Cockroach Killer Powders

Not all of these work the same way, and they are not interchangeable. Insecticidal dusts carry an active substance that acts on cockroaches walking through it. Diatomaceous earth works by physical action instead, with no active substance - it is only effective while dry, so it needs to be kept dry and undisturbed once applied. Check which type you are buying on the product page.

Into the gaps, not across surfaces. Puff a light dusting into cracks and crevices, behind and underneath the fridge and cooker, into motor housings and pipe voids, behind kickboards and along skirting gaps. A thin, barely visible layer works better than a pile - cockroaches walk around a heap and through a film.

Apply them where children and pets cannot reach or disturb them - inside voids, cracks and gaps rather than on open floor - and follow the label directions for the product you have. Avoid raising dust as you apply, and do not treat food-preparation surfaces unless the label allows it. Where the range includes a food-grade diatomaceous earth, that is stated on the product page.

In line with the label, and whenever the powder has been vacuumed up, cleaned away, or has got damp. Diatomaceous earth in particular stops working once it is wet, so anywhere near a leaking pipe or a damp cupboard base needs drying out before re-dusting is worth doing.

Look at whether the dust has been walked through. If it is undisturbed, cockroaches are not using that route - move the treatment closer to where the traps are catching. If it has clearly been crossed and you are still seeing them, the population is bigger than the treated area: dust the remaining voids around the same appliance rather than adding more to the same spot. Allow for egg cases, which survive most treatments, so a repeat in line with the label is normal.

A powder gets into voids a spray cannot coat properly - motor housings, wall cavities, the space behind a kickboard - and stays there while it is dry. A spray is easier on flat runs and skirting edges. Most jobs use both, plus monitoring traps to show whether numbers are falling.

If activity continues after a full treatment cycle, if it has spread beyond one room, or if you are in a flat where the source may be a neighbouring property. German cockroach infestations are particularly hard to clear as a DIY job. Look for a qualified pest controller with BPCA membership.