Residual Insecticide Sprays

A residual insecticide spray is designed to keep working after it dries, leaving a treated film on surfaces that insects cross later. That makes it the choice for ongoing control when pests are hiding or activity keeps returning, rather than for spraying insects you can see. For those, a fast knockdown spray does the job better.

On insect pathways and harbourages, not into the air. Focus on:

• Skirting edges, corners and the edges of rooms
• Cracks and crevices, gaps around pipes, and under units
• Behind and under appliances, under sinks, and cupboard gaps
• Entry points such as door thresholds and window frames, where the label allows

Aim for a light, even application on the surfaces insects are likely to cross.

It depends on the product and the conditions, but in general a residual keeps working until the treated surface is disturbed by heavy cleaning, moisture or abrasion, or until the label's stated period is up. Check treated areas regularly - if activity returns, or the surfaces have been washed down, reapply in line with the label directions.

These are insecticides. Keep children and pets out of the room while you spray and until treated surfaces are completely dry, ventilate as you work, cover fish tanks and move pet bowls and bedding out first. A residual is meant to sit on the surface rather than be wiped off, so apply it into cracks, edges and the backs of units rather than across open floor and worktops, and do not treat food-preparation surfaces unless the label specifically allows it.

Look at which of these fits. If insects are still appearing in the same spot, they are probably not crossing the treated surface - a residual only works where they actually walk, so move the treatment to the routes the monitoring traps show are active. If numbers dropped and then recovered, the surface has likely been washed down or the label period has run out, and it needs reapplying. If you are seeing new small insects rather than adults, the life cycle is still running and an IGR alongside the residual is what breaks it.

If activity continues after a full treatment cycle in line with the label, 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.