Butterflies a Warning Sign for Alfalfa Growers

Alfalfa caterpillar butterflies.

Photo buffs might get a thrill taking snapshots of colorful yellow and white butterflies fluttering around amber fields of alfalfa.

Growers, though, might get a chill when they spot dozens of these insects in their fields. It’s a warning sign that alfalfa caterpillar populations are on the rise.

Sweep the net in a 180-degree arc.

With summer around the corner, I’m starting to see an increase in worms around Valley alfalfa fields as growers prepare to cut their crop for the third time this season. If left unchecked, caterpillars and armyworms can eat away a grower’s profits as they feed on leaves and stems and defoliate the crop.

I can’t stress it enough: Monitoring your fields pays off. It’s a smart way to manage pest threats effectively and economically while helping you determine when to schedule treatment. In the long run, you avoid unnecessary treatments and save money because your decisions are based on need instead of the calendar.

 As soon as caterpillars are spotted in the field, you can use this as a warning sign and begin regular sweeping for alfalfa caterpillars, western yellowstriped armyworms and beet armyworms. 

To make the task easier, the University of California Statewide Integrated Pest Management program offers a monitoring form (100 kb pdf) to record your observations and tips about sampling with a sweep net. The monitoring form includes treatment thresholds.

Here’s a summary of alfalfa caterpillar and armyworm monitoring guidelines from UC IPM: With a sweep net 15 inches in diameter, you should take weekly samples in fields where plants are at least plants 6- to 10-inches tall. Divide each field into four sections and take five sweeps per area – for a total of 20 sweeps. Lets confirm that these are taken from the site correctly.

Western yellowstriped armyworm larva.

After finishing your sweeps, identify, count and record the number of healthy and parasitized caterpillars (caterpillars with fatal parasitic wasps living in them) caught in the net. Then you divide the total by the number of sweeps. Use the monitoring form to list the average number per sweep. Here’s how you determine a parasitized caterpillar: Pull apart young worms (at least a half-inch long) and see if a white or green parasitic wasp larvae comes out. Population estimates are based on the average taken in a field, counting only armyworms that are at least a half-inch long.

Full-grown alfalfa caterpillar.
UC IPM photos by Jack Kelly Clark

If you’re not scheduled to cut soon after monitoring, you should follow these guidelines for scheduling treatment:

• An average of 10 or more nonparasitized alfalfa caterpillars per sweep.
• An average of 15 or more nonparasitized armyworms per sweep.
• An average of a combined 10 or more nonparasitized alfalfa caterpillars and armyworms per sweep.

Be sure to talk to your pest control advisor about organic and or targeted reduced risk materials that won’t harm beneficial insects or the environment.