I am sorry to say this is the second largest killer of Earthworms.
You might think I would be saying the number one killer of Earthworms is Insecticides or Pesticides. You would be wrong. Although they do the most damage, the number one killer of Earthworms (believe it or not) is a fast freeze.
A very important point must be pointed out here! Many Earthworm cocoons can survive a fast freeze, but no Earthworms will survive because of pesticides.