Recently I was asked if Red Wigglers (Eisenia fetida) are invasive and therefore unsafe to place in the compost pile.
I did some initial research and learned that wigglers are not invasive or dangerous if released outside. For one thing, they have a very limited temperature range. Red wigglers cannot survive below 40 degrees and above 90 degrees. So, in northern climates like mine (Wyoming), Red Wigglers would not survive the winter. In southern areas, the summer temperatures would be too hot. Other worm species survive temperature extremes by burrowing deep into the soil. However, red wigglers are not burrowers.
I DID discover a scenario where imported earthworms ARE invasive and dangerous. Northern forests (Minnesota in particular) developed in an environment free of earthworms (which were killed off during the ice age). Europeans who settled in the area introduced worms accidentally, which is affecting that forest ecosystem. Still, while researchers in Minnesota found a number of earthworms causing these forest problems, but did not find ANY Eisenia fetidas outside of compost piles.
I also learned that, The Brooklyn Botanic Garden website agrees that wigglers not a “problem species. Maryland’s Extension Service says these worms “do poorly” in average soil. I read other bloggers who have conducted their own investigations. One communicated with a red worm specialist (Dr. John Reynolds) who noted:
“No. Eisenia foetida [note: alternate spelling] because of its high optimum developmental temperature requirements it is not now found widely in nature. Since the decline of horses in agriculture, most populations are in articifial situations. If you looked at three of my recent monographs covering eastern North America you would see the limited distribution of Eisenia foetida.”
So I say, until new research shows conflicting findings, I feel that composting with red wigglers is safe in my local ecosystem and a great way to recycle household wastes! I may even throw in some horse manure since that seems to be a favorite.