Scorched Earth - Oasis of trees and an explorer note around 10 Lat 16.5 Lon (top left corner).

Travel around the dunes north/northeast of the oasis, you should find plenty of worms. Alphas spawn far more often here than anywhere else it seems, but are still rare. ~ 3 - 8 black pearls for normal worms (not guaranteed drop), 75 black pearls per alpha (guaranteed).

This is based on experience in ASA, no clue how similar ASE spawning is.