Galina, I feel your pain. A few years ago I had purchased several conifers and had the pots waiting for planting. I went out to water them one morning and hundreds of grasshoppers came out of the soil of the pots. In less then a week all the plants were dead!
Here's grasshoppers of Florida. See if you can id which you have.
You should also find this helpful.
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.