They'll hitch a ride on anything that moves, be it dust, Canada geese or water.
But the main way soybean cyst nematodes spread? Maybe the answer's in your mirror.
"The biggest way is through impatient farmers who are out working fields that are too wet," says Pat Donald, a University of Missouri extension nematologist.
"They get mud on everything and just carry dirt from field to field."
To keep SCN from spreading, work fields not known to have nematodes first, then move your rig into infested fields.
Or power-wash equipment between fields. Just don't move soil back and forth between fields, she reiterates.
"I never knew I had a problem, so I didn't clean my equipment off," admits Dave Broghamer, Decorah, IA. He's in his first year of fighting SCN on 60 acres.
"I just assumed this year that I had already spread it to all the other fields, so all the soybeans I planted are nematode-resistant," says Broghamer.
"Some think soybean cyst came into this country with soil that was brought in as inoculant," Donald says.
Others surmise that SCN has been around all along and that it was surviving on weeds, says Jamal Faghihi, a Purdue University research nematologist. "When soybean cultivation became widespread, it started showing up in different places," Faghihi adds.
Once it was found in North Carolina, it was discovered throughout the Southeast and later into the Midwest, reports John Ferris, a Purdue University nematologist.
"We first found it in 1970 in southern Indiana. Eight years later we found it in the northern border of the state. Then we found it all over."
However it got here, SCN is here to stay. Canada geese and other waterfowl are active carriers of the costly pest. So is water, says Donald.
"We know that water moves it; I documented in the flood of '93 that it was being brought intodifferent areas along the Missouri River."
Blowing soil also carries SCN, says Walker Kirby, University of Illinois plant pathologist. He, too, suggests scrubbing and spraying tillage tools, tires and fender wells, for example. Custom harvesters should especially be asked to wash equipment because "you have no idea where they are coming from."
"If you take time to do this, it will reduce the spread," Kirby says