For instance, electrostatic forces around leaves can repel drops of pesticide. ''If the dropss so fine that its mass can't overcome the electric field, it will curve away,'' Dubois said. To test ways of best delivering the spray, the researchers simulate aerial spraying in a handmade 12-foot tower. One afternoon last week, Huntley shook up a flask of Bt, which looked like cafe au lait. Below, four saplings rotated on little platforms, a movement that is supposed to simulate wind eddies around branches. Huntley poured a touch of liquid into a special nozzle, then sprayed.
Manufacturers prepare the bacterial killer in large fermentation vats.
Dubois said the germs are inactive until they reach the gut of a caterpillar, which is alkaline - unlike human and lobster stomachs, which are acidic. Triggered by the alkalinity, an agent in the Bt formula ruptures the caterpillar's stomach lining, enabling the bacteria to invade and infect the bloodstream. Dubois said Bt is found naturally in soils, but not in sufficient quantity to kill an epidemic of caterpillars. Bt has been around for decades. It was first registered as a pesticide in 1954.