How does spray droplet size affect pesticide coverage?

When applying pesticides, droplet size directly influences the movement of the spray in the air, its ability to reach the target surface, and the resulting coverage and deposition.

Smaller spray droplets generally offer greater potential coverage, but are also more susceptible to wind and evaporation. Larger droplets generally have a higher mass and momentum per droplet, often resulting in a lower risk of drift; however, fewer droplets are formed per unit volume of liquid, which may lead to insufficient coverage in certain application scenarios.

This means that selecting spray droplet size is not simply a matter of prioritising finer or coarser droplets, but rather striking a balance between target coverage, pesticide deposition, canopy penetration and drift control.

For pesticide importers, distributors, agricultural technical service providers and large-scale farming enterprises, understanding these fundamental principles helps explain why the same pesticide may exhibit different application outcomes under varying nozzle, pressure and application conditions.

This article will explain how to interpret these differences by examining droplet size, spray coverage, target deposition, equipment parameters and real-world application scenarios.

What is spray droplet size?

Spray droplet size refers to the diameter of the droplets produced by spray equipment, typically expressed in micrometres (µm).

When the spray liquid passes through the nozzle, droplets of varying sizes are formed under the combined influence of factors such as pressure, orifice geometry and the physical properties of the liquid.

In practice, a spray typically does not consist of droplets of exactly the same size, but rather contains a continuous size distribution, known as the droplet spectrum.

For example, a single spray may contain both smaller and larger droplets simultaneously. Even if two spray regimes have the same median droplet diameter, their proportion of small droplets and overall size distribution may differ.

Therefore, when evaluating spray quality, one should not rely solely on a single droplet diameter value, but should also consider the complete droplet spectrum and the actual application conditions.

What is the Volume Median Diameter (VMD)?

The Volume Median Diameter is commonly referred to as VMD and is also frequently denoted as Dv0.5.

It is the median diameter calculated based on the volume of spray liquid represented by the droplets: approximately half of the spray liquid volume comes from droplets smaller than this value, whilst the other half comes from droplets larger than this value.

It is important to note that VMD is not the arithmetic mean of all droplet diameters, nor does it imply that the majority of droplets have this diameter.

For example, two sprays may have the same VMD, but one may contain more fine droplets whilst the other has more large droplets. There may still be differences between the two in terms of drift risk and actual coverage.

Therefore, whilst VMD is an important indicator for understanding the droplet size distribution, it cannot be used on its own to judge the final spray quality.

What is the difference between spray droplet size and pesticide formulation particle size?

These two concepts are often confused, but they describe entirely different things.

For example, solid particles in an SC may have a specific particle size distribution; however, after passing through the nozzle, these particles are still carried into the air by the spray droplets and deposited onto the target surface.

The particle size in SC formulations cannot be used to directly infer spray droplet size.

For further technical information on formulation particles, please refer to: How does the particle size of SC pesticide formulations affect product performance?

How does droplet size affect the coverage of pesticides?

Droplet size influences coverage primarily through the number of droplets, their spatial distribution, their trajectories and their deposition behaviour on the target surface.

Assuming the same spray volume, ideal conditions and spherical droplets, smaller droplets generally mean that more droplets can be formed.

For example, a spherical droplet with a diameter of 200 µm has a volume of approximately one-eighth that of a droplet with a diameter of 400 µm. Therefore, given the same liquid volume, theoretically more droplets with a diameter of 200 µm can be formed.

However, this is merely a geometric relationship and does not imply that actual field coverage will necessarily increase eightfold.

This is because droplets must also successfully reach the target and remain on its surface in order to form an effective deposit.

Smaller droplets: offering greater potential coverage

Where other conditions are similar, smaller droplets can generally form in greater numbers, offering the potential to increase coverage density per unit area.

This is of particular value in application scenarios requiring relatively uniform foliar coverage.

However, smaller droplets also have their limitations:

• They are more prone to having their trajectory altered by air currents;
• They may evaporate more rapidly under high-temperature, low-humidity conditions;
• They may drift before reaching the target;
• They are more susceptible to the influence of canopy airflow and spray equipment settings.

Consequently, the fact that spraying produces more fine droplets does not guarantee that all of these droplets will effectively deposit onto the target.

Larger droplets: generally carry a lower risk of drift

Larger droplets typically have greater mass and inertia; when subjected to the same airflow disturbances, their trajectories tend to be less easily altered than those of fine droplets.

This is why some pesticide labels specify the use of a particular droplet size to reduce the risk of drift and exposure in non-target areas.

However, larger droplets also present issues that need to be considered.

If the number of effective droplets per unit area is insufficient, or if the target surface is difficult to cover adequately, uneven coverage may occur.

Furthermore, droplets may bounce off, splash or run off after striking foliage. Under certain conditions, larger droplets may also lead to localised pooling or runoff.

Therefore, larger droplets do not necessarily mean greater deposition of the active ingredient, nor do they imply that coverage will inevitably be poorer.

What is the difference between Fine, Medium and Coarse droplets?

Terms such as Fine, Medium and Coarse are commonly used to describe relative categories within a spray droplet spectrum.

Specific classification should be based on applicable standards, nozzle manufacturer’s data and pesticide labels, rather than directly applying a fixed micrometre range that is suitable for all equipment.

The comparisons provided here are general guidelines, not product recommendations.

When applying pesticides in practice, one cannot determine whether a particular pesticide is suitable for a specific spraying method based solely on droplet size. It is also necessary to consider label requirements, crop canopy, target location, nozzle performance and weather conditions.

Why is coverage not the same as spray deposition or ultimate biological efficacy?

These are three distinct concepts that must be distinguished when evaluating spray quality.

Coverage refers to the extent to which the target surface is covered by the spray; deposition refers to the amount of spray or active ingredient that actually reaches and remains on the target; biological efficacy, on the other hand, refers to the ultimate control outcome.

For example, two spraying schemes may produce similar leaf coverage, but one scheme may concentrate the spray solution more on the upper surface of the leaves, whilst the other may result in a different distribution within the target canopy.

For products that require contact with the target, the actual deposition site may be of critical importance.

For active ingredients that need to be absorbed by plants, the absorption process following deposition must also be taken into account.

Therefore, when assessing the quality of an application, attention should be paid to coverage, deposition and actual control efficacy, rather than merely to droplet size or the visual appearance on the leaf surface.

For further information on how wetting and spreading affect coverage of the target surface, please refer to Differences between wetting, dispersibility and suspension rates in pesticide formulations.

How does droplet size affect canopy penetration?

Canopy penetration refers to the ability of a spray to penetrate the crop canopy and reach the target leaves or other parts of the plant.

In crops such as maize, wheat, cotton and fruit trees, the canopy structure can significantly influence spray distribution.

Smaller droplets are more susceptible to airflow and may be carried within the canopy or deflected before reaching their target.

Larger droplets generally possess greater inertia and, under certain conditions, can penetrate parts of the leaf structure more directly; however, they may also tend to deposit primarily in the upper canopy.

Therefore, it cannot simply be assumed that fine droplets are always more likely to penetrate the canopy, or that coarse droplets will inevitably deposit only at the top.

Actual results are also influenced by the following factors:

• Canopy density and leaf arrangement;
• Nozzle orientation and spray angle;
• The speed of the spraying equipment;
• The distance between the nozzles and the target canopy;
• Whether airflow assistance is used;
• Wind speed, wind direction and internal airflow within the canopy.

For crops with dense canopies, appropriate spray direction, nozzle configuration and equipment settings are often just as important as droplet size.

In certain scenarios, even altering droplet size cannot fully compensate for issues such as inappropriate spray direction or insufficient canopy coverage.

How does droplet size affect drift and evaporation?

Spray drift refers to the phenomenon whereby the spray solution leaves the intended target area and moves to other locations.

Droplet size is one of the key factors influencing drift, but it is not the only one.

Why do fine droplets drift more easily?

Fine droplets have a relatively low mass and are more easily deflected by airflow.

If strong winds are present during spraying, some of these fine droplets may be carried away from the target area.

Temperature and humidity also affect droplet evaporation. Under conditions of high temperature and low humidity, fine droplets may shrink more rapidly, thereby further altering their behaviour.

Consequently, under conditions where the risk of drift is high, one should not continuously reduce droplet size solely for the purpose of increasing coverage density.

Why cannot coarse droplets completely eliminate drift?

Larger droplets generally have a lower potential for drift, but this does not mean there is absolutely no risk of drift.

The height of the spraying equipment, wind speed, nozzle angle and application method may still cause the spray to deviate from the target.

Furthermore, actual spray consists of a complete droplet size distribution, rather than droplets of a single size.

Even if the overall droplet spectrum is coarse, smaller droplets may still be present.

Therefore, drift control should take into account a combination of nozzle selection, pressure settings, equipment height, weather conditions and label requirements, rather than simply adjusting a single droplet size parameter.

How do nozzles and operating pressure affect droplet size?

Nozzles are a key component in determining the spray droplet spectrum.

Different nozzle designs may produce different droplet distributions. Even at the same operating pressure, different nozzles do not necessarily produce the same droplet size distribution.

Nozzle Types

Common agricultural nozzles include fan nozzles, hollow cone nozzles and low-drift nozzles utilising an air-induced design.

Different designs may vary in terms of droplet size distribution, spray angle and droplet momentum.

For example, some air-induced nozzles help reduce the risk of drift under specific conditions by forming larger droplets, thereby lowering the proportion of fine droplets.

However, this does not mean that air-induced nozzles are suitable for all pesticides and application targets.

The actual choice should still be based on the nozzle manufacturer’s technical data, label requirements and target coverage needs.

Operating Pressure

Operating pressure may also alter the spray droplet spectrum.

For many common nozzles, increasing the pressure may produce finer droplets; however, the specific changes depend on the nozzle design and operating range, and this principle cannot be generalised to all nozzles.

Changes in pressure may also affect flow rate, spray angle and equipment output; therefore, pressure should not be increased arbitrarily in the pursuit of finer droplets.

A more sensible approach is to select a pressure range that complies with label requirements, based on the nozzle performance curve and the manufacturer’s recommendations.

Nozzle Height and Application Equipment

The further the nozzle is from the target, the greater the flight distance the droplets typically travel before reaching the target.

All other things being equal, a longer flight distance may increase the likelihood of droplets being affected by wind disturbance and evaporation.

However, the nozzle height must also meet requirements for spray coverage uniformity and equipment safety; it must not be arbitrarily lowered solely to reduce drift.

Therefore, nozzle type, pressure and height must be evaluated as a whole.

How should droplet size be considered for different pesticide applications?

Different pesticides and target crops may have varying requirements for spray coverage; consequently, it is not possible to specify the same optimal droplet size for all insecticides, fungicides and herbicides.

Contact fungicides

Contact fungicides typically require the formation of an adequate protective deposit on the plant surface.

Consequently, the uniformity of foliar coverage may be an important evaluation factor.

However, one cannot assume that finer droplets will necessarily provide better protection. It is equally important whether the droplets reach the target, remain on the leaf surface and meet the product’s application requirements.

Foliar insecticides

For foliar insecticides, the selection of droplet size must take into account the actual location of the pests.

If pests are primarily distributed on the undersides of leaves or within the canopy, simply increasing the coverage density on the upper leaves may not resolve the issue.

Therefore, in addition to the droplet size distribution, the nozzle orientation, spray angle and canopy penetration conditions should also be assessed.

Systemic herbicides

Systemic herbicides need to be absorbed by the plant under appropriate conditions.

However, systemic action does not mean that spray deposition or drift risks can be ignored.

Actual application must still comply with the product label’s requirements regarding droplet size, weather conditions and application methods.

Particularly when near sensitive crops or non-target vegetation, priority should be given to complying with relevant drift control requirements rather than simply pursuing higher coverage density.

What should be checked when selecting spray droplet size?

During actual application, it is recommended to make judgements in the following order.

It is important to emphasise here that field observations can help identify issues such as uneven coverage; however, one should not judge whether the droplet size distribution is appropriate or whether the control efficacy meets requirements based solely on whether the leaf surface appears wet.

Where pesticides requiring strict drift control are involved, priority should be given to meeting the droplet size class and equipment requirements specified on the label and in local regulations.

Frequently Asked Questions

Does a smaller droplet size result in better pesticide coverage?

Not necessarily. Smaller droplets typically have a higher potential coverage density, but they are also more prone to drift and evaporation. If they do not effectively reach the target, a greater number of fine droplets does not necessarily translate into better actual coverage or control efficacy.

What is VMD?

VMD stands for Volume Median Diameter. It indicates that approximately half the volume of the spray comes from smaller droplets, whilst the other half comes from larger droplets. VMD is not the arithmetic mean of droplet diameters, nor does it alone reflect the complete droplet size distribution.

Do larger spray droplets necessarily reduce drift?

Generally, larger spray droplets have a lower potential for drift; however, drift also depends on wind speed, nozzle height, spray direction and equipment settings. Actual spray consists of droplets of varying sizes; therefore, it cannot be assumed that coarse droplets can completely eliminate drift.

Do contact fungicides always require fine droplets?

Not necessarily. Contact fungicides generally require adequate foliar coverage, but the specific droplet size should be determined in conjunction with the product label, target crop, nozzle and application environment. A uniform optimal droplet size cannot be prescribed solely on the basis that a product is a contact fungicide.

What is the difference between spray droplet size and the particle size of an SC formulation?

Spray droplet size describes the droplets produced by the nozzle; the particle size of an SC formulation describes the solid particles suspended in the liquid. As they affect different aspects of the application, one cannot directly infer the coverage and drift behaviour of spray droplets from the particle size data of an SC formulation.

How can drift be reduced whilst maintaining adequate coverage?

This should be assessed by considering the droplet size class specified on the product label, suitable nozzles and pressure, a reasonable spray height, and appropriate weather conditions for application. Increasing droplet size or adjusting pressure alone cannot resolve all coverage and drift issues.

How to balance pesticide coverage, deposition and drift control?

Spray droplet size is a key factor in the quality of pesticide application, but it is not the sole variable determining control efficacy.

Smaller droplets may increase potential coverage density but are more susceptible to the effects of air currents and evaporation; larger droplets generally help to reduce the risk of drift, but it is still necessary to ensure that the spray solution reaches the target and forms an appropriate deposit.

For different pesticides, crops and application equipment, an appropriate droplet size distribution must be selected by taking into account target coverage requirements, product labels, nozzle performance and actual operating conditions.

A truly effective spraying programme does not involve seeking the smallest or largest droplets, but rather ensuring that the appropriate amount of spray reaches the correct target location, whilst complying with label and safety requirements.

If you are evaluating pesticide products with different active ingredients and formulation types, you may consult the POMAIS Pesticide Product Centre to gain further insight into the active ingredients, formulations and technical specifications of various commercial products, and to assess them in light of the registration and usage requirements of your target market.