Using permethrin dust to treat large areas such as subfloors and roof cavities is pretty satisfying and gets good results, provided the job is carried out properly.

For me, one of my favourite methods for urban pest control is cavity dusting. And by that, I mean the application of permethrin dust to large, enclosed spaces such as in the roof, subfloor or wall void cavities. Nothing is more satisfying than standing next to the client after the treatment and watching black ants raining out of the downlights or cockies doing backflips as they spew out of the weep holes! As with most application methods, cavity dusting is an acquired skill not without its pitfalls, so here are a few tips I thought I might share.

Tip 1. Always do a site pre-check

By this, I mean a quick walk around to look out for any potential hazards. I can’t stress this one enough; I can personally confess to being a victim of my own lazy pre-check.

I once had a trainee who started the job by dusting the subfloor cavity for me while I made a start on the internals. Neither of us had bothered to walk round the back to see the fishpond up against the building. Unfortunately, the dust had filtered out through a decent wall vent and had settled across the surface of the pond. I had no idea what the LC50 for permethrin dust on goldfish was but I did know that fish pretty much only have to look at SPs and go belly up. So I quickly grabbed the hose and proceeded to flood the fish pond in the hope that the dust floating on the surface would run off. I notified the owners and asked them to let me know should any fish die. Supposedly no casualties.

Another time I was dusting a roof cavity and I was quite pleased at how well the dust was dispersing to the other end of the house; a quick pre-check would have revealed another manhole at the other end of the house with the cover off!

And again, dusting the subfloor of a partially renovated brick veneer home with suspended floor, I was pleased with how well the subfloor was taking the dust. A pre-check inside would have revealed that the base of the fireplace had been removed and dust was being sucked inside.

Poor dusting in a subfloor

Tip 2. Do a test dust

Cavities often have their own airflow direction, so do a quick blast with your duster to see which way it is going to flow. There is nothing worse than finishing dusting a roof cavity only to have the dust pour back down through the manhole opening as you scramble to get out and close the cover. You can often change the airflow simply by opening the closest external door, which creates an upward draft.

Tip 3. Know the limitations of your duster

There are many different types of dust applicators on the market these days, all with varying volume outputs. Know how far your dust will disperse – just because you can no longer see the other end of the roof space through your dust, doesn’t mean it has dispersed all the way to the end.

Think about the pros and cons of your dusters. Years ago, I used a petrol-powered air compressor fixed to my ute. As powerful as it was, dragging the air hose through a house was not particularly easy. These days, like most, I use one of the rechargeable blowers. Even the most powerful of dusters will not bend dust around a corner, so if the house is L-shaped, go for a crawl and work your way backwards.

A cut down plastic silicone cartridge tip prevents damaging narrow weep holes

Tip 4. Seek out hidden dusting spots

Often we encounter roof cavities with no visible access, and most pest managers will simply exclude it from their treatment. Downlights, unducted exhaust fans and the undersides of corrugated roof sheets (or even partially removed roof sheets) are all potential dusting points. This is where you, as a professional, need to weigh up the physical risks versus the pest issue, with the owner’s permission of course!

Tip 5. Use a dust that suits your duster

Not all permethrin dusts are created equal; choose a dust that gives the best flowability with your dust applicator.

Tip 6. Don’t be lazy

One of my pet hates is climbing into a roof cavity and seeing a thick layer of insecticide dust radiating from the manhole, only to find that once I climb in, the dust fades to nothing just a few metres in (see main picture above as an example). Don’t be lazy – go for a crawl to ensure a good, even coverage.

As with any skill, we all improve with practice; even mistakes can be valuable when we learn from them, and live to share those experiences with our peers!