Proper Pesticide Application Takes Precision

By Elena M. Toro|April 1, 2013

Pesticides are widely used on agricultural crops, in the home, for mosquito control, in yards, and in public places. The types of pesticides commonly used include insecticides, herbicides, fungicides and rodenticides. Some of the benefits of pesticides are increased crop production, preserving produce, combating insect infestations and controlling exotic species. Pesticides are designed to be harmful to pests. When not used properly or if off target movement of pesticide occurs, pesticides can cause harm to humans, animals, or the environment.

According to Dr. Fred Fishel and Dr. Jay Ferrell from the University of Florida Agronomy Department drift is a significant legal concern in Florida. During 2009–2010, the Florida Department of Agriculture and Consumer Services (FDACS), which is the state pesticide regulatory agency, initiated 39 investigations in response to allegations of drift. Where significant drift does occur, it can damage or contaminate sensitive crops, poison bees, pose health risks to humans and animals, and contaminate soil and water in adjacent areas. Applicators are legally responsible for the damages resulting from the off-target movement of pesticides.  Scientists recognize that it is impossible to eliminate drift totally but also know it is possible to reduce drift it to a legal level if the directions on the pesticide label are followed.  The laws that address drift focus on preventing substantial drift.  Consequently, pesticide applicators need to understand the factors influencing drift and use common-sense solutions for minimizing potential drift problems when pesticide applications are conducted.

Types Of Pesticide Drift

What are the types of drift? In Florida, drift can also have legal meaning, as there are penalties for damage caused to sensitive crops by certain types of herbicides. Off-target movement can be in the form of:

• Spray droplet drift
• Vapor drift
• Particle (dust) drift

Spray Drift

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Spray drift refers to the off-target movement of a pesticide during a liquid application. This is the result of small spray droplets being carried off-site by air movement. Spray drift occurs more frequently than the other two types of drift because almost all spray applications result in some off-target movement.

Vapor Drift

Vapor drift refers to the movement of pesticides as gaseous vapors from the target area. Some pesticides are volatile and can change readily from a solid or liquid into a gas under the right conditions. This most commonly occurs with high air temperatures. Pesticides that have volatilized into a vapor or gas may drift farther and for a longer time than they would have as spray droplets. Only those pesticides that are able to volatilize are susceptible to vapor drift. As air temperatures increase, the likelihood that these pesticides will volatilize and drift also increases.

Whenever possible, choose a pesticide formulated as a low-volatility product. Avoid applying volatile pesticides on hot days. Some products can even volatilize several hours after application, so beware if high temperatures are predicted for later in the day. Many products carry precautions against applying these products when temperatures are above 85°F or expected to reach 85°F. Remember to check label precautions for product-specific concerns about vapor drift.

Some herbicide formulations are sufficiently volatile to cause plant injury from drift of vapor. For example, 2,4-D esters may produce damaging vapors, while 2,4-D amines are essentially nonvolatile and can drift only as droplets or dry particles. Herbicide vapor may drift farther and over a longer time than spray droplets.

Particle Drift

Particle drift refers to the movement of solid particles from the target area by air during or just after an application. These solid particles may include pesticides formulated as dust or soil particles to which pesticides are attached. Some pesticides can remain active on soil particles for long periods after they are applied. If particles are blown off the target area, contamination or damage to sensitive areas can occur. Be sure to close all windows, vents, and turn off all circulating fans, forced-air heating systems, and air-conditioning units to prevent particle drift from nearby outdoor pesticide applications from entering a building.

Influencing Factors

Drift can occur in all forms of pesticide application: using agricultural airplanes, ground sprayers, airblast sprayers, or irrigation systems. In general, drift can be influenced by factors in one of these four categories:

• Spray solution characteristics
• Weather
• Application equipment
• Applicator decisions

Pesticide Spray Characteristics

Droplet Size

The overwhelming factor influencing drift is droplet size. Small droplets do not have enough mass to drop quickly, so they remain airborne and exposed to air movement longer than larger droplets. Droplets are measured in microns, or micrometers (µm). One micron is equal to one millionth of a meter—the equivalent of 1/25,400th of an inch. Without magnification, particles less than 100 µm in diameter are practically invisible. Table 1 presents a relative perspective of droplet sizes.

The term, volume median diameter (VMD), is used to indicate the relative droplet size of a volume of spray from a nozzle. A VMD of 400 µm means that half the volume of spray will be droplets that have a diameter of less than 400 µm, and the other half of the volume of spray will be droplets larger than 400 µm. Because smaller droplets have much less volume than larger droplets, most of the droplets will be smaller than the VMD.

Table 1.  Relative size of spray droplets.

(Source: Fishel and Ferrell, 2010)

The larger the droplet, the faster it will reach its target. Larger droplets will fall faster and be less exposed to wind. The higher a droplet is released and the stronger the wind, the greater the chance that a droplet will travel downwind and drift. Droplets that are smaller than 150 µm are considered drift-prone.

Droplet size becomes an issue when choosing a particular setup that will effectively reduce drift potential but still maximize coverage and penetration. In general terms, there is greater coverage from many small droplets than by a few larger droplets. The relationship between a droplet size and its volume is cubic; therefore, when comparing two droplets, for example a 250 µm and a 500 µm droplet, the latter carries 8 times more volume than the former.  Using large droplets may be detrimental to some contact insecticides and fungicides that depend on coverage and penetration to be effective. On the other hand, systemic herbicides can be effectively managed using large droplets since coverage and penetration are not so critical.

Pesticide Formulation

The viscosity, or thickness, of the liquid affects droplet size. The viscosity of a liquid is a measure of its resistance to flow. For example, mayonnaise is more viscous than water. As the viscosity of the liquid increases, so does the droplet size, thus reducing the potential for off-target movement. Formulations, such as invert emulsions, have a pasty consistency that aids in reducing drift. Other formulations produce some spray drift when water droplets begin to evaporate before reaching the intended target. As a result, these droplets become very small and light and may move from the target site. Thus, invert emulsions have less water loss, and more of the pesticide reaches the target.

Drift Retardants

Drift retardants are usually added to the spray mixture in order to increase the viscosity of the spray solution. Increasing the viscosity of the spray will reduce the number of small droplets (the ones smaller than 150 µm). There are several different drift retardants in the market with various degrees of efficacy. Although drift retardants are a tool to be used to decrease drift potential, their contribution is limited. Research has shown performance of drift retardants to be inconsistent. The best management practices to minimize drift potential are to correctly manage nozzle type, height, and operating pressure. Do not rely solely on drift retardants; there is no drift retardant that will eliminate drift.

Crop Protection And Weather Considerations

Wind Speed

Wind speed is the most important weather factor influencing drift; but unfortunately, applicators have no control of it. High wind speeds will move droplets downwind and deposit them off target. On the other hand, dead calm conditions are never recommended because of the likelihood of temperature inversions. Drift potential is lowest at wind speeds between 3 and 10 miles per hour—that is, a gentle but steady breeze, blowing in a safe direction away from sensitive areas.

The most effective way to check wind speed is to use a wind meter. Depending on the level of accuracy desired, models on the market range in price from around $20 to $300. To be accurate, wind meters should be used in places with no obstructions and not in places, such as near buildings or large trees that may mask wind speed.

Wind Direction

Wind direction will influence where off-target spray droplets will be deposited. A careful operator will try to apply pesticides whenever the wind is blowing away from sensitive areas. The use of a spray buffer downwind to protect sensitive areas and crops is an effective way to minimize drift.

Florida law should also be kept in mind regarding wind speed and direction. The Organo-Auxin Rule (discussed in-depth later in this article) was designed to protect sensitive crops from injury caused by drift of these herbicides. Depending upon the location of a sensitive crop, there are defined wind speeds and direction that may prohibit the application of some herbicides that are prone to drift.

Air Temperature and Relative Humidity

High air temperature and low relative humidity go hand-in-hand in creating a worst-case scenario for pesticide drift. Use special caution when relative humidity is below 50 percent and when temperatures are high. Drift is more likely during the hottest part of the day. Hot, dry conditions reduce droplet size through evaporation and thereby increase drift potential. Under these conditions, spray droplets can evaporate very fast and become more susceptible to wind forces. Air temperature can also influence atmospheric stability and off-target movement of spray droplets.

Temperature Inversions

A temperature, or thermal, inversion is a condition that exists when the air at ground level is cooler than the temperature of the air above it. Temperature inversions occur naturally and are part of a daily atmospheric cycle, occurring in the early morning hours when the ground cools the air layer immediately above it. Such a condition is conducive for pesticide drift. Inversion conditions result when warmer air above traps cooler air located near the surface of the ground; this can be readily visible if dust or smoke rises little from its source and fails to dissipate (Figure 7). These conditions are more likely to occur in the early morning or evening. Applications made under low-wind conditions can sometimes result in more extensive drift than those made under high winds. Drift that occurs over long distances (over a mile) is most often the result of applications made under stable atmospheric conditions such as temperature inversions.

Except in the case of temperature inversions, the early morning and evening are often the best times to apply pesticides because windy conditions are more likely to occur around midday when the temperature warms near the ground. This causes hot air to rise quickly and mix rapidly with the cooler air above it, favoring drift. During stable conditions, a layer of warm air can stay overhead and not promote mixing with colder air that stays below, closer to the ground. Inversions tend to dissipate during the middle of the day when wind currents mix the air layers. It is very important that applicators recognize thermal inversions and do not spray under those conditions.

Pesticide Application Equipment

Nozzle Selection

Select the best nozzle type and size for each type of application you make and use drift reduction nozzles. Remind yourself that nozzles are cheap compared to the cost of materials and cheap compared to the yield losses or litigation costs that can result from a poor application. Most improved nozzle designs really do have superior drop size characteristics. Drift reduction nozzles are designed to create larger droplets at the same flow rate and operating pressure as comparable standard flat-fan nozzles. To help applicators select nozzles according to droplet size, spray equipment manufacturers are including drop size charts with their respective catalogs and with Web sites that use color codes set by the American Society of Agricultural and Biological Engineers (ASABE). Pesticide labels state specified droplet sizes to use when applying the product

Boom/Nozzle Configuration

For ground sprayers, keep the boom close to the target (soil surface or canopy) in accordance with the nozzles being used. Nozzles with wider spray angles allow lower boom heights. Do not spray at greater heights than those recommended by the spray tip manufacturer.

Spray Height

The spray release height will influence how far droplets will travel downwind. This may not be as critical for ground sprayers, but it is very important for aerial application. With aerial applications, pilots should try to maintain an optimum distance from the crop, generally with nozzles about 8 feet above the ground or crop, compared with forestry or rangeland applications that are sometimes made at 20 feet or higher.

Spray Pressure

Pesticide product labels contain generic statements concerning the use of the lowest possible spray pressures while achieving adequate droplet size and coverage. In general, higher pressures will produce more fine droplets and increase the drift potential of an application. Nozzle tips are available that, when operated at relatively high spray pressures, will produce coarse to very coarse droplets as compared to other designs of nozzles that would produce more fine droplets at the same pressure.

Pressure has been demonstrated to not be an effective way to increase coverage and canopy penetration. Furthermore, increasing pressure is not an efficient method to make significant increases in output volume. In order to double output, pressure would have to be increased four-fold. For example, increasing flow from 5 gallons per acre (gpa) at 10 pounds per square inch (psi) to 10 gpa would require that pressure be increased to 40 psi. Such an increase would increase the number of fine droplets produced as well. When significant changes in output volume are necessary, changing to a nozzle tip with a larger orifice is most practical. Pressure gauges should be checked periodically for accuracy.

Calibrate The Sprayer And Replace Worn Nozzles

Calibration is the process by which the amount of pesticide being applied per a unit of area is determined. Calibrate your pesticide applicators on a regular basis to ensure that output from each nozzle is consistent, and the desired application rate is achieved.

Worn nozzles produce spray patterns and poor droplet distributions that result in poor coverage, increased drift, or both. Some nozzle materials wear faster than others, and applying certain abrasive pesticide formulations, such as wettable powders, increases the rate of wear.

Local Pesticide Rules And Regulations

Florida’s Organo-Auxin Rule

Chapter 487 of the Florida Statutes states “It is unlawful for any person to apply a pesticide directly to, or in any manner cause any pesticide to drift onto, any person or area not intended to receive the pesticide.”

The Florida Organo-Auxin rule applies to the application of organo-auxin herbicides anywhere within the state as there are many agricultural plants sensitive to organo-auxin herbicides.  Organo-auxin herbicides have different formulations possessing certain characteristics. Amine and ester formulations are the most popular although other forms of phenoxys exist. As a general rule ester formulations are more active than amines. This difference in control activity has made ester formulations very popular due to the fact that about one half the rate of the amine formulation can be used to achieve the same weed control level. Therefore, growers can buy less total herbicide in the ester form and do the same job as a larger amount of a phenoxy in the amine form.

Although ester formulations are more active herbicidally than amine formulations, they do have serious drawbacks associated with their use. Specifically, ester formulations are typically very volatile and possess the ability to move away from the target site up to several days after the initial herbicide application has been made. Volatilization problems have led to the complete destruction of nearby sensitive crops if weather conditions were favorable for volatilization to occur. Sub-lethal doses of organo-auxin herbicides cause very visual effects, indicative of hormonal action.

Specifics of this rule are included in Table 2.

Table 2.

FLORIDA ORGANO-AUXIN HERBICIDE RULE NO. 5E-2.033 ORGANO-AUXIN HERBICIDES

A suggested recordkeeping form developed by FDACS is available for applicators of organo-auxin herbicides to record their data. Although this specific form is not required, it does contain spaces for providing the required data to be recorded.

References

• Fishel, F. (2012). Why do we use pesticides? http://edis.ifas.ufl.edu/pi140
• Fishel, F. and Ferrel, J. (2012). Managing Pesticide Drift. http://edis.ifas.ufl.edu/pi232
• Fishel, F., Ferrell, J. , McDonald, G. and Brecke, B. Florida’s Organo Auxin Rule- 2012. (2012) http://edis.ifas.ufl.edu/wg051

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