Most of the indoor surfaces that you treat are porous surfaces, meaning that insecticide can soak into tiny pores in the material, leaving less residual insecticide on the surface for insects to contact. This is good if the pests you’re trying to control are below the surface (like larvae in carpets), but not so good if the pests you want to control remain on the surface (like cockroaches or ants), but your treatment doesn’t.

Some examples of porous surfaces are bare wood, brick, carpet, fabric, plasterboard, concrete, and cinder block. Examples of nonporous surfaces that do not absorb insecticide are stainless steel, glass, ceramic tile, and plastic. A porous surface that has been sealed with paint or a finish like polyurethane will be somewhere between porous and non porous in terms of its ability to absorb insecticide.

You can control the degree of absorption on porous surfaces by choosing either an insecticide formulation that stays longer on a porous surface, or one that is more easily absorbed into the surface. Wettable powders, microencapsulates, baits, and dusts last longer on top of porous surfaces and resist absorption. Emulsifiable concentrates tend to penetrate porous surfaces better and will hold their residual within the material instead.

Fabric – Some fabrics are more porous than others. Natural fabrics like cotton will hold more insecticide than synthetic fabrics. For example, cotton twill which is commonly used as furniture covering holds insecticide within the fabric weave rather than on the surface. Furniture or drapery fabric is often treated with stain or water-repellent materials which may mean that little or no insecticide will penetrate the surface.

Carpet – The insecticide absorption of carpets is affected by both the carpet’s composition and the pile height. Most carpet insecticide applications are to kill flea larvae hidden deep in the pile. If you use a fine fan spray on the surface, 93% of the insecticide remains in the upper 1/3 of the pile (Robinson). Increasing the application pressure or decreasing the spray height does not increase the insecticide penetration. Spraying closer to the carpet doesn’t help penetration much either. For best control, make sure you treat the carpet thoroughly and evenly, overlapping application bands.

Wood – There are few porous bare wood surfaces indoors. Most indoor wood surfaces are painted, varnished, or stained and will resist insecticide penetration. Porous wood surfaces may be found on the interior of cabinets, drawers, and the voids behind and beneath kitchen and bathroom counters. These surfaces may absorb insecticide to slightly below the surface when treated with liquid or foam insecticides.

Insecticide Application to Porous Surfaces