Steve Broadbent, regional director of Ensystex, explains how wettable powders work and gives tips on when to use them for best results.

A wettable powder (WP) is a solid pesticidal formulation supplied in a micronised powder form. Typically, wettable powders consist of the active ingredient adhered to a mineral clay carrier. A dispersant and a wetting agent are also added. The dispersant ensures the particles do not clump together in the spray tank and the wetting agent ensures a more even spray coverage of the target surfaces.

Wettable powders generally provide the longest residual performance, especially on porous surfaces, but suffer from the fact that they leave a visible residue.

In discussing wettable powders I am perhaps more than ever showing my age! When I entered the pest management industry in 1980, the latest concept in pesticide formulation technology was wettable powders (WP). Fison’s had launched Ficam (bendiocarb) and shortly afterwards Bayer launched Baygon (propoxur) as a wettable powder. The emulsion concentrate (EC) version of the latter had been one of the most commonly used products in the industry. Now, in Australia, propoxur has almost disappeared as an active and Ficam, now a Bayer product, is probably the only wettable powder still used by professionals.

The key benefits wettable powders brought to the market in those days were improved residual performance and more importantly, they were odourless. This lack of odour and indeed the lack of the flammable oil-based solvents used in ECs, provided significant benefits to pest managers. Although there were always clients back then who would complain at the lack of a noxious smell – a common misconception in those times was that if the chemical didn’t smell awful, then it couldn’t be very good at killing insects! How times have changed.

So we saw emulsion concentrates replaced as surface sprays for general insect pest control, by the odourless and more residual wettable powders. The negative aspect to wettable powders was that they left a visible residue. Whilst technically they did not stain, as the residue could be wiped off most surfaces, they did leave an unsightly residue unless applied deep into cracks and crevices. The less concentrated the product, the more visible the residue (as more product and therefore more carrier needs to be applied to achieve the correct dose on the surface).

Wettable powders still generally provide the best residual performance for a given insecticidal active constituent, especially on porous surfaces such as concrete, timber, bricks, and especially on mud bricks and the like. So wettable powders are still very commonly used as surface residual sprays for public health purposes in third world nations, to combat various vector insects such as mosquitoes and tsetse flies.

In developed nations their use now tends to be confined to crop protection uses and around animal housing, where their improved residual performance and lower cost structure is important, and visible residues are not such an issue. For professional users, suspension concentrate (SC) technologies have largely taken over. SCs offer good residual performance, without the drawback of leaving visible residues.

Essentially, a wettable powder is a solid pesticidal formulation supplied in a micronised powder form. This is diluted with water and applied as suspended particles, after dispersion (dilution in water). The basic ingredients are a solid or liquid active ingredient, an ionic dispersant, a wetting agent (which can be either anionic or non-ionic) and carrier particles (typically mineral clays), to which the active is sorbed. The ionic dispersant will ensure the particles do not flocculate (clump together) in the spray tank and that they remain in suspension. The wetting agent ensures a more even spray of the target surfaces. Products intended for use in crop situations would generally have a greater amount of wetting agent present.

Good agitation (mixing) is needed in the spray tank to maintain the WP in suspension. Good wettable powders spray well, and do not clog screens, but they can be abrasive to pumps and nozzles. The powdery nature of a wettable powder does present an inhalation hazard to applicators during mixing.

Steve Broadbent, Regional Director, Ensystex

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