Nahum changes how he uses the image of locusts halfway through this verse. Now he compares the people of Nineveh to locusts. Multiply yourselves like locusts.
In other words, multiply your army in the hope that it will withstand the Babylonians. But of course, it will not.
15 There will the fire devour you; the sword will cut you off. It will devour you like the locust. Multiply yourselves like the locust; multiply like the grasshopper!