Question
I have a 40 x 50 lawn that has no worms, how many would you recommend that I put down after I aireate?  

Answer
Hi Brian;
I have never bought worms for my yard, just let them come there naturally, so I don't know how many you need to start with, but I would imagine the more the merrier.
They will reproduce, so their numbers will grow rapidly.
You want some lizards and toads too.
Check with nurseries etc to find out what kinds of lizards do well in your area.
Toads love slugs, and grubs.
Lizards eat all the aphids off my roses, and the tent catapiullars that used to drop out of my trees on me.
Grass snakes love ants, and all sorts of insects.
I have enough toads, lizards and grass snakes that they have a very good life, and reproduce and take care of ALL the insects that I don't want. Never have to bother with insecticides.
I have no idea how many earthworms I have, but there are other beneficial insects too, like cockroaches.
Yep, they are beneficial.
The normally live in the soil, and feed on other harmful insects. They also tunnel through the soil and actually aerate better then earthworms do.
We put down pesticides, and it kils off their food supply. They come inside the houses to get food and get away from the poisons.
If I go out in the yard and turn on a light  at night, I see them scattering.
But I put a fresh sprig of rosemary on each pantry shelf, cabinet shelf, under appliances, everywhere roaches can come in or hide. I change it once a month.
I never see roaches, silverfish etc in my house.
Charlotte