Ice cream? Some peas, string beans, or broccoli florets? Maybe a container of overripe bananas that you've ambitiously stored for a future smoothie or fruit bread? Our kitchen freezer looks about the same, but an auxiliary freezer tucked away in the storage room harbors hidden treasure of a decidedly unpalatable sort: worm poop (castings, in more refined vernacular).
Why do we do this? Well, for a couple of reasons, actually.
The best composting worms, red wigglers (Eisenia fetida) are not native to North America. It is a European species that is imported and sold for composting because it is a surface dwelling worm that lives, eats, breeds, and works in the leaf litter and debris that collects on the forest floor. Adaptation to this surface environment makes red wigglers better composters that their native earth burrowing relatives, but their status as non-natives means a responsible composter should avoid introducing them into the wild.
Introduced species can cause problems. They are usually less vulnerable to native diseases and predators, which have co-evolved to be particularly effective at attacking the native species. This means the new environment can serve as a safe haven where they can thrive and reproduce, potentially resulting population explosions such as those documented for zebra mussels. Such proliferation can drive native species to localized extinction and alter ecosystem conditions making the habitat unsuitable for other native plants and animals.
The question is whether the species is non-native or invasive. Unfortunately the answer to that is not simple and typically habitat specific. Japanese Beetles and zebra mussel, most of us can easily label as invasive. Horses are also non-native to North America; are they invasive? Think about some of the coastal islands and areas in the West where they run wild? Humans...certainly invasive if the plants and animals could vote.
Susan Harris, a master gardener in the DC suburbs, offers a thoughtful assessment on the status of red wigglers on her web site Sustainable Gardening:
We choose to freeze our compost because we feel it is our environmental responsibility to reduce the threat of introduction. Freezing also provides an added benefit because it kills the eggs and larvae of other critters that can inhabit worm bins: millipedes, soldier flies,... We feel we can not in good conscience deliver compost that might result in an indoor infestation of soldier flies (http://en.wikipedia.org/wiki/Hermetia_illucens) after incubating in a member's pampered houseplant. They are entirely harmless and effective composters in their own right as larvae (maggots), but they are wasp mimics that look disturbingly like their stinging cousins.
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