When locusts swarmed in to eat Mormon crops what followed and ate them?

It was the Mormon Cricket, not locusts, that swarmed and began to destroy the Mormon's crops. The "Mormon Cricket" is actually not a cricket at all, it is a migratory shieldbacked katydid common in many western states. Mormon Crickets are known to periodically form large swarms, which is exactly what happened to the Mormons.

The Mormon's crops were saved by California Gulls, which came in and ate the crickets. Today, Utah's state bird is the California Gull, in honor of their saving the Mormon pioneers in this "Miracle of the Gulls".