There are different types of nematodes that kill insects. However, many of them are difficult to mass produce or have very narrow host ranges.

Are there good and bad nematodes?

Most nematodes are harmless, but a handful of troublesome species attack the outside surfaces of plants, burrowing into the plant tissue and causing root, stem, folar and even flower damage. Other nematodes live inside the plants for part of their lives, causing damage from the inside out.

How many nematodes are there?

Wikimedia CommonsMore than 400 quintillion nematodes inhabit the Earth. Although nematodes are roundworms, they aren't true worms like the ones most people know about. They're microscopic multicellular organisms that pretty much rule the Earth. Nematodes are the most numerically abundant animals on the planet.