| County | Region† | 2013 | 2014 |
| Fields sampled | Fields sampled | ||
| Benson | NE | 3 | - |
| Eddy | NE | 1 | 1 |
| Foster | NE | 2 | 1 |
| Ramsey | NE | 1 | - |
| Steele | NE | - | 1 |
| Walsh | NE | 1 | - |
| Wells | NE | 1 | 1 |
| Renville | NW | 2 | 2 |
| Ward | NW | 3 | 1 |
| Barnes | SE | 3 | 2 |
| LaMoure | SE | 3 | - |
| Stutsman | SE | 2 | 3 |
| Sargent | SE | - | 1 |
| Stark-Billings | SW | 3 | - |
| Total |
| 25 | 13 |