How to Manage Pests
UC Pest Management Guidelines
Western Yellowstriped Armyworm
Scientific Name: Spodoptera praefica
(Reviewed 1/17, updated 1/17)
In this Guideline:
Description of the Pest
Western yellowstriped armyworm may be abundant in alfalfa fields in the Central Valley from June to early September.
The caterpillar is usually black, with two prominent stripes and many narrow bright ones on each side. At maturity it is approximately 1.5 to 2 inches long. Eggs are laid in clusters on the upper side of leaves and covered with a gray, cottony material. Eggs hatch in a few days and larvae reach full size in 2 to 3 weeks. Larvae pupate on or just under the soil surface. Adults are brown moths that primarily fly at night but may be encountered flying up as you walk through the field.
There are at least five generations per year in the low desert and four generations in the Central Valley.
Armyworms skeletonize leaves, leaving veins largely intact.
Armyworms are frequently controlled by natural enemies and are more or less cyclic, occurring in large numbers only every few years. Early harvest, border cutting and biological control are important management methods that prevent damage from armyworms.
Natural enemies can provide good control of armyworms in many fields. Predators include bigeyed bugs, spiders, minute pirate bugs, damsel bugs, and lacewings. The parasitic wasp, Hyposoter exiguae, is believed to be the most important of at least 10 parasites attacking this pest. Sample for parasitism by pulling the heads from older caterpillars and squeezing the body contents out toward the head end. Hyposoter larvae are a light, translucent green color. Viral diseases can also be important.
Fields may be cut to avoid damage.
Organically Acceptable Methods
Biological and cultural control methods, as well as sprays of Bacillus thuringiensis (e.g., Xentari, Agree), are acceptable for use on an organically certified crop.
Monitoring and Treatment Decisions (View photos for identification of caterpillars)
In early summer start sweeping fields with adequate plant height 2 to 3 times per week to monitor for caterpillars; monitoring can be discontinued after September. Divide each field into 4 sections and take 5 sweeps per section with a 15-inch diameter sweep net, for a total of 20 sweeps. For information on sampling, see SAMPLING WITH A SWEEP NET.
Combine monitoring of armyworms with monitoring for alfalfa caterpillars and leafhoppers 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, apply a pesticide if there is an average of:
UC IPM Pest Management Guidelines: Alfalfa
Insects and Mites
L. D. Godfrey, Entomology, UC Davis
Acknowledgment for contributions to Insects and Mites:D. R. Haviland, UC Cooperative Extension, Kern County
M. Rethwisch, UC Cooperative Extension, Riverside County (Blythe)
C. G. Summers, Entomology, Kearney Agricultural Center