I'll take a guess at answering your question, the root maggots might be cucumber beetle larvae, if you have noticed any beetles early on they may have taken advantage of your plants, excerpt from googling :

"...Striped cucumber beetles lay eggs at the base of cucurbit plants and their larvae then feed on the roots of these plants. ... Larvae then pupate in the soil for about a week before emerging as adult beetles."

You may have hit 2 generations of CBs? Maybe using the DE around the base of the plants will deter them from laying eggs.

Thank you! I know I can't believe it's gotten all my cucs! The zucchini in the ground nearby hasn't gotten attacked at all. Some of the 2nd gen cucs I started from seed have done OK because I used DE around the stem. However, not all of them have made it so I'm going to start some new seeds and before I plant them I'm going to do a French of beneficial nematodes. The war is far from over! ;-)

Z