23 Sep Pesticide dusts and repellancy

Pyrodust PCP # 13074 is a common pesticide dust applied by homeowners for bed bugs and can be purchased in many stores. Unfortunately when people use this product they often apply it inappropriately causing more harm than good. For example, I often see about a line of dust around the baseboard about a quarter inch thick which simply keeps the bugs in the baseboard or they move through the wall to another area.

I have used this product twice in my experiments. The first time I wanted to entice bed bugs from one side of my teflon pan to the other side of the pan via an experimental contraption. The contraption did not seal against the side of the teflon pan so I put copious amounts of dust in the corners in the hope that large amounts of pesticide would repel the bugs and force them onto the contraption. I dropped a few bed bugs onto one side of the pan and noticed that in their effort to flee the open area ran right through the mound of pyrodust and eventually died. After the bugs settled down a bit they tended to avoid the heavy concentration of pyrodust. So we learn that bed bugs in flight mode will run through a large pile of pyrodust but will normally avoid the product.

In another experiment i placed progressively greater amounts of pyrodust onto a cloth to see if they would willingly cross the pesticide. I found that as long as the pyrodust was applied lightly to the point it was barely visible the bugs had no difficulty crossing the pesticide. Once the cloth started turning white with pyrodust the bugs would be repelled.

So a good rule of thumb when using this product is to apply it very lightly so that you do not even notice it is there – if you can see it easily it is too heavy. If you apply too heavily the bugs will be repelled which is not conducive to getting rid of bed bugs.

Additionally, pyretherin, the active ingredient in pyrodust, is not known to have a good residual effect and it also breaks down very quickly in sunlight. I suppose there might be greater residual if applied very heavily but then the bugs are repelled. Therefore it is unclear to me how this product could be used effectively on a baseboard to kill bed bugs.

I did go to a bed bug seminar a few years ago and a prominent presenter stated that pyrodust would have a 50 year residual in an enclosed area such as a electrical box. I do use pyrodust in electrical boxes in my own practice but often wonder at the effectiveness. I asked my chemical supplier with 30 years experience as an exterminator and he could not provide a good explanation of why pyretherin, which is not known to have any residual properties, would have a residual effect even in ideal conditions. If someone could explain that to me, or point me to a study, I would listen very carefully.