Hi Keep adding food but then add a large layer of dry paper on the top. This will allow the worms something to eat while the paper on top will cut back on the flies. Here is a video I made on this exact issue.

Hopefully you are using " apple cider vinegar" and not "apple cider" there is a big difference. If the flies are able to get to the food scraps they are in heaven. Do your best to create a barrier. A dry paper barrier over bedding and food scraps should help.

I recently went thru a bout with fruit flies and can share my experience. First off, I don't think a "healthy" worm bin will produce "any liquid" but I am by no means an expert, I do know that mine does not.

I used a spray bottle of rain water to "moisten" the bedding (shredded paper) when I started my bin. The red wigglers seem to like it moist but not dripping wet.

On fruit flies: When the fruit flies started I tried laying paper on top and spritzing it down with rain water. While the worms seemed to like it so did the flies. When I thought about it there is no reason these tiny little flies can't go down and around the paper. So now after more searching and reading I started doing something I said I wouldn't bother doing - freezing the worm food before putting in the bin. When putting any food in the bin make sure it is covered completely.

I used things like chopped or crumpled up leaves, shredded paper, and/or unfinished compost I had from an outside pile that went cold to completely cover the food.

As of this writing the flies are gone, the worms are happy, and there is no liquid leaking, dripping, or visible anywhere.

If the worms aren't happy they will crawl up and out. Those black spots you see on the sides of worm bins are castings from unhappy worms trying desperately to escape.

That could be caused by no air, too hot, or let's say the wrong temp, too wet, or even NO food, or no food they like.

It seems that fruits and veggies purchased from the store come complete with fruit fly eggs (who knew) and washing any unused fruits and veggies with soap before use then freezing the scraps is key to reducing then eliminating these pests. Dish or hand soap is fine, use what you have on hand. Of course reducing any excess moisture in/around the bin is helpful.

The freezing will kill the eggs. I imagine 30 seconds or so in the microwave (depending on the amount) will also kill the eggs. Using the microwave does cost more than freezing since the freezer is (and will be) running all the time. I live on the east coast and plan to put the worm food outside to freeze it when the temps drop down.

I started collecting containers to keep kitchen scraps in before freezing - so any stray flies don't lay eggs. Some folks use zip lock bags to put in the freezer. Right now I just keep the food in the same container when placing in the freezer.

HTH

If I have time in the future I will post some pictures of the fruit fly traps I made.

Right now I am getting ready to find a lazy mans way to harvest my hungry worms.

All this to say -

1. Freeze (or microwave) before adding any food.

2. Be sure the food is completely covered (buried).

3. Add enuff dry bedding to make sure things aren't dripping wet in the bin.

Fruit Flies are one thing but no need to worry about not getting any liquid. Ive been worming for years and never get any liquid drainage but get plenty of good vermicompost. To me if you have drainage your bin is too wet , but thats just me.

I have been having a similar issue in my worm bed as the op after adding some leaf lettuce that was over ripe. Tomorrow we shall be getting a newspaper, shred it and add that to the bed. Wife suggested adding some peat moss as well.