Medium Transport
Situational Awareness
Conflict NMAC
Flight Crew Took Evasive Action
Human Factors
We were descending into Sioux Falls and had been told to descend to 6;000 feet. As we were descending through roughly 7;000 feet I looked ahead and noticed an airplane headed directly towards us. It was a yellow crop dusting aircraft. Right about the time I saw the airplane and prepared to respond to what I assumed to be an upcoming RA the cropduster began a climbing turn towards our right. The airplane passed off our right hand side probably within 500 feet. We never received any traffic advisory from ATC and never received either TCAS RA or TA. After the plane had passed we advised ATC of the aircraft and they told us they didn't show any primary targets anywhere near our position. We continued on and had a normal landing at FSD. Cropdusting aircraft most likely did not have a transponder and was not talking to ATC. I do not believe anything done by us could have prevented this from occurring.
On Approach into FSD and with approach control I was the pilot monitoring and the pilot flying first noticed the traffic but there was no RA received as we both concluded that the aircraft was non transponder equipped. We descended below the aircraft at approximately 500 feet separation. I advised approach control of the aircraft and to alert any other inbounds.
A flight crew reported an NMAC with a non-transponder equipped aircraft on descent into FSD.