How to Manage Pests

UC Pest Management Guidelines

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Alfalfa

Alfalfa Caterpillar

Scientific Name: Colias eurytheme

(Reviewed 11/06, updated 11/06)

In this Guideline:

Description of the pest
Damage
Management

DESCRIPTION OF THE PEST

The yellowish-orange or whitish butterflies of the alfalfa caterpillar lay eggs on the short, new growth 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 a year of alfalfa caterpillar, and each generation is closely synchronized with the hay-cutting cycle so that the caterpillar pupates before cutting occurs.

DAMAGE

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.

MANAGEMENT

The most important way to control the alfalfa caterpillar is to preserve and encourage its natural enemies by avoiding unnecessary insecticide applications for aphids or weevils in late spring or by using nonselective insecticides against caterpillar pests in summer.

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 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 than normal in color, 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.

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 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.

Common name

Amount/Acre**

R.E.I.+

P.H.I.+

(trade name)

(hours)

(days)

The following materials are listed in order of usefulness, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider environmental impact.

BACILLUS THURINGIENSIS ssp. KURSTAKI#

(various products)

Label rates

MODE OF ACTION: A microbial (Group 11.B2)¹ insecticide.

COMMENTS: Bacillus will give satisfactory control of the alfalfa caterpillar, does not affect beneficial species, and leaves no undesirable residue on the hay. Upon ingesting Bacillus the caterpillars cease feeding but may remain on plants 3-4 days before dying.

INDOXACARB

(Steward) 1.25 SC

6.7–11.3 fl oz

MODE OF ACTION: An oxadiazine (Group 22)¹ insecticide.

COMMENTS: Make no more than one application per cutting. Not for use in alfalfa grown for seed or for sprouts for human consumption. Do not apply more than 45 fl oz/acre/crop season.

+	Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment until the field can be grazed or cut. In some cases the R.E.I. exceeds the P.H.I.; the longer of these two intervals is the minimum time that must elapse before harvest may take place.
#	Acceptable for use on an organically grown crop.
¹	Modes of action are important in preventing the development of resistance to pesticides. 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. 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 is assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at http://www.irac-online.org/.

PUBLICATION

UC IPM Pest Management Guidelines: Alfalfa
UC ANR Publication 3430
Insects and Mites
C. G. Summers, Entomology, Kearney Agricultural Center, Parlier
L. D. Godfrey, Entomology, UC Davis
M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
D. R. Haviland, UC Cooperative Extension, Kern County
P. B. Goodell, Entomology, Kearney Agricultural Center, Parlier
R. F. Long, UC Cooperative Extension, Yolo County

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