Complex sentence: Once they reach the burrow, the wasp lays a white egg, about 2 mm long, on the roach's abdomen and proceeds to fill in the burrow entrance with pebbles, more to keep other predators out than to keep the roach in.
Split into simpler sentences: Once they reach the burrow, the wasp lays a white egg, about 2 mm long, on the roach's abdomen. It then exits and proceeds to fill in the burrow entrance with pebbles, more to keep other predators out than to keep the roach in.