Diatomaceous earth (DE) is the fossilized remains of diatoms, an algae-like single-celled creature. It feels like powder to mammals, but to wax-shelled insects it might as well be razors. It erodes the shells of many species, causing them to lose internal fluids and essentially bleed to death. How to spread DE to combat insects depends upon the type of insect, but the common denominator is to pour it in a line that is placed so the insects are force to cross it.
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Kill ants by lining windowsills, crevices and wherever you see them entering. Ants leave a path as a communication device with other ants. If you can tell what path they are all following, you can determine where to spread a small line of DE that will force them either to get cut up or lose the trail.
Kill roaches with a line of DE along the baseboards, where they instinctively crawl. A roach could crawl the full length of a baseboard, with DE scraping its underbelly the whole time.
Kill flies and fleas on pets and farm animals by applying DE directly to their coats. Because ingestion of DE can kill worms in the GI tract, even application to self-grooming cats is acceptable, as long as it is food-grade. (Adding food-grade DE to animal diets is a practice used in many zoos.)
Kill bed bugs by putting DE in any tears in your mattress. Then form a barrier by putting your bed legs in large cups. Pour DE into the cups. Bed bugs are poor climbers and can’t generally climb up walls to drop onto beds, so they need to go up the bed legs.