HELP! Pests that prey on new seedlings?

Have any of you encountered pests that just eat the tender leaves of seedlings? My friend has a green house and have lost almost 200 seedlings - something is only eating the tiny leaves of the petunias and portalacas leaving the stems so she has tiny little spikes!

Slugs or snails would be my first guess. Any signs of slime trails anywhere?

Ecrane I wish it were that easy - no signs of either - no silvery trails - it's in the greenhouse in Oklahoma, the temp doesn't get below 50 degrees - can get much warmer during the daily sun. Are there warm loving pests that may do this. Also the stalks are untouched but the leaves are definitely eaten and these stalks are tiny (petunias).

A few years ago, I had something eating the Datura seedlings in my coldframe. The bug ate round holes in the leaves. It turned out that it was a tiny green worm-like bug. There were many of them. Evidently some kind of insects layed eggs somewhere in the coldframe and they hatched into larva feeding on leaves.

I never wait to figure out a bug. I go for my mix of Malathion spray and hit the plants with it before too much damage. The larva was hard to find since they blended in with the color of the leaves.

Malathion is my standard remedy for all damaging bugs. I squirt a bit of liquid dish soap in the mix to make it stick to the leaves, otherwise it will just run off. It will kill many common insects found on ornamental plants including aphids and spider mites.