Worms used for composting, or vermicompost as it is sometimes called, can tolerate a range of temperatures -- after all, the worms' natural habitat is outdoors. Whether you keep the worm bins outdoors during winter or move them to a warmer spot, such as a garage or basement, depends upon the likelihood of freezing temperatures. If freezing or near-freezing temperatures are common, the worms will be better off in a warmer area unless you insulate the bins for overwintering.
Ideal Temperature Range
Like people, worms prefer certain temperature ranges. If it is too cold or too warm, they may die. The worms used most often for vermicomposting, red wigglers (Eisenia fetida), thrive in temperatures between 55 and 75 degrees Fahrenheit. They'll tolerate other temperatures and survive beyond this range, but will slow down at temperatures below 50 F. The temperature within the bin itself is likely warmer than air temperature in winter, or cooler during summer. Insert a thermometer to find the temperature within the bin to find the exact temperature. If the bedding temperature is above 84 F, the worms will likely die. Similarly, temperatures hovering around freezing within the bin are harmful and may kill the worms. If you live in an area that gets hot in summer, you may need to move the worms when it gets too hot outside.
Worm Bins in Winter
If the ground freezes outdoors where you live, your best bet is to keep the worms in a warmer area during the winter. Move the bins to a basement, pantry, closet or garage that is above freezing for the entire winter. Place a tarp beneath the bin structure to keep dirt and debris off the floor. Make sure the place you keep the bins is also in a suitable temperature range -- 55 to 75 F -- to keep the worms in good health. An area next to a furnace or heater is not a good idea as it may overheat or kill the worms.
Overwintering the Bins Outdoors
If keeping the worm bins outdoors is the most viable option for you, even in a cold climate, there are ways to insulate the bins so worms are less likely to die. Dig a hole in the ground deep enough to house the bottom half of the worm bin. Pack the area beneath, around and above the bin, once set into the hole, with straw or leaves to insulate the structure.
Optional Insulating Methods
If you live in an area that gets a good deal of snow, insulating the worm bins with snow itself is an option. Snow is an excellent insulator against wind when packed around worm bins. Bury the bin in snow, or in dirt and snow, to insulate it against the cold. Sheets of polystyrene foam packed around the outside of the bin will also insulate it; covering the entire structure with a plastic tarp creates a bit of a greenhouse effect for the bin. Monitor the temperature within the bins using a thermometer, checking every few days at first until you're confident the temperature within the bin is suitable for the worms. Add more insulating materials if the bin temperature is below 50 F.