Residual Insect Control Powders

A residual powder is one that stays in place and keeps working after you have applied it, rather than acting only at the moment of treatment. Diatomaceous earth works by physical action, with no active substance, and only while it is dry - so it holds its position in a dry crack or void, and stops working as soon as it gets damp or is disturbed.

As a thin, targeted dusting in the harbourages and travel routes insects use:

• Skirting gaps, corners and floor edges
• Cracks and crevices around cupboards and units
• Pipe entry points under sinks and behind toilets
• Behind kickboards and into voids, where you can safely reach

Do not over-apply. A light dusting works better than a pile, because insects walk around a heap and through a film.

It stays effective as long as it stays dry and undisturbed, and it is not used up as it works. Reapply if the powder has been vacuumed or cleaned away, covered over, or has got damp. Somewhere prone to condensation or a slow leak needs drying out before re-dusting is worth doing at all.

Apply it into cracks, gaps and voids where it cannot be reached or disturbed, rather than across open floor. Avoid raising dust as you apply, and do not treat food-preparation surfaces unless the label allows it. Where the product is food-grade, that is stated on the product page. Follow the label directions.

A powder gets into cavities a spray cannot coat properly - wall voids, the space behind a kickboard, a skirting joint - and stays there while it is dry. A residual spray is easier on flat runs and open surfaces, and is unaffected by damp once dry. Choose by where the insects are travelling rather than by which sounds stronger.

Check whether the powder has been walked through. Undisturbed dust means insects are not using that route, so move the treatment closer to where activity actually is - monitoring traps will show you where. Powder that has clearly been crossed is doing its job, and what matters then is whether sightings fall week on week.

If activity continues after a full treatment cycle, if it has spread beyond one room, or if you are dealing with bed bugs or German cockroaches, which are both hard to clear as a DIY job. Look for a qualified pest controller with BPCA membership.