How to Manage Pests
UC Pest Management Guidelines
Alfalfa Caterpillar and Armyworm Monitoring
(Reviewed 1/17, updated 1/17)
In this Guideline:
Start sweeping for beet armyworm, western yellowstriped armyworm, and alfalfa caterpillars in the early summer (late May or June, as soon as you see caterpillars in the field) and continue through early fall. Large numbers of yellow and white butterflies during late spring or summer is a warning sign that alfalfa caterpillar numbers may be increasing. See SAMPLING WITH A SWEEP NET for more details on sweeping.
Use a monitoring form with treatment thresholds to record observations.
How to monitor (View photos to identify caterpillars)
To determine if caterpillars are parasitized, pull young worms (at least 0.5 inch long) apart to see if white or green parasitic wasp larvae are inside. Base your population estimates on the average of all sweeps taken in that field, counting only those armyworms collected in sweeps that are at least 0.5 inches in length.
Treatment action threshold
If cutting is not practical or not scheduled soon after monitoring, apply an insecticide if there is an average of:
UC IPM Pest Management Guidelines:
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 Research and Extension Center