Oct 9, 2012 – What Makes It Residual?


Please explain to me how some of today's "residual insecticides" actually have a residual effect on bed bugs? I certainly have my doubts that there are residual properties of insecticides outside of DE or insecticide dusts that penetrate the exoskeletons of these bugs.


Don't you just love terminology, particularly when there may not be any clearly defined rules regarding certain terms? Within pest management and the use of insecticides we may loosely divide products into those that have no residual and are expected only to kill or affect the pest "on contact", and then to rapidly disintegrate so that they no longer are available. The top prize here probably goes to pyrethrum, which we know breaks down rapidly when exposed to UV light, heat, etc. Or, we place insecticides into the group we call "residual" products, meaning they stick around for some highly variable length of time, and this length of useful residual varies wildly depending on what they are and where they are applied. The old chlordane applied to the cool, dark, dry soil under a slab might remain there at a level high enough to kill termites for 30 years or longer, but applied to the hot asphalt driveway in the sun might break down within a few days. 

While our current insect control chemicals do not approach the residual life of the old chlorinated hydrocarbons they still may remain available on a treated surface for anywhere from a couple of weeks to several years or longer, again depending on the molecule and the conditions where it is applied. The stability of that active ingredient also depends on what kind of formulation it comes in. A pyrethroid in a dust formulation, probably dusted into a dark void, is going to remain effective far longer than the same active ingredient in a spray application or an aerosol. The cyfluthrin in a microencapsulated formulation will last longer than one in a WP. It is just so variable that I really don't think a clear definition is given for what constitutes a "residual". But, in general we take it to mean a product that remains on the treated surface for some days or weeks after it is applied, and at a level high enough (strong enough) to kill insect pests that expose themselves to that a.i. by resting or walking onto that surface. This works great for cryptic insects like cockroaches that spend most of the 24-hour day hiding in a crevice that we treated, but less so for the spider that quickly walks over a surface and then makes a web that it spends the rest of its time on. 

For bed bugs this gets complicated, because of the high level of resistance these insects have or can acquire to many of our current insecticides. But, bed bugs are not "immune" to any of our active ingredients, and thus what is required to kill them is either a higher concentration of the a.i. or a much longer exposure period that allows a greater amount of the a.i. to make its way into the bug. For this reason baseboard treatments for bed bugs will likely be ineffective, as the bug quickly walks over the treated surface without acquiring the necessary amount of the a.i. Treating directly into their harborage sites ensures a much longer length of exposure to products that offer some level of residual. 

The majority of our residual insecticides kill by being absorbed through the exoskeleton of the arthropod, ending up in the nervous system where they so badly disrupt things that the bug's important systems fail. Yes, these "contact" insecticides (as opposed to stomach toxins that must be ingested, like boric acid) are absorbed through the cuticle of the bug, finding their way in through pores and tiny openings at leg joints or seams and plates on the exoskeleton. In this sense they definitely are contact products. They do not need to be eaten or inhaled by the bug, but can make their way inside purely by getting onto the outside of the bug. 

DE and silica gel dusts are also contact insecticides, but work with a different mechanism than the nervous system toxins of most of our products. These active ingredients are desiccants that physically abrade the exoskeleton, causing tiny cuts and openings that then allow the body fluids to leak out, killing the insect by dehydration. Thus, they are defined as well as contact products that only have to get onto the outside of the bug to work. 

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