How to Manage Pests
UC Pest Management Guidelines
Scientific Name: Colias eurytheme
(Reviewed 11/06, updated 7/13)
In this Guideline:
Description of the Pest
The yellowish-orange or whitish butterflies of the alfalfa caterpillar lay eggs on the new growth of alfalfa that is less than 6 inches tall. Eggs hatch into green caterpillars in 3 to 7 days. Full-grown caterpillars are about 1.5 inches long and are distinguished from other common caterpillars on alfalfa by their velvety green bodies and white lines along their sides.
Caterpillar populations usually result from a flight of butterflies into the field when the alfalfa is less than 6 inches tall. 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 Imperial Valley. Factors contributing to economic populations are slow and uneven growth of the crop, lack of parasites, and 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 consume entire leaves. The larger larvae are most destructive. In contrast to armyworms, alfalfa caterpillars do not skeletonize leaves and will consume the midrib.
The most important way to control the alfalfa caterpillar is to use nonselective insecticides against caterpillar pests in summer and preserve and encourage its natural enemies by avoiding unnecessary insecticide applications for aphids or weevils in late 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 consumes the body contents of the caterpillar. A parasitized caterpillar dies before it reaches 0.5 inch in length. It is recognized by being 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.
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 economic levels of alfalfa caterpillars kills a large number of caterpillars, preserves crop yields, and avoids reducing the natural enemy population. 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 .
If cutting is not practical or not scheduled soon after monitoring, treat if there is an average of ten or more nonparasitized alfalfa caterpillars per sweep, fifteen or more nonparasitized armyworms per sweep, or 10 or more nonparasitized alfalfa caterpillars and armyworms combined per sweep.
UC IPM Pest Management Guidelines:
Insects and Mites