4.2 Minimizing Pesticide Drift in Orchards Part I

Disease and insect control is a critical factor in most commercial orchards. While such control may, in some seasons, be a small proportion of crop value, there is a demand from growers for increased efficiency of spraying. Attention to detail is necessary to improve efficiency of deposition, reduce drift and increase sprayer output. Spray drift of pesticides is an important and costly problem facing pesticide applicators. Drift results in damage to susceptible off-target crops, environmental contamination to waterways and a lower than intended rate to the target crop, thus reducing the effectiveness of the pesticide. It can also lead to strained relations with adjoining landowners.

There are two types of drift, particle drift, often very noticeable, and vapor drift. The amount of vapor drift will depend upon atmospheric conditions such as humidity, temperature and the product being applied, and can occur days after an application is made. Drift is influenced by many interrelated factors including droplet size, nozzle type and size, sprayer design, weather conditions and, last but not least, the operator.

4.2.1 Drift Factors

Droplet Size - In the past, trees were sprayed with high volumes and coarse droplets at 200-400 gallons per acre, resulting in trees dripping with excess pesticide. High application volumes leads to environmental pollution, such as soil contamination. It also requires an excessive number of tank loads per acret.

Using lower volumes may result in smaller droplets being sprayed, although there is a limit to droplet size because of drift concerns. Droplets under 150 microns generally pose the greatest hazard; droplets less than 50 microns have insufficient momentum for impaction and remain suspended in the air indefinitely or until they evaporate. Research in England concluded that a 100-micron droplet takes approximately 11 seconds to fall 10 feet in still air. A similar size droplet released into a 5 mph wind will drift about 75 feet before hitting the ground.

Operating pressure affects droplet size. The higher the operating pressure, the smaller the droplet; the lower the pressure, the larger the droplets. While large droplets are less susceptible to drift, they may bounce off the target. Traditional air blast sprayers produce many small droplets, which often move off-target. Certain spray surfactants are designedto reduce the number of drift-prone droplets.

Nozzle Type and Size - Modern nozzles, such as air inclusion nozzles, are designed to produce larger droplets than conventional cone nozzles. Large droplets normally roll off the leaf, but air inclusion nozzles create air bubbles within the larger droplets, which “explodes” on contact with the leaf, dispersing the liquid. Research in England and Germany has shown promising results using air inclusion nozzles with air blast sprayers in trees and bushes, although further trials are necessary in apple orchards.

Rotary atomizers create smaller, more uniform droplets, which would normally drift. When used in conjunction with a tower and cross-flow fan design, the smaller droplets are actually directed into the canopy. This type of sprayer, referred to by some as controlled droplet application, produces 95-98% of its droplets all of the same size. The size produced depends on the speed of the spinning cage. Advantages include less water, resulting in better timeliness and a more targeted spray. Research in the US and Europe shows that small droplets on a target are effective at controlling diseases and insects.

Sprayer Design - Tower sprayers and tunnel sprayers are better at targeting the spray into the canopy, reducing drift and increasing deposition. The conventional air blast sprayer sends droplets in an air blast from a central fan upwards into the canopy. The tower sprayer, using an air curtain and rotary atomizer, was developed by agricultural engineers at Michigan State University several years ago and has shown excellent results at disease and insect control. Horizontal penetration into the canopy is preferential to vertical penetration from an air blast sprayer. Tunnel sprayers, developed in Europe and the US, have tremendous advantages in orchards using trellis designs and dwarf trees. The use of a spray collection device at the base of the tunnel canopy results in the ability to recirculate spray, saving on pesticide and reducing drift.

Drift problems increase when a gap occurs within the row. Air blast sprayers, with or without a tower, can be fitted with ultrasonic or laser canopy sensors. The sensors also detect the shape of a tree and adjust the spray pattern accordingly. Advantages to these systems include reduced drift and ground deposition, reduced pesticide use, and improved logistics.

Herbicide drift should not be forgotten. Shielded herbicide sprayers can be used to prevent drift from contaminating apples and damaging leaves. Shields vary from the simple to the complex, use hydraulic flat fan nozzles to controlled droplet applicators using reduced herbicide rates. Shielded sprayers also allow growers to apply herbicides in variable weather conditions.

Sprayer Calibration - Correct calibration will ensure that all the nozzles are discharging the correct amount of liquid at the correct distance and angle to the target and at the correct forward speed. Operators must set the air deflectors correctly to confine airflow, spray, and disturbance to the tree canopy.

Weather - Wind speed and direction, relative humidity, temperature and atmospheric stability affects drift. Applying the correct product to the correct target at the correct time with the correct equipment is the key to good spraying.

Research in England and New Zealand has shown the effectiveness of shelterbelts (windbreaks). Natural and artificial belts were used and drift is reduced closer to the shelterbelt. Shelter belt height and density will affect drift, and may, in certain conditions, create additional air currents and eddies. There are many variables, such as topography and wind direction, that it is difficult to conclude that research at one site is transferable to another.

Forward Planning - “Forward planning is the key to good management”, a phrase often used by successful business managers, also applies to orchard management. Choose the correct size sprayer with good back-up support to ensure that spraying may be done in a timely manner. Far too often, growers are racing around orchards in an attempt to apply pesticides after a problem disease or insect attack has occurred. Good logistical support reduces the need to return for frequent refills. The use of orchard field cards detailing the block, pesticide required, application rate, quantity required per tank fill, etc., will improve efficiency and safety. Integrated pest management (IPM), including monitoring of pests and disease forecasts, should be conducted to allow sufficient time to apply a needed pesticide in the area to be sprayed.

Continuing development of spray application techniques will improve orchard spraying efficiency. Many factors affecting application and drift are interdependent. Airflows must be optimized, particularly where smaller droplets are used; crop canopies and water volumes must be carefully considered if growers are to take advantage of new technologies. Investment levels in modern technology must be maintained if the grower is to remain competitive.