Remember that crazy story I have in my head? Specifically remember the Toxins which end up being the ultimate evil race that holds control over the whole universe? Yeah I have been wondering recently about them and I need help fixing this logical hole:

-The toxins go around, patrolling galaxies searching out for intelligent civilizations to dismantle and destroy before they become a threat, they don't kill an entire civilization, just enough to throw it into turmoil.
-For every planet they take over, they kill all life and lay sort of their egg on the planet. The toxins have an incredibly hard time reproducing, and their young need to feed on an entire planets biomass to reach maturity. Remember their population is so sparse that each ship only has one Toxin controlling it, and their is only about one Toxin ship for every galactic cluster.

The problem is, if their population is so sparse, what happens to all the planets they seed? Sure it it only one planet for every Toxin, but that still leaves possibly hundreds of toxin young per galaxy. Perhaps they die off as young, or when they start patrolling galaxies themselves, but if that is the case then the toxins become far less powerful, and their young are more easily killed off.

Overall, how can I have the toxins have a low population when they seed life on every planet they kill off? I need some creative ideas.