Marine creatures of Kwejian which came ashore in swarms after the Burn altered the planet's tides, devouring its harvests and leaving its people dependent on the Emerald Chain.
Sea locusts are swarming creatures native to the oceans of Kwejian, the homeworld of Cleveland Booker. For most of history they stayed at sea. After the Burn, changes to the orbit of the planet's moons shifted the tides and drove them on to land, where they stripped the crops and caused famine.
The Emerald Chain offered a repellent that kept the swarms away, and in return took Kwejian's trance worms and its obedience. The arrangement is the background to the episode The Sanctuary, in which Osyraa comes to the planet to reclaim an escaped prisoner.
Book and his estranged brother Kyheem finally solve the problem themselves. With Discovery amplifying their empathic abilities, they reach the locusts and guide them gently back to the ocean.