Exactly what sort of "worms" are you getting?

Root Maggots or Bulb Mites or Nematodes.

The controls for each is somewhat different and they attack the onions at very different stages of development.

Dave

maggots i think

Yellowing tops on onions can mean there is damage to the plant, but yellowing and then browning of the tops is also 100% normal as the bulb growth comes to an end for the season which happens in the summer though if you are in zone 3 I would think it a tad early.

Why not dig one up and inspect the bulb and roots? If possible take a picture and post it.

You could even carefully dig around an onion with your finger, taking care not to damage the skin over the bulb, and look at both the bulb and the soil around it. I check mine periodically this way.

The way to avoid onion maggots is to keep the flies from laying eggs around the plants in the first place. Mulch heavily (I use aspen shavings) around the plants so the flies can't lay eggs in the soil up against them; and sprinkle some dry dill weed around the plants now and then to mask the scent of them.

I'm in a zone 5 and my onions aren't quite big enough to harvest yet, so yours probably aren't either, depending on when you planted them. But if the tops were healthy-looking and green but are now turning yellow and starting to lean, it could just mean they are growing closer to being ready for harvest. The tops will turn brown and lay down when they are ready to pull.