Alfalfa Caterpillar

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.

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.

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.

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

(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.