Alfalfa weevil coming on strong

Tam Moore
Oregon Staff Writer

In this wet spring with repeated showers, alfalfa growers in far Northern California and parts of Southern Oregon have found it a tough beginning to their haying season.

The job has been made tougher as farmers battle an insect pest, the alfalfa weevil. It has been a good year for the weevil, said Steve Orloff, a University of California extension farm advisor based in Yreka. North of the state line, Rodney Todd, an Oregon State University extension agent in Klamath Falls, also hears lots of weevil reports.

In Northern California’s Siskiyou County, where weevil larvae started showing up in late April, weevil control was the centerpiece of a mid-May field day.

“When I first got here 13 years ago,” said Orloff, “weevils were a periodic thing. ... Now they seem to be here every year.”

UC is reviewing decades-old information on economic thresholds for weevil treatment in the face of the surging populations and a change in available insecticides.

The longstanding advice is to take a sweep net, like one of those things children use to capture butterflies, and swing it through the top of the alfalfa stand. If it averages 20 or more larvae – the caterpillar stage of the weevil – there’s enough eating of leaflets to pay for a spray program.

Orloff and others, under the supervision of Larry Godfrey, a UC extension entomologist, are in the second year of testing that 20-larvae-per-sweep advice against current infestations of the weevil.

Godfrey said by telephone from his office in Davis that preliminary data indicate that the treatment threshold should come down a bit. The experiments, carried out in cooperating farmers’ alfalfa stands, are being done at several California locations.

Growers should hear the results during winter meetings.

There’s no scientific explanation for changes in weevil populations. Godfrey said while farmers speculate that a series of mild winters might have something to do with it, that doesn’t make sense because the weevil overwinters in areas with severe cold temperatures.
What may be significant is the change in insecticide materials. As organophosphates with a broad killing range are phased out, many growers switch to pyrethroids, which are more specific on their target insect.

Also significant for control, Godfrey said, is the extended season for emergence of young larvae. That means that little worms can appear over weeks, thwarting periodic scouting efforts and hopes of control through a single application of insecticide.

Tam Moore is based in Medford, Ore. His e-mail address is tmoore@capitalpress.com.