5.2 Drift Management

What Drift Is

Drift is the airborne movement of pesticide away from the target site. It takes two main forms:

• Particle drift - the physical movement of spray droplets (or dust from granular applications) through the air during and after application.
• Vapor drift - the movement of pesticide vapor after the spray has dried, a particular concern for volatile products such as 2,4-D esters, dicamba, and some fumigants.

Both forms can injure off-target crops, pollinators, and people, and both can trigger a FIFRA misuse violation even when the applicator did not intend harm.

Droplet Size and the ASABE Classification

Droplet size is the single most important controllable factor in particle drift. The American Society of Agricultural and Biological Engineers (ASABE) S572 standard classifies spray droplets by Volume Median Diameter (VMD), measured in microns (one micron = 0.001 mm). One-half of the spray volume is in droplets larger than the VMD and one-half in droplets smaller.

Section 6.1 gives the micron boundaries for each category. Use the same S572.3 categories everywhere: a label that says "apply as a Coarse or coarser spray" is naming this classification, not a general description.

Droplets smaller than about 150 microns are considered "driftable fines" - they can remain airborne for long distances. Larger droplets (Coarse and above) settle faster and drift less, but they reduce coverage on small targets such as insect pests or fungicide targets. Always follow the label's droplet-size requirement when one is specified.

Wind, Temperature, and Humidity

• Wind speed - 3 to 10 mph is the generally accepted application window. Calm air (under 3 mph) often signals a temperature inversion and is unsafe; wind over 10 mph (or below the label's stated maximum) increases drift and may violate the label.
• Wind direction - never spray when wind is blowing toward a sensitive site (beehives, organic fields, schools, surface water, or a neighbor's susceptible crop).
• Temperature - high temperatures increase evaporation, shrinking droplets into driftable fines and increasing vapor drift for volatile products.
• Humidity - low humidity speeds evaporation, shrinking droplets; high humidity preserves droplet size but may slow drying on the target.
• Temperature inversions - a layer of warm air aloft traps cooler air near the ground, and any spray released into this calm layer can drift for miles with no predictable direction. Inversions typically form near sunset, persist through the night, and break up shortly after sunrise. Do not spray during a suspected inversion.

Drift Reduction Technology (DRT)

Drift Reduction Technology is any equipment, adjuvant, or practice that reduces off-target movement. EPA recognizes DRTs as part of its ESA and label mitigation strategies, and some labels now require a specific DRT category.

Buffer Zones

A buffer zone is an untreated strip between the application area and a sensitive site. Buffer widths appear on the label for many products and are enforceable under FIFRA. Buffer widths typically scale with application rate, droplet size, and the sensitivity of the off-target feature (e.g., a wider buffer for a public water supply than for a pasture).

Iowa Bee and Pesticide Notification Rules

Iowa Administrative Code 21-45.31 (the Iowa "Bee Rule") governs application near registered apiaries. The key provisions are:

• Between 8 a.m. and 6 p.m., commercial applicators shall not apply pesticides labeled toxic to bees to blooming crops when located within one mile of a registered apiary.
• Applicators are responsible for maintaining the one-mile distance from apiaries listed on the registry on the first day of each month.
• Beekeepers register hive locations through BeeCheck (ia.beecheck.org) or DriftWatch (ia.driftwatch.org); registration expires December 31 each year and must be renewed annually.
• Applicators can look up registered sites through the FieldCheck tool.

Practical Drift-Management Checklist

• Use the largest droplet size the label allows for the target pest.
• Spray when wind is 3-10 mph and blowing away from sensitive sites.
• Stop spraying during temperature inversions (calm, hazy, dewy conditions near sunrise/sunset).
• Keep the boom as low as practical while maintaining even coverage.
• Use a drift-control adjuvant when the label permits.
• Observe all label buffer zones.
• Check the Iowa Sensitive Crop Registry for beehives and specialty crops before each job.