If cilantro were a sure way to prevent hornworms and that became known to the world, what a day that would be.

But my take on the situation is that many folks plant this or that herb or flower and say it does this or that, as in companion planting,and while something may work for one, it doesn't for another.

If someone sat down and planned some experiments where they actually used controls and also planted test crops in areas where the pest is KNOWN to exist, that would be a wonderful addition to the body of what I call, in general, anecdotal information.

Carolyn

Good points, Carolyn. I can conduct an experiment maybe next year. Unfortunately, the obvious lack of sphinx moths over here, is not helping.
It's just that I would like to know that cilantro [or any other companion plant] would work so that anyone NOT wanting manducas in their nightshades could deter them long before their inviting crops become a veritable death-trap for them.
It's unfotunate that many of our cultivars [including tomatoes] are simply more pungent and appetizing than the low-keyed wild natives. In the wild, Manduca larvae are never a problem and I'd sure like to see a way to just keep them on "their side of the fence" so both man and wildlife can co-exist more peacefully. The moth is a splendid creature and quite beneficial, but it's too bad she gets lured by the scent of introduced cultivars and she oviposits thereon. It's not like the larvae just voluntarily left their wild food plant and trekked across field and meadow just to eat our tomatoes and eggplants. They were born there!
There's pros and cons to these hornworms. Some people here, even on this forum like them as much as I do, but still most often, they are killed--without even any chance of relocation to any unwanted weeds they are perfectly capable of keeping in check. If kept on their side of the fence and made to eat their wild food, most people would see their true benefit to man and nature.