Complex approach to organic alfalfa pest control

Because there are fewer agricultural chemicals available, and they are frequently less effective than non-organic products, it is particularly important in organic alfalfa production to choose varieties with good resistance or tolerance of target pests.

In the case of weevils, the most important insect pest in alfalfa, the judicious use of sheep in the winter can also make the difference in sparing the next spring crop.

"Sheep off during the winter—the sheep eat the old stand where the weevils lay eggs," said Rachael Long, University of California Cooperative Extension farm advisor.

Long wrote the chapter on organic production in the UC Alfalfa Production Manual. She discussed the complexities of organic alfalfa pest control at a seminar on Organic Production Healthy Plants sponsored by the Organic Fertilizer Association of California and the California Association of Pest Control Advisors in Woodland.

The threshold for economic damage from weevils is around 15 per sweep with a sweep net, she said.

"Weevil is our worst pest; it affects the first and second cuttings. There are parasitic wasps, but they are just not effective. You can harvest early to avoid damage, but you lose yield. You can also overseed clovers or grasses to make up for weevil loss, but this reduces your feed quality and limits your markets," Long said.

Entrust 80W is one organic material that can give good weevil suppression, according to Long.

But even the most effective organic insecticides will not give the knockdown effectiveness of synthetic materials.

"I've always had products that cost more and did less, so you have to integrate them into a program," said Tom Quick, global sales manager for Grow More, a Los Angeles-area supplier of organic fertilizer products.

In one sense, organic alfalfa pest control involves going back to the future by incorporating tools that were widely used before the development of synthetic insecticides.

"Prior to 1945, everything was IPM (integrated pest management) because it was mostly on-farm inputs. Post-1945, we had the Green Revolution with synthetic pesticides and calendar spraying. We ran into problems with resistance and other issues, and that's when California took the lead with the notion that you need trained entomologists supervising your spraying," Quick said.

Parasitic wasps may not help much with weevils, but they can be more effective in controlling armyworms, according to Long, and a ratio of one wasp for every 10 armyworms should be adequate.

Sweep counts of around 10 per sweep indicate that armyworms could cause economic damage.

"Bt works best. Early harvest can limit the damage, but at $230 a ton, can you afford it?" Long asked.

Pyganic or Neemix are organic materials that should help with aphid control, but variety selection is particularly important in managing aphids.

"For aphids, resistant varieties are the best. Last year was very bad for aphids in alfalfa," Long said.

Later in the season, cutworms can damage alfalfa by limiting or even stopping regrowth.

"Cutworms were bad in 2012. You could see the plants weren't regrowing after September. Apply Bt near sundown, when the cutworms are active," Long said.

With even fewer organic herbicides available than organic insecticides, sheep can play an important role in weed control.

"Probably the best method of weed control in establishing fields is grazing, or sheeping off. Pre-irrigate and harrow to reduce weeds. Select more non-dormant varieties for better competition with weeds," Long suggested.

And another old farmers' friend can limit the damage gophers do to an alfalfa crop.

"A family of five barn owls," Long said, "can eat 1,000 gophers a year."

(Bob Johnson is a reporter in Monterey. He may be contacted at bjohn11135@aol.com.)