How to Manage Pests
UC Pest Management Guidelines
Alfalfa Caterpillar and Armyworm Monitoring
(Reviewed 11/06, updated 11/06)
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 populations 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, count, and record the number of healthy and parasitized caterpillars caught in your sweep net and divide that total by the number of sweeps taken. Record the average number per sweep on a monitoring form. 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 pop out. 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.
UC IPM Pest Management Guidelines:
C. G. Summers, Entomology, Kearney Agricultural Center, Parlier