The hosts had fallen behind early on to a converted try before Thomas Holliman and Luke Ashton went over as Dees led 10-7.
A penalty brought the visitors level as the teams changed ends at 10-10.
In the second period, Aaron White dotted down with Stuart Hall following suit to make it 20-10.
St Ives hit back with a try of the own before Dees ran riot as Mattie Anderson crossed, with Kevin Boland’s boot seeing Dees lead 27-15.
Ashton had his second try of the afternoon, with Boland adding the extras, while Duncan Holmes then went over, as Boland did the rest once more.
The scoring was completed late on by Matthew Knight, with Boland making it 48-15.
The result, Dees’ second victory in three matches, saw the Bidwell Boys climb to fifth in the table.
This weekend, Dunstablians visit St Neots,