Pesticides choice affects alfalfa runoff

Using pyrethroid pesticides rather than the more commonly used organophosphates is more effective for controlling Egyptian alfalfa weevil and reduces the amount of pesticide that could run off the field into natural waterways, reports farm advisor Rachael Long in the September-October 2002 issue of California Agriculture magazine.

However, Long cautions that, even though pyrethroids stay put in the field, accidental spray drift onto surface waters can adversely affect beneficial species and is highly toxic to fish at low concentrations.

To assess the impact of insecticide choice on water quality, Long and her colleagues collected tailwater samples from alfalfa fields in the northern Sacramento Valley over a three-year period. Samples were collected during irrigation after organophosphate and pyrethroid sprays were applied. To test toxicity, the scientists placed water fleas (Ceriodaphnia dubia), organisms that are extremely sensitive to toxins at very low concentrations, into the water samples.

“In nearly all sites where organophosphate insecticides were used, all of the water fleas died within 24 hours,” Long said. “Very few of the water fleas died in the water collected from fields treated with pyrethroids.”

Organophosphates — including Lorsban, Lock-On, Imidan and Malathion — are a class of fairly water-soluble pesticides that have been available for decades. The more newly developed pyrethroids — such as Warrior and Baythroid — are not water-soluble and tend to bind to soil particles.

Low solubility

“Not all alfalfa fields have tailwater that drains into natural waterways,” Long said. “If farmers know they drain into surface water, I would suggest pyrethroids or other insecticides with low water solubility. If they have a tailwater return system that doesn't drain into natural waterways, they should use whatever they feel is appropriate as farmers.”

After Long's field trials began, the use of the organophosphate pesticide chlorpyrifos decreased significantly in Solano County. In 1998, almost 12,000 pounds of chlorpyrifos were applied to alfalfa for pest control. In 2000, the amount was less than 3,000 pounds.

Long's research comes at an appropriate time. On Jan. 1, 2003, regulations that exempt farmers from reporting on their runoff will sunset. The California Water Code, adopted in 1982, requires those who discharge wastewater into California's streams and rivers to submit a report to the Regional Water Quality Control Board. Industries and municipalities have been submitting the reports, however all of California's regional boards adopted waivers for the return of agricultural drainage water.

The waivers are conditional. The discharger must make sure discharge doesn't cause toxicity to fish and wildlife and that sediment and temperature are controlled. However, because of budget and staffing limitations, the waivers have not been actively enforced.

Under the new provisions, contained in legislation signed into law in 1999, the water boards must review waivers at least once every five years and ensure compliance before granting a renewal.

While state agencies are working with agriculture industry officials to develop an interim plan and review options for long-term policies, Long is working with growers to find on-farm solutions.

“Our livelihood depends on being ecologically balanced,” said John Garner, a Glenn County farmer who chairs the California Farm Bureau Federation's water advisory committee. “Farmers are problem-solvers. We want to sit down with people involved in the process, come up with a workable solution, do it and get on with farming.”

Long's seven-page article, complete with tables with detailed water analysis information, is available on the California Agriculture Web site at or by requesting a copy of the magazine by calling (510) 987-0044.

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