Boric acid has long been recognized as an effective pesticide; when used properly, it can control cockroaches, ants, silverfish and fleas. The list includes any insect that grooms itself, but it does not include mosquitoes, lice, bed bugs and any other insect with a proboscis to piece skin and drink blood.
Borax is related to boric acid, but it has a different chemical composition and crystalline structure. You may have success using it to control insects, but it isn't as effective as boric acid and isn't classified as a pesticide.
What's the Difference?
Both borax and boric acid occur naturally, and both contain boron. The white powder that semi-mythical 20-mule teams haul out of California deserts is borax -- a salt of boron; boric acid -- an acid of boron -- is common in mineral springs, the oceans and even in some plants. Because borax is more available, manufacturers derive boric acid from it by
How Boron Kills Insects
The boron in both borax and boric acid is fatal to insects -- it probably affects the insects' stomach lining and interferes with digestion. Although the exact mechanism isn't completely understood, it's well documented that the insects must ingest it for it to be fatal. The belief that it lacerates their exoskeletons and desiccates them is an urban myth.
The crystalline structure of boric acid is much smaller than that of borax, and it forms a finer powder that's easier for insects to ingest. Once ingested, the smaller structure is probably easier for the digestive system to break down. No doubt because of these characteristics, boric acid is also more toxic to humans than borax.
Baiting With Boric Acid
Baiting with boric acid is a common treatment for cockroach, silverfish and ant infestations. The typical strategy is to mix a low concentration of boric acid with sugar and water -- perhaps adding flour or corn meal to stiffen the mixture. One recipe for controlling sugar-eating ants is to mix 1 teaspoon of boric acid and 3 tablespoons of white sugar with 1 3/4 cups water. To make a simple cockroach bait, mix boric acid with sweetened condensed milk to make a paste. You must keep the concentration low -- less than 5 percent for ants -- to avoid repelling the insects or killing them before they can feed the colony.
Can I Use Borax?
You can use borax to make baits in the same way you use boric acid, but it may not be as effective, primarily because the larger crystals are more difficult for the insects to digest. If you already have some borax around the house, you can't hurt anything by using it. Make some experimental bait formulations, and you may eventually have success. Borax has the advantage of being safer for children and pets, although you should still keep the baits out of their reach. If elimination of the infestation is a priority, use boric acid.
Boric acid dust can kill cockroaches -- as the insects walk through it, tiny electrostatic charges attract the dust to their legs, and the insects later ingest it when they preen themselves. The secret to effective dusting is to lay down a super-fine layer, or the insects will avoid it. Because it's a coarser product than boric acid, borax isn't as suitable for dusting. If you use it, you'll probably just create a barrier that the roaches walk around on their way to your pantry. Even if they get some on their legs, it's likely to fall off before the insects get around to cleaning themselves.