Complex sentence: Here also, the winter moth was controlled by the parasitoid wasp and fly at high densities, but at low densities, a more important factor seems to have been the native beetles that ate the moth pupae.
Split into simpler sentences: Here also, the winter moth was controlled by the parasitoid wasp and fly at high densities, but at low densities. A more important factor seems to have been the native beetles that ate the moth pupae.