Bug Counts Up: Time for Border Strips in Alfalfa fields

There’s one way I can tell when the spring will begin getting warmer and sunnier. The bugs start coming out.

With daytime temperatures now in the upper 70s and low 80s, I’m starting to see more pests in the alfalfa fields. Specifically, looper worms and alfalfa caterpillars are showing up in greater numbers as I scout various fields around the San Joaquin Valley. I’m also noticing lygus counts are on the way up.

Monitor  for  looper  worms.      Photos by  Jack Kelly Clark

Last week, my sweep net collected an average of 5 to 7 worms for every 50 sweeps in the alfalfa. That’s up from an average of 1 to 3 worms the previous week. So far, the sampling indicates were in a safe mode, which means the pest pressure isn’t great enough to warrant pesticide treatment. As the days grow warmer, I expect the pest numbers will increase.

Right now, most growers are preparing for the second cut of alfalfa. A few are even poised for their third cut. The recent rains had growers on edge and mulling the timing of their next harvest.

It’s never too early to think about creating border strips. It’s good to have small strips of uncut alfalfa this time of the year. I recommend a leaving quarter swath – roughly a 2 to 3 feet wide strip – at the end of the field. These strips will retain loopers, alfalfa caterpillars and lygus, preventing them from migrating to nearby fields and damaging those crops, especially cotton. Alfalfa strips also will attract natural enemies such parasitic wasps.

Keep an eye out for alfalfa caterpillars in the fields.

This farm management practice can save time and money because you’re holding off spraying pesticides until absolutely necessary. That’s where field monitoring and pest sampling comes in. Here’s a rule of thumb: Consider treatment when after 50 sweeps per stop, you collect 10 or more alfalfa caterpillars, or 15 armyworms, or a combination of 10 alfalfa caterpillars and armyworms. I usually make 3 to 4 stops in each field.

In early summer start sweeping fields with adequate plant height 2 to 3 times per week to monitor for caterpillars and continue through fall.  Early cutting will give satisfactory control if the infestation appears late in the cutting cycle.
Combine monitoring of armyworms with monitoring for alfalfa caterpillar 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 (68 KB, PDF).
If cutting is not practical or not scheduled soon after monitoring, treat if there is an average of 10 or more nonparasitized alfalfa caterpillars (those not infected by parasites) per sweep, 15 or more nonparasitized armyworms per sweep, or 10 or more nonparasitized alfalfa caterpillars and armyworms combined per sweep.

When you need to spray, consider selecting from a variety soft pesticide materials on the market. They’re effective and good for the environment. Talk to your pest control advisor about the available materials, especially those that won’t harm beneficial insects.

There are great resources available from the University of California Statewide Integrated Pest Management program. Check out UC IPM’s website for more tips about pests and alfalfa.