He fills up his army slots with tons of ant shadows, as there are thousands of those on the island and they are strong. He also converts the queen, but this makes so his command over the ants gets relegated to the queen, its her natural power to command them. Because he doesnt wants to keep her just to order the ants around (and doesnt likes the idea that his shadows obbey someone else) he releases the queen shadow. Yes, he can just 'erase' any shadow he doesnt wants so it doesnt occupies a slot but if he does that he cant get that shadow back anymore.