Tobacco Hornworm

July 11th, 2008 by david laferney Leave a reply »
tobacco horn worm

actual size believe it or not

Sooner or later you are going to encounter this little beast – the Tobacco Horn worm (Manduca sexta) munching on your tomatoes or peppers. He has a close relative called the tomato hornworm which for all practical purposes is the same thing, and despite the names they can infest a variety of your garden plants. For example that tobacco worm in the picture is on one of my celebrity tomato plants – you might also sometimes see them on eggplant, and potato plants. They can do a lot of damage in a short time so keep an eye out for missing foliage like this…

tobacco worm damage

… or worm poo like this…

If you see those things then a horn worm is somewhere on that plant rapidly defoliating it. Despite the jumbo size they really blend in well – they take on the color of their diet, but if ye seek ye shall find. Tip: they thrash around when sprayed with water and are easier to see.

Organic Control of the Tobacco HornWorm
The best way to control this little monster is to just stay alert and pick them off and squash ’em. Despite the large size – commonly up to three inches long – and ferocious looking red tipped stinger thingy, they are completely harmless. Usually hornworms don’t occur in large numbers, and hand picking really is a practical control. If you happen to come across one that looks like this –

parasitized horn worm

Just leave it alone to die in peace, those white things are cocoons of parasitic wasps, which will hatch out and do your work for you if you let them. Yet another reason not to blindly spray insecticide around your garden. Parasitic wasps are common just about everywhere even though they are so small that they mostly go unnoticed. As long as you don’t wipe them out by spraying every thing with pesticides they will naturally be attracted to your yard and garden by many kinds of herbs and weeds with tiny flowers – clover, mint, fennel, Queen Anne’s Lace, Joe Pye Weed, etc. Unless your yard is a chemical soaked golf-green-like monoculture of bluegrass and azaleas you’ll have parasitic wasps around.

If you simply must spray something for life to be complete then Bt – Bacillus thuringiensis – is a completely safe biological control that is effective against all kinds of caterpillars – especially Bacillus Thuringiensis Kurstaki or BTK. When caterpillars eat foliage that has been sprayed with BT their digestive system shuts down and they’re unable to eat and they die within a few days. Compost tea apparently has a similar effect. BT can be found in most serious garden centers under various trade names in both dust and liquid concentrate forms. Our local big box doesn’t have it – I guess they didn’t have room for it once they stuffed the shelves with petrochemical poisons.

If you feel experimental you might try this trick that my Dad used to do – liquefy some of the worms (or Japanese Beatles or whatever the pest du jour is) in a blender along with a cup or so of milk (Yum yum!) and dilute with water, and let it ferment for a few hours before spraying with a garden sprayer. I’ve never actually tried this, but it makes sense that it could work by growing a culture of worm disease pathogens, and basically making them sick with some kind of horn worm Ebola. Or it could just be so disgusting that they move on. I’m sure if someone did that to me it would have one of those effects. In any event during the process you pick off some of the worms and get to play with a kitchen appliance.

Once the caterpillar has eaten it’s fill it will burrow a couple of inches into the ground to metamorphose into the adult phase. Cultivation of the top layer of soil to expose and damage the big brown pupae before they can emerge as moths to lay eggs and restart the cycle is also an effective control reportedly resulting in up to 80% mortality. If you use a no till gardening method (Ruth Stout, “lasagna”, or other deep mulch method) and experience problem levels of hornworm infestation you might consider a preemptive program of spraying BT starting in early summer to hit the larvae while they are still young and relatively weak. Once the caterpillars get big they are much harder to kill, and inflict damage at a much higher rate. I bet that free range chickens would pretty much keep them under control by scratching out the pupae to eat. No wonder free range eggs are so tasty.

Common paper wasps prey on hornworms among other garden pests so think twice before destroying wasp nests near your garden. You might even consider intentionally providing a nest shelter near the garden, but safely located for people.

Carolina Sphinx moth - adult phase of tobacco hornworm

Sphinx Moth Photograph by: John Capinera, University of Florida

In case you were wondering – when they grow up tobacco hornworms become Carolina Sphinx moths, a large brown nocturnal visitor to porch lights all over the south which is actually an important pollinator of night flowering plants. Interestingly enough (if you are a nerd as I am) Sphinx moths, humming birds, and a bat (the name of which escapes me) are the only creatures adapted to hover while sipping nectar. Sphinx moths are sometimes mistaken for nocturnal humming birds. A minor nuisance in the Garden, but beautiful in their way.

The excellent photo of a Shinx moth dining on a moon flower was shot by JohnnyBigFish

This excellent photo of a Sphinx moth dining on a moon flower (a night blooming variety of morning glory) was shot by JohnnyBigFish

The entire hornworm/sphinx moth life cycle can happen as quickly as 30 to 50 days and several cycles may occur in one season given proper conditions. Later in the summer an increasing number of pupae will go dormant in the soil – a phenomenon called diapause. These pupae will overwinter to emerge as adult moths to restart the process next year.

Happy Gardening!


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