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Under certain weather conditions,  sprays don’t always stay where they are intended.

Under certain weather conditions, sprays don’t always stay where they are intended.

Pesticide residues are detected in many places where they are not intended, including fish-bearing surface waters, groundwater and wells, and sensitive areas near application sites. But growers can take practical steps to reduce pesticide drift, which may help retain the use of certain pesticides sometimes found “off target.”

Waterways across the nation, particularly those that are home to endangered or threatened fish species, have been the focus of pesticide and water quality monitoring programs. In many surveys, herbicides were more frequently detected compared to other pesticides; however, insecticide concentrations are detected, too, with diazinon, chlorpyrifos, and carbaryl frequently found in water surveys. Pesticide detections do not mean that concentrations found were at levels known to cause adverse effects to humans or aquatic ecosystems as most detections are below regulatory limits.

“Drift happens,” said Dr. Paul Jepson, director of Oregon State University’s Integrated Plant Protection Center. Although most people think of drift as spray drift that happens when droplets are too small to land on the intended surface and are carried in the air, pesticides can also enter waterways from surface runoff, soil erosion, and leaching into the groundwater.

“Everyone thinks of drift as wind related, with the wind pushing the pesticide away,” Jepson said during a recent workshop held in Toppenish, Washington, that was sponsored by the Integrated Soil, Nutrient, and Pest Education Project called iSNAP, a collaborative effort of university and agency specialists from Oregon, Washington, and Idaho. “But high temperatures and low humidity are highly conducive to drift occurrences.”

He believes that growers following pest management practices can reduce pesticide residues detected in water and help preserve the use of chemicals like chlorpyrifos, which is often found in surface waters in tree fruit growing regions.

Weather conditions

The perfect conditions for spraying are neutral breezes (greater than three miles per hour but less than nine mph), wind direction away from sensitive areas, and cool and humid conditions. “There has to be some mixing of the air. Absolutely still days are not good days to spray,” Jepson said in describing optimum spray conditions. Perfect spray conditions tend to occur more often in the spring than summer.

Thermal drift occurs when warm weather (temperatures 70 to 80°F) generates thermals and eddies. He noted that he used afternoon thermals to create drift and spread pesticides further from his backpack sprayer while he was in Africa years ago spraying for locust.

Inversion drift occurs from cool, stable air that has low surface air mixing due to only slight winds. He explained that the small pockets of inversion can trap pesticides, and the high humidity prevents the droplets from evaporating.

Application management

Planning ahead before spray season can help growers protect sensitive areas from unwanted residues, he said, highlighting the following steps that should be part of a drift reduction plan:

• Sprayer tune-up—Spray equipment maintenance is the easiest way to prevent off-target drift. An OSU survey of grower spray equipment found only 6 out of 44 sprayers inspected to be in good working order, with worn nozzles the most common problem. Other sprayer issues were boom heights too low or uneven, excessive pressure, and dripping check valves. Spray equipment should be calibrated regularly to prevent overapplication, with nozzles changed every year.

• Farm maps—Map out your farm, identifying fields, sensitive areas, and the direction of predominant winds.

• Weather data summaries—Develop monthly tables to summarize weather data and drift risk in specific farm locations, with pests and monthly activity noted. Include data that can be gathered from agricultural weather stations—average wind speeds, relative humidity, and daily maximum temperatures. Your table will show what months are more hazardous to spray due to conditions from thermals and inversions or high wind speeds.

• Strategy for marginal conditions—Use weather forecasts to guide your spraying when sensitive areas are involved. Knowing the forecasts of wind speed and direction a few days before spraying can help you schedule locations that will be upwind of sensitive areas. The National Weather Service Web site has national graphical forecasts that show wind direction and speed at different times of the day that can be accessed to help plan spray applications.

• Nozzle technologies—Try using different nozzle types designed to reduce drift—air induction, air inclusion, air injection, or venturi nozzles. Field trials should be done when first using different nozzle types to ensure material sprayed is still efficacious. Bubble-type droplets may reduce drift but may not provide good enough coverage to be effective for some of the newer pesticides.

• Spray technologies—Drift reduction spray technologies have been developed to more accurately target sprays, including tower sprayers, sensor-equipped sprayers, and sprayers with tunnel or reflector shields. Attaching plywood donuts to the air intake of airblast sprayers has been found effective by OSU researchers to significantly reduce drift.

• Air-blast sprayers—Wider buffers and greater attention to drift are needed when using airblast sprayers. Minimize drift from an airblast sprayer by reducing ground speed, spraying inward at borders near sensitive areas, turning off nozzles at the end of rows and at field edges, increasing droplet size, and paying particular care when dormant spraying.

“If you plan ahead and implement management practices designed to minimize drift, you can get up to a 90 percent reduction of your spray drift,” Jepson concluded, adding that farm maps, spray plans, and using weather data can help determine which solution works best in different conditions.

He encouraged growers to work regionally to develop best management practices for drift reduction. An example of a pesticide handbook developed by tree fruit growers is the Best Management Practices Handbook for Pesticide Use developed by the Hood River Grower-Shipper Association and OSU.

To view the handbook, go to the Web site:

To use data from the National Weather Service graphical forecasts to help manage your application strategy, visit: gov/forecasts/graphical/ sectors and click on the weekly view.