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SIZE DOES MATTER!

Jay Turner of Laguna Pest Control goes behind the scenes to learn what it takes to make a successful bait formulation.

Typically, the first thing most pest managers look at when choosing an insecticide is the active ingredient and its cost. But as I’ve recently learnt, there is a lot more to it, and as with any product or service, the quality can vary!

By no means do I profess to being any sort of chemical engineer or formulation chemist, but having visited both the Sherwood Chemicals and Renovo Technologies manufacturing plants, I’d like to think I now have a grasp of the basic fundamentals of how the products we use are put together. Both these companies highlighted the importance of consistently producing a high quality liquid formulation. (And just for those that are unfamiliar with Renovo Technologies, Renovo are the developers and producers of the liquid product range for Sundew Solutions.)

There are many factors that go into producing a quality formulation but by far one of the key parameters is consistent active ingredient particle sizes.

Here’s how it works. Active ingredient molecules often bind together to form particles, and the size of these particles can then affect the efficacy of the active ingredient. Typically, smaller particles have a quicker knockdown but shorter residual and bigger particles typically have a longer residual but the knockdown effect is compromised. It’s a little more complex than this and there are other factors to consider but that is the basic concept.

So, in an ideal world a liquid insecticide should possess both these qualities – a quick knockdown effect and as well as a long residual. How to achieve this, though? The simplest answer would be to have a big variety of particle sizes, but it isn’t quite as simple as that. There is a limit as to how much active you can put into a formulation before it simply falls out of suspension (remember there are lots of other additives that also have to t into the formulation) and there is also a limit on how big or small you want the particles. Too small and particles will degrade too quickly, or get absorbed into the treated surface; too big and particles will create a whole host of issues, such as wear and tear on equipment, visible residue, sedimentation issues and of course the fact that all your active will be bound up in these bigger particles, making it easier for pests to detect.

So obviously there is an optimum particle size range – thus the trick is to consistently produce active ingredient particle sizes all within this narrow range.

Mark Linton showing Jay Turner a particle size analysis graph

For those that love spreadsheets and figures we can actually use a particle size calculator to illustrate the effects that changing the size of the particles can have on the physical aspects of a suspension.

Particle calculations for 1ml of 0.079% solution of Bifenthrin SC

Active particle size

20μm

5μm

0.1μm

Surface area

0.0013mm2

0.000079mm2

0.000000031mm2

Number of particles per ml of spray mix (approx)

190,000

12 Million

1,500 Billion

Total surface area of particles

2.38cm2

9.52cm2

476.2cm2

Number of spray droplets per ml of spray assuming 50μm droplet size

15 Million (approx)

Number of particles per spray droplet

0.012

0.794

99206

% Droplets without active

98.80%

20.60%

The table above summarises the particle size calculations for three different particle sizes of bifenthrin, at 20μm, 5μm and 0.1μm. Firstly, we can see how decreasing the particle size also decreases the surface area of the particles but, overall, the total particle surface area is greatly increased (because there are more particles per spray droplet). Now, as we know, pyrethroids are contact insecticides, so contact with the product should deliver a lethal dose. Thus, the smaller the particle size, the greater total surface area, thus more potential for contact.

Secondly, we can see that increasing the particle size decreases the number of active particles per spray droplet. So, for larger particle sizes, many spray droplets will not have any active. For example, if the suspension was made up of purely 20μm active particles with a 50μm spray droplet then 98.8% of spray droplets will not have any active. An alternative way of looking at it would be that, on average, for every 81 spray droplets only one of them would contain an active ingredient particle. In contrast, for smaller active particles there can be many thousands of active particles per spray droplet.

However, for those who are like me and prefer to use analogies to visualise a concept, imagine enlarging the whole scenario so that insects were the size of humans, and large active ingredient particles were the size of beach balls, but small active ingredient particles would be the size of bean bag balls. Now imagine one beach ball in a room and a person having to negotiate their way from one side of the room to the other – chances are that they won’t come into contact with the beach ball! Now imagine swapping the beach ball for hundreds of tiny bean bag balls that make up the same volume as the beach ball. If you spread them out across the floor, the chances of the person walking from one side of the room to the other without treading on the tiny balls is not very likely, if not impossible.

Jay displaying two different sizes of zirconium milling beads

So how do our chemical manufacturers control the size of the particles? Through a process called milling (main picture, above) where basically a slurry of the active ingredient is fed through a mill full of specifically sized hardened beads until the particles reach the desired size range. A sample of the slurry is then placed in a laser scattering particle size distribution analyser, which can then display the particle size variance as a graph (Figure 2).

Figure 2: Distribution of particle sizes in a sample of bifenthrin slurry

A good manufacturer will test this slurry multiple times throughout the milling process until the desired particle size range is reached. These companies have tight parameters for particle size variance unlike a poorly manufactured product, which often has particle sizes well beyond the optimum range, and therefore poor efficacy.

Hopefully this article has given you a greater appreciation of the importance using an expertly formulated insecticide as there is a lot more to a formulation than just which active it contains. For me, it is important that I use a product that I am con dent will deliver consistent performance on all my jobs.

I would also like to take this opportunity to thank Renovo Technologies (Sundew Solutions) and Sherwood Chemicals for their hospitality, allowing me to see behind the scenes and answering my millions of questions.

If you are interested in this and want to nd out more, Mark from Renovo Technologies will be at this year’s Rapids Solutions EduCON conference on the Gold Coast and will be happy to go into greater depth about this or other formulation topics.

Jay Turner, Laguna Pest Control