Via Bruce Schneier comes this article in USA Today:

Scientists at a Georgia laboratory have developed what could be a low-tech, low-cost weapon in the war on terrorism: trained wasps.

The tiny, non-stinging wasps can check for hidden explosives at airports and monitor for toxins in subway tunnels.

"You can rear them by the thousands, and you can train them within a matter of minutes," says Joe Lewis, a U.S. Agriculture Department entomologist. "This is just the very tip of the iceberg of a very new resource."

...

The wasps are trained with sugar water by using the classical conditioning techniques made famous by Pavlov's dogs. Rains says the wasps are sensitive to a host of chemical odors, including 2,4-DNT, a volatile compound used in dynamite.

To do their work, five wasps — each a half-inch long — are placed in a plastic cylinder that is 15 inches tall. This "Wasp Hound," which costs roughly $100 per unit, has a vent in one end and a camera that connects to a laptop computer.

When the wasps pick up an odor they've been trained to detect they gather by the vent — a response that can be measured by the computer or actually seen by observers.

Lewis says the wasps, when exposed to some chemicals, "can detect as low as four parts per billion, which is an incredibly small amount."

I admire the innovation in the research laboratory but I am skeptical of success in the real world. The wasps apparently have to be trained for each specific volatile chemical. The 2,4-DNT mentioned in the article as being present in dynamite doesn't exist in other explosives such as ammonium-nitrate/fuel-oil mixtures. Ammonium nitrate by itself doesn't really have any volatile byproducts other than, in some cases, ammonia which would result in the obvious problem with false positives. Fuel oil sensing would also have similar problems with false positives as well as being easily replaced with almost any hydrocarbon including such things as diesel, alcohol, and powdered sugar.

The ATF as well as foreign regulatory agencies require plastic explosives to be manufactured with a small percentage of volatile chemicals such as Ethylene glycol dinitrate, 2,3-Dimethyl-2,3-dinitrobutane, para-mononitrotoluene, or ortho-mononitrotoluene. This is to make it feasible to easily detect the presence of the explosives.

It would be overly optimistic to assume terrorists would conform to these requirement in the manufacture of their own explosives.