The University of Nebraska had students map out DCA's with 
helium weather baloons tied to heavy fishing poles.  Each 
balloon had a capsule of queen phermone dangling via a 
string.  They located them mostly at about tree height in 
places where there were "straight runs" like fence rows or 
rows of trees or someother structures forming lines and 
believe found they could identify "most likely" areas from 
aerial photographs.
Dave

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