Depending on your climate, insulating and/or heating your outdoor bin during the winter months is necessary to keep your worms active & eating optimally. I highly recommend that you monitor the bin temperature by inserting a compost thermometer into the side of the bin. I use one with a 19″ stem (approx. $20), though a 5″ stem, costing only $7, might work as well. Drill a hole, the same diameter as the stem, where the stem will enter into the center of the bin.
If you use soil warming cables (described below), place the thermometer stem a few inches above the cables, because you want to make sure no area is getting too hot.
Here’s a graph showing optimum temperatures for worms. It’s to be used only as a general guideline, as I compiled this from several sources, not all of which agreed exactly. You can download this graph, print it, & keep it near your bin for reference. Right-click on the graph, select Save Image As, & it will download.
Though it’s always a good idea to keep a 2-3″ layer of leaves on the surface of the bin, it’s especially important to do so during the cold winter months. The seasonal timing is perfect; leaves are most plentiful just when you begin to need them.
You can also attach rigid foam insulation board to the sides of your bin. I use 1.5″ board. Or place straw bales around the sides. I’ve not tried this, but as long as you can keep rats & mice from climbing on board, it sounds like a great idea.
Soil warming cables
For the past two winters, I’ve successfully used Gro Quick Soil Warming Cables from Wrap-on Co. I’m not sure if heating a worm composting bin fits within the manufacturer’s intended use or not, so before buying, please read their installation pdf, with their warnings & cautions. That said, my experience has been positive. I would not rely on the built-in thermostat, however. I nearly cooked my worms on one occasion. Since then, I’ve used a timer to maintain the desired temperature.
Where I live in northern California, night time temps below the mid to upper 20’s F are unusual. In such a climate, if you regularly feed your worms, and are using an insulated 50-60 gal bin, I doubt the bedding temp will ever fall to a live-threatening sub 50 degree F. But the lower the temp, the less they eat. In my experience, after installing soil warming cables, there’s no longer a winter slow down. I can add scraps at the same rate in the winter, as summer.