You'll need to match the species of insect-parasitic nematodes with the insect you wish to control. And the timing of the nematode application is very important, as is the suitability of your environment and the quality (viability) of the nematodes.

Yes, insect-parasitic nematodes are safe to use in the garden....it is pretty much up to you to make sure that their use will be effective or a waste of time and money.

I've not heard of nematodes being used to control hornworms. Are you talking about the big Tobacco hornworm so commonly found on tomatoes, peppers, potatoes, etc.? How many plants are you trying to protect (defend)?

I have 35 tomatoes 50-60 peppers potatoes cucumbers squash melons.. All sorts of stuff. The cut worms first got one of my smallers potatoes. I didn't think anything of it. Then two nights ago it dented one tomato and I built dirt up around the stem to try and save it. Today I went outside and it had destroyed the same plant from yesterday (thank god it was only one so far). And last year they were the big green worms with a spike on their butt. I've heard there are tomato horns and tobacco worms. All I know is these were huge, bright green and did a great deal of damage. :( I thought the nemoatodes might help with those too since they are used for cut worms too :)

You need to protect your plants from hornworms with a biological agent called Bacillus thuringiensis kurstaki (Bt -k) . Bt -k has been used for generations to control caterpillar pests of trees, shrubs, garden plants of all kinds, and agriculture.

Bt works by infecting the gut of any caterpillar that ingests it...the younger the better. I would apply a liquid formulation with that many plants. The larvae feeds on the Bacillus, becomes infected and can no longer eat leaves but won 't die right away.

Bt is safe to use around pets and children and won't harm the birds that love to feed on the hornworms.