Holy Cow – Worm Population Surges in Alfalfa Field

Damien Jelen walked into a sun-baked alfalfa field, swinging his sweep net to the left and then to the right. Swoosh. Swoosh. Swoosh. Swoosh.
 

Beet arrmyworm. (UC IPM photo)

He opened up the net, revealing dozens of squirming worms. “The numbers are bumping up,” Damien says, citing this month’s relentless heat wave as one reason for the population explosion.

Yes, it has been extremely hot in the Valley. As July nears a close, all but three days this month have recorded triple-digit high temperatures, averaging a sizzling 102 degrees – hot even by Valley standards. Historically, the average high for the month is 98.4 degrees.

Since late spring, field scouts and growers have been on the lookout for beet armyworm, western yellowstriped armyworm and alfalfa caterpillars. Yellow and white butterflies flying around the fields are telltale signs alfalfa caterpillars are on the upswing.

Alfalfa pests can significantly reduce yields, stand life and forage quality.  Alfalfa serves as an important food for the state’s $6 billion dairy industry.

Western yellowstriped armyworm butterfliers.

Armyworms leave foilage looking like skeletons while alfalfa caterpillars will gobble entire leaves. Most of the time natural enemies such as bigeyed bugs and lacewings are plentiful enough to keep these pests at bay. Or other growers may opt to cut their crop a little early.

But with two or more months of harvesting still left in the season, some growers are spending the money to treat their fields. “They had to spray. They can’t wait,” Damien says.The pest counts were above the treatment threshold.

Alfalfa caterpillar. (UC IPM photo)

During the summer, UC IPM recommends scouting a field two to three times a week. To come up with good pest counts, UC suggests dividing each field into four sections and taking five sweeps per section for a total of 20 sweeps.
 “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,” UC IPM says.

Growers should consider treating their fields under these thresholds:

• 10 or more nonparasitized alfalfa caterpillars per sweep,
• 15 or more nonparasitized armyworms per sweep, or
• 10 or more nonparasitized alfalfa caterpillars and armyworms combined per sweep.

FIELD DAY: If you want to learn more about latest

 trends in alfalfa field and pest management and pesticide regulations, come to Tuesday’s Alfalfa Field Day in Firebaugh. The free event will be from 9:30 a.m. to 12:30 p.m. at the Firebaugh-Mendota United Methodist Church, 1660 O Street, Firebaugh. Featured speakers are:

•          Lynn M. Sosnoskie, agronomy and weed science advisor, for the University of California Cooperative Extension in Merced and Madera counties, who will discuss weed control in alfalfa and the chemical, cultural and biological factors that can affect success and failure.
•      Tom Casey, pest control operations official for the Fresno County Agricultural Commissioner’s Office, who will cover current pesticide use and regulations.
•        Nicolas Clark, agronomy and nutrient management advisor at UCCE Kings, Tulare and Fresno counties, who will offer tips about insect pest management in San Joaquin Valley alfalfa hay.

Continuing education credits have been approved and include one hour of regulations. The field day issponsored by the San Joaquin Sustainable Farming Project, which provides farmers with valuable strategies to improve yields while becoming better environmental stewards in today’s challenging economic and regulatory climate. For more information about the field day, contact Project Director Marcia Gibbs at (530) 370-5325.