BLACK CANKER is the name given by husbandmen to a caterpillar which commits great devastation among turnips. The best method of destroying these insects is, to turn a body of ducks into the fields infested by them. In the year 1784, Mr. Coke purchased four hundred ducks, and set them at liberty on thirty-three acres of turnips, which they completely cleared of the caterpillar in five days. In a relative proportion, twenty or thirty might be employed upon a small farm, with considerable effect.