well, I know that rosemary essential oil is a great ant killer....

I'd say it's worth a try--why not. Can you let us know how it works if you do?

I have a collection of herb spray recipes. It depends what bugs you want to get rid of, as to which herb you use. Here's a basic recipe. Don't become too spray-happy, though, will you? Herbal sprays will kill many good bugs as well as the bad guys. I regard them as remedies for plagues only.

HERBAL SPRAYS
Herbal sprays are made by mashing or blending 1-2 cups of fresh leaves with 2-4 cups of water and leaving them to soak overnight.

I've also just posted the recipe for Garlic Spray on another thread.

Or pour 2-4 cups boiling water over 2-4 cups fresh or 1-2 cups dry leaves and leaving them to steep until cool. Strain the water through a cheesecloth before spraying and dilute further with 2-4 cups water. Add 1/4 teaspoon liquid soap (not detergent) to 1-2 litres water to help spray stick to leaves and spread better. Try tansy for cabbage worm, wormwood or nasturtium for aphid, wormwood or nasturtium to repel aphid, catnip, chives, feverfew, marigolds or rue for leaf-eating insects.

Essential oils of herbs can be diluted with water to make a spray. Experiment with proportions, starting with a few drops of oil per cup of water. Try sage, thyme, hyssop, rosemary and white clover for caterpillars, butterflies and moths.

Combine the oils for peppermint, spearmint, lavender lemongrass and citronella with water to create a good repellant for gnats, ants aphids, moths and many flies.

Here is a link that might be useful: garlic spray

No worries; to be honest, I've only rarely used sprays. I've been lucky to have nothing worse than Japanese beetles and black spot strike regularly. The beetles I pick off, and the black spot... I try to plant around.

I have had problems with scale on my citrus, but I think I was more intrigued by the idea of plants to defend plants more than anything else. I know lavender has antiseptic properties, as well as a sweet smell -- I like the idea of it cleaning up the bad guys, and leaving just a lingering pleasant smell!

This might be a little OT, but "herbs as an insecticide". . . . Having recently moved to the Houston area (where, it's TEXAS!, and the state bird is the mosquito) the mosquitoes are HORRIBLE! Researching led me to the old farmer's remedy to keep the mosquitoes from attacking the horses/mules/etc. They used crushed Beautyberry leaves under the harnesses. I bought a house with Beautyberries in the yard. Crushing the leaves and rubbing them on my skin really does seem to help more than DEET! Now, if I could just figure out a way to make a spray from the Beautyberries to drench the yard. . . . .. :-) Pat
Oh, Beautyberries are zone 6-10. They are GORGEOUS plants! Mine are berrying right now --- AWESOME!

LAVENDER OIL:
Place a handful of flowers into a wide-mouthed jar and cover with any plain oil. Leave in a sunny spot for several weeks, shaking from time to time. Strain and use. Pure lavender oil is poisonous, so take precautions. Lavender spray will deter aphids. You might prefer to make a tea instead of using oil. Just pour boiling water over lavender flowers (a few leaves won't hurt), leave until cool, then spray.

This is info I've been looking for. I've been interplanting herbs with all my veggies and flowers, but needed a spray recipe for infestations. Thank you everyone!

Not really an insecticide, but BuggOff is based on cedar and I've successfully used catnip to keep mosquitos off me. These are repellents.

I've heard that a tobacco solution keeps stuff off your garden; I've never used it.