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)
- Take a weekly sweep net sample in fields that have adequate plant height to monitor for beet armyworm, western yellowstriped armyworm, and alfalfa caterpillars.
- Divide each field into 4 sections and take 5 sweeps per area with a 15-inch diameter sweep net, for a total of 20 sweeps.
- 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 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:
- 10 or more nonparasitized alfalfa caterpillars per sweep.
- 15 or more nonparasitized armyworms per sweep.
- 10 or more combined nonparasitized alfalfa caterpillars and armyworms per sweep.