Eric Hess, managing director of Agserv, explains why less is more when it comes to spray droplet size. 

Droplet size is extremely important in the pest control industry. Getting your droplet size right can greatly increase the effectiveness of your applications and enhance your overall coverage, as well as reduce application costs and ensure a more satisfying outcome for your customers.

Understanding droplet sizes

Droplet sizes are understood in the following ways:

  • Droplet sizes are measured using microns (or micrometres); 1 micron = 0.001mm = 1μm
  • Microns are calculated by considering the diameter of the individual droplet
  • Droplet size spectrum (DSS) is the distribution of droplets to form classes, as delivered from the spray nozzle
  • The size of droplets can be controlled by using different sized nozzles or devices, calibrating machinery or manipulating the pressure of the application (or a combination of the above).

Getting droplet sizes right

Active chemicals, such as insecticides, fungicides, disinfectants and deodorisers, achieve the best results when they are applied in small droplets between 10 and 100 microns (variances in between will depend on the chemical type).

However, the best droplet size within this range will depend on the formulation type, the target surface, spray situations and safety considerations. Discussion of all the possible combinations is beyond the scope of this article, but it is definitely getting input from your chemical and equipment suppliers as to the best nozzle for the job.

From a safety point of view, spray drift, particularly outdoors, needs to be taken into account as part of the risk assessment before spraying. Smaller droplet sizes are more prone to spray drift, with the smallest droplets being affected by the slightest of winds. When considering personal safety, safety of others and the environment for outdoor surface treatments, a larger droplet size should be used and spraying should not take place under windy conditions.

Space spraying

Using various, smaller droplet sizes, is hugely beneficial when it comes to space spraying. Space spraying is a very useful method that is used to eliminate flying insects (both in indoor and outdoor spaces). Generally speaking space sprays require smaller droplet sizes for good ‘hang time’, creating an insecticide mist which the flying insect cannot avoid.

Space spraying is generally carried out using one of the below methods.

1. Fog spraying

Fog or thermal spraying, using thermal foggers, uses high temperatures to produce large quantities of fog without degrading the active chemical. Thermal foggers are great if you want to quickly produce a large quantity of very small droplets.

Best droplet size: Around 10 microns.

2. ULV spraying

Ultra low volume (ULV) spraying is administered by fogging machines that use large volumes of air at low pressures to transform liquid into droplets for dispersing. This is ideal for producing very small droplets and when treating pests like mosquitoes, birds, midges and other flying insects.

Best droplet size: 1-50 microns.

3. Mist spraying

Misting or cold fogging is used commonly in pest control applications. While it is similar to ULV spraying, mist spraying often has the capacity to produce larger droplets than ULV applications, meaning your droplets can have a much heavier weight and are therefore less susceptible to wind and spray drift.

Best droplet size: 10-150 microns.

4. Aerosol spraying

Aerosol sprays also see widespread use and the main benefit of aerosols is their ability to produce tiny droplets. Aerosol spraying first entered the Australian market in 1950, thanks to Mortein. The Pestigas product produced by BOC is a common example of a high pressure, non-flammable aerosol spray currently on the market; Pestigas has good safety profile and uses liquid carbon dioxide in its application.

Best droplet size:1-50 microns.

Misting and fogging may look similar but the droplet size is quite different; misting (10-150 microns), fogging (around 10 microns)

Surface spraying

For surface spraying, it is all about getting good coverage on the surface to be treated. Droplet sizes for surface sprays tend to be a lot larger than for space sprays – there is no need for the droplets to hang in the air and the larger droplet sizes help reduce spray drift in outdoor spraying.

Typically there are three methods of application, outlined below.

1. Backpack mister

Produces a fine mist, generally best suited to outdoor surface applications, particularly for residual mosquito treatment applications.

2. Hand-held sprayer

The 5L hand-held pressure spray is the mainstay of the pest control industry. With its wide selection of nozzle types it has the flexibility to be used in a wide range of situations.

3. Truck-mounted hand hose

For those involved in termite treatments, truck mounted sprayers are typically used to flood trench treatments or inject drill holes. For such equipment it is less about droplet size and more about accurate flow rate and speedy application.

Of course a large range of different nozzles are available for most devices, each nozzle being ideally suited to a different application situation, providing benefits in terms of accuracy of application and reduction in spray drift.

Agserv staff can assist with advice on what droplet size is best for your next application.

Eris Hess, Managing Director, Agserv