 4 0
      COMMENTS: To reduce the development of resistance, do not make more than two applications of any group 28 insecticides in a crop year.
      (Steward EC) 6.7–11.3 fl oz 12 7
      COMMENTS: Make no more than one application per cutting. Steward EC can be used for alfalfa grown for seed, but seeds cannot be used for sprouts intended for human consumption or livestock feed. All seed must be tagged, “Not for human or animal use.”
      (Intrepid 2F) Label rates 4 see label
      COMMENTS: Make no more than one application per cutting. Not for use in alfalfa grown for seed or for sprouts for human consumption.
    ** 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 personal protective equipment. 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.
    # Acceptable for use on certified organic crops.
    1 Group numbers for insecticides and miticides are assigned by the Insecticide Resistance Action Committee (IRAC). Rotate pesticides 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; insecticides with a 1B group number should be alternated with insecticides that have a group number other than 1B.
    Text Updated: 04/24
    Treatment Table Updated: 04/24