The U.S. government has lifted its two-day ban on flights by American crop-dusting aircraft. But evidence that suspected terrorists showed interest in using the aircraft - possibly to deliver chemical or biological weapons - has put the nation's agricultural aviation industry on high alert.

America's fleet of 4,000 crop dusters is flying again. Agricultural aviators are scrambling to make up for two days of lost business, spraying pesticides and other farm chemicals on crops during one of the industry's busiest seasons. James Callan, the executive director of the National Agricultural Aviation Association, says the government-ordered grounding was extremely costly to the nation's 2,500 mostly small, family-run cropdusting businesses. And while Mr. Callan says most ag-aviators understand the security concerns that prompted the nationwide grounding, he believes the crop-duster airplane doesn't really lend itself that easily to an act of terror. "The belief of most of the folks I've talked to in our industry is that it would not be a very efficient means of [widely] dispensing deadly chemicals. They are not really designed for that. They are designed for precision applications, precision spraying," he says. "The nozzles usually are wound fairly tight so the spraying is extremely accurate it's in your economic interest to have it that way. I can't say that there is no threat at all from them, that's not something that we're comfortable with saying.. But clearly, there has never been any kind of an incident where they've been used, thankfully, for any terror attack in the United States."

In addition, Mr. Callan says, the specially-designed or modified crop-duster planes are more difficult to fly than regular single-engine aircraft. Licensed pilots must complete as many as forty hours of additional flight training to earn an agricultural aviator's license.

Still, some experts in chemical warfare and bioterrorism believe the crop-duster airplane could be a dangerous weapon in the hands of a skilled terrorist. "These aircraft when used in conjunction with the expertise to actually make the biologic agent in a way that could be released from a plane - could be very, very important," says Dr. Michael Osterholm is director of the University of Minnesota's Center for Infectious Disease Research and Policy, and the author of several books on bioterrorism. "One does not need to look at the spray devices from airplanes as the ideal vehicle," he continues. "The problem is that even if they're not ideal, they're still damned effective at infecting potentially large numbers of people. And that's why we have to understand that we have to be prepared to respond."

But other experts in the field argue that it would be extremely difficult, if not impossible, for terrorists to deliver infectious agents using crop-dusters or any other vehicles. They note that bacterial or viral toxins are not only hard to acquire but are also extremely perishable and not easily transported. And the wind and many other environmental conditions, they say, would have to be perfect for any aerial spraying to be deadly.

Still, National Agricultural Aviation Association director James Callan says the scare has made the nation's crop-dusting firms a lot more careful about their planes. "In the vast majority of cases the planes are in a locked, steel hangar on their property, in their backyards or near their homes. They are also doing extraordinary measures, like chaining their props, disabling the engines, some of the newer planes have ignition keys and obviously they are not leaving those in the planes, and certainly there is a heightened sense of alert," he says. "We have been cooperating with US law enforcement to help them get the word out to agricultural aviators. And we think the word is getting out, and we are urging people to continue to keep an eye out, to continue to be vigilant, to watch their chemicals, to watch their materials, and to keep an eye on their planes."

But Mr. Callan adds that agricultural aviators are hoping that whatever new evidence turns up about terrorists' interest in crop-dusting planes, it won't lead to another nationwide grounding.