Worms will gain access to any compost bin that does not have a base, i.e. is open to the ground, and eventually those with a base and sometimes to tumbler bins where the drums are up off the ground. However, worms do not have magical powers allowing
them to appear in a compost bin out of thin air they and their eggs and casts are transferred when organic material from the garden is added to the bin e.g. in soil on roots, on plant material. When they gain access, they
play an active role in the composting of the organic material. With hot composting system the worms, and their eggs cannot survive temperatures over approximately 35°C and will if possible move from the hot area to any cooler zones in the bin (often
at the base), or retreat into the soil if the whole of the heap exceeds the temperature at which they are happy. They will reappear during the cooler maturation, curing or aging stage.
There are occasions when worms appear
at the top of the heap or bin larger numbers. This often occurs in the spring where the compost has provided a warm shelter for the worms during the winter months and a readily available food supply when the warmer weather at the start of spring
encourages breeding and a subsequent increase in numbers. A breeding surge can result in the addition of hundreds of baby worms in a bin. Adults found at the top of the bin may just be trying to escape from overcrowding in the bin and given a little
time the population will balance itself without any problems. Alternatively, the surplus worms can be removed to another bin.
On other occasions, practically during the summer, when even cold composting systems will warm up, due to
the addition of green material and rise in ambient temperature, worms may can be found crawling up the walls and over the lid of the bin to escape to a cooler environment. Some composters put a crumpled up empty compost bag in
the top of the bin to provide a temporary home to these worms.