Some earthworms are beneficial to gardeners through soil aeration (earthworms) or composting (red wigglers). However there is a growing problem with a new invasive species (Amynthas spp) that is particularly destructive to soil structure and soil chemistry. Commonly called invasive jumping worms, crazy worms or snake worms (previously referred to as Asian jumping worms), they are both extremely fast spreading and destructive. They can drastically reduce the productivity of your plot , particularly with plants that are not established by mid June.
Good garden hygiene is the best way to avoid the introduction of this species. This means either isolating your tools to your bed only, or washing tools between use on different beds. Never bring an unwashed tool from home to the garden, or transplant soil from elsewhere. Try to check that bagged soil products introduced to plots are marked as “sterile”. This can be difficult, though, because companies are slow to recognize the issue.
How do you know if you have jumping worms? Testing the soil can only be done effectively after spring eggs hatch, and grow to a size that's easily seen, probably in mid-June:
Mix ⅓ cup of ground hot yellow mustard seed (look for Chinese or Asian hot mustard) into 1 gallon of water and pour half of the liquid slowly over a 1 square foot of soil you want to test. Wait a few minutes and pour the rest. This will make worms (any earthworms) come to the surface.
Once the worms are big enough to see on soil surface, you can hand pick them to reduce numbers. Place the worms in a jar or bucket with a mixture of 2/3 water and 1/3 vinegar or rubbing alcohol. They die quickly. They can climb out of buckets of water, so you need to kill them. Don't dispose of eggs or live worms down drains or toilets, as they will get into rivers and spread.
Check the Resources page for links to more detail about current research, prevention and mitigation measures.