How does worm charming trick earthworms to rise up to the surface? Researcher Ken Catania of Vanderbilt University found the answer: worm charmers create vibrations similar to that of the worms' predator, the mole:
"Hundreds of large earthworms rapidly emerged from the ground for a distance of up to 12 metres from the vibrated stake," says Catania. The closer they were to the stake, the more earthworms emerged, and the worms stayed on the soil surface for between 4 and 15 minutes before beginning to burrow back down again. [...]
Finally, Catania compared the vibrations produced by worm grunting and those of a mole burrowing.
He found considerable overlap between the two, although moles produce a wider range of vibrations that peak at around 200 Hz and worm grunting vibrations are more uniform and concentrate near 80 Hz. Playing a recording of mole digging through a speaker into the soil, also drove worms to the surface.
"The results support the hypothesis that earthworms have a stereotyped escape response from foraging moles, and that bait collectors have unknowingly learned to mimic digging moles to flush worms," says Catania.