[last-instar larvae:]
Middle pair of dorsal abdominal setae not set on tubercles; spatula almost entirely black; from P. strobus ... C. candidipes
Middle pair of dorsal abdominal setae set on tubercles; spatula brown; from hard pines ... [the other two species with dorsal tubercles - C. piniinopis and C. brevispatula]

...in pitch mass on P. strobus...

C. candidipes is known only from white pine (P. strobus). I have seen specimens from Connecticut, Illinois, Maryland, New York, Ontario, Pennsylvania, and Quebec...
If larvae of C. candidipes do leave the pitch masses to pupate, their behavior would fit that of the other two Nearctic species with larval tubercles, C. piniinopis and C. brevispatula.

Larvae may be single to gregarious. They pupate in a white cocoon some distance away from the resin mass in which they feed...
Larva in running resin; on white pine, Pinus strobus...Cecidomyia candidipes Foote.
Several larvae are usually found in the same pitch mass. Host: P. strobus...