The Insidious Tobacco Hornworm

Good gawd, these are impressive creatures. A couple of days ago, I picked a handful of cherry tomatoes and admired my growing heirlooms. Both plants were full of fruit, full of life, and just about as tall as I am. I mowed the next day, and both plants had withered, looking like I hadn’t watered in a week. Or over watered. I can never tell which is which.  Most leaves had dropped, and those that remained were damaged. It took a few minutes before I spotted the culprit – Manduca sexta, the infamous Tobacco Hornworm. I saw one, and then a dozen, and knew I had picked my last tomato. These things are voracious: their Latin name means ‘glutton,’ and they live up to the moniker.

I’m always surprised at my evolutionary imprinting and how it instinctively overrides any learning or reading – I’m not going to just grab this thing. I see that spike on the rear and wonder if it will try to poke me. Are they poisonous? Irritating to the touch? Surely something that big and luscious-looking has some kind of defense against hungry animals?

And they do. The Tobacco Hornworm – viola! – eats tobacco leaves, which contain poisonous nicotine. But the hornworm carries a gene that allows the animal to digest the toxin so that it is absorbed by the hemolymph: bug’s blood. My guess is that, even by caterpillar standards, they taste pretty gross. Once in the hemolymph, the nicotine metabolite converts back to nicotine and is released through the spiracles (breathing tubes) as ‘toxic halitosis’, which, by the way, would be a great name for a band. A single puff of this toxic breath keeps spiders and bugs away.

So why aren’t tobacco fields wiped out in a year? After seeing my two plants crumble in a day, I can hardly imagine what damage hundreds or thousands of these things can do. As you might guess, the tobacco plant isn’t overly fond of this chomping and devastation. These worms, as I can attest, strip the plant in a day or two. In defense, the plant emits chemicals called Green Leaf Volatiles that change composition in response to predation and the predator’s saliva. This chemical signals to anything listening, that, Hey! There’s a big, fat, juicy caterpillar over here ready for eating! Other predators swoop or climb in and, in Disney terms, perpetuate a fascinating circle of life.

I don’t know why my plants didn’t call out to the birds in the field behind us, and I don’t know how fast this sensory response takes. Maybe it happened at night?

Still…it’s fascinating stuff!

Hornworms as pests…

A natural history of the hornworm.

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Have you ever seen one? Ever picked one up?