The mechanism by which boric acid kills cockroaches may be a bit mysterious, but there's no question that it works, and people have been using it for years with great success. The mystery lies in the exact mechanism whereby the white powder, which is derived from the mineral borax, acts on the insects' internal organs and nervous systems. There is also uncertainty about the effects of the powder on the exoskeleton. There is no mystery, however, to the fact that, if you can get a cockroach to walk through boric acid or eat a boric acid bait, the roach is done for.
A Pesticide to Handle With Respect
Borax is the white mineral that comes to American laundry rooms via the famous 20-mule teams that haul it out of California deserts. It's also known as sodium tetraborate decahydrate, and it's a salt of boric acid, which is derived from the mineral in the laboratory. Both contain boron, and both are pesticides, but boric acid is a finer powder and is usually more effective. As far as humans are concerned, boric acid is a Category III toxin, meaning that it is moderately toxic. A dose of 2 to 3 grams, if ingested by an infant, could be fatal. For children, the fatal dose is 5 to 6 grams, and for adults it is 15 to 20 grams. Consequently, boric acid should be handled with caution and kept out of the reach of children.
Toxicity for Cockroaches
A 1995 study found that boric acid destroyed the stomach lining of the German cockroaches (Blattella germanica) that ingested it. The study speculates that this caused the insects to die of starvation. The insects typically ingest boric acid after walking through it and subsequently preening themselves. It is also probable that some of the powder gets absorbed through the insect's bodies. Folk belief has it that the powder desiccates the exoskeletons of roaches that walk through it, but this could be due to a confusion between the effects of boric acid and those of diatomaceous earth, another insecticide known for its ability to lacerate exoskeletons.
Baiting with Boric Acid
Roaches must come into contact with boric acid and ingest it before they can succumb to its toxic effects. An easy way to ensure this is to spread a thin layer in places where they walk. The layer must be very thin -- almost invisible to the eye -- or the roaches will likely walk around it. You can also bait the insects by forcing them to walk through a layer of boric acid to get to a strategically placed treat or incorporating the boric acid into the treat itself. For example, one recipe consists of making balls from a mix of 16 ounces of boric acid, 1 cup of flour, 1/4 cup of sugar, 1/2 cup of shortening and a small amount of water. Spread dust or deploy baits in dark crevices and corners where roaches have been seen and never in places where children or pets can find them.