Monday, September 5, 2011

Macro Monday: Hornworms' Lament

Fish gotta swim and birds gotta fly. Insects, it seems, gotta do one horrible thing after another.

-- Annie Dillard, Pilgrim at Tinker Creek

Our friend Andy spotted these two critters on the currant tomato that is growing rampantly in the washtub by the grape arbor.

They are larvae of Manduca quinquemaculata -- the five-spotted hawk moth -- aka tomato hornworms.

And they have munched their last nightshade leaves.

The stuff that looks like a a threadbare white shag carpet are parasitoids -- the pupae of a braconid wasp. As larvae, they already ate most of the insides of the hornworms, which maintain just enough life to cling to the branch where they stopped. (I took these photos on Saturday, and the worms are still where we left them.) Soon they will hatch out into adults and go hunting other hornworms on which to lay their eggs. I've never seen a hornworm with this heavy a parasitoid infestation. I guess they aren't any more doomed / dead than one with a few pupae.

I'm ambivalent about this particular horrible thing. The hornworms can really play hell with the tomatoes, and the little wasps are very effective at controlling them. But the adult moths are magnificent -- easy to mistake for hummingbirds when one first sees one -- and we have lots of tomato foliage. When I find a healthy hornworm, I tend to just stick it on a robust volunteer tomato somewhere away from the garden, or else on some wild nightshade.

The life cycle of parasitic wasps is the stuff of nightmares and the inspiration for some viscerally horrifying speculative fiction.



  1. As much as I believe in letting nature take its course, I have to draw the line at termites and tomato hornworms. To me they are totally evil and I can't believe anyone would allow them to breed.

    But then I've never seen the adult moth. I might change my mind, but it would have to be totally magnificent.


    I don't know if this is the exact same species, but here you have the general idea.

    I'll sacrifice some tomato foliage so I can have these guys in my garden.

  3. jane at scout's run farmSeptember 5, 2011 at 10:28 PM

    hi from southern ontario,
    i smushed 587 hornworms in ten days, stopped counting and kept squishing. we have 325 tomato plants, bad ratio, infestation to be sure.
    unfortunately, we saw no wasp larve, none.
    weird. any info on similar trends around the 49th would be appreciated.

  4. Jane, my guess is that you've got an infestation of the larvae because you have so many plants -- a small monoculture.

    You will only see the moths if you have the kinds of flowers they like.

    However, logically, with that many hornworms you should have parasitoids. Has anyone sprayed any pesticides on your place or a neighbor's place? Small wasps can be very sensitive to them.

    If you have chickens, don't squash the hornworms, harvest them for the girls!

  5. Some of the spots on that hornworm look disturbingly like eyes.

    ::goes off to have nightmares::

  6. Yes, chickens love hornworms, unlike the larva that eat the parsley and dill, even carrots and arugula. That "worm" grows up to be a black swallowtail butterfly! I still don't want to lose my herbs to it, however. Chickens won't eat the suckers so I have to drown them. I refuse to squash them, too gross.

    Margaret P

  7. Hate the hornworms (and yes, those spots are supposed to look like eyes so that the birds won't eat them), but I have planted extra parsley and dill to keep the swallowtail butterfly.

    It just seems easier to grow enough to share the parsley and dill than to share the tomatoes. Of course, I'm not saving seed for Seed Savers Exchange for the parsley and dill, so that might make a different -- need every one of those seeds of the rare tomatoes I used to grow.


  8. Actually, Heather, tobacco hornworms are much more common - larva - 7 diagonal stripes and red hiney horn. M. sexta

    There's some indication the wasps use a virus to confuse the host immune system so they don't get axed.

    The adults, commonly called hawk moths, are often mistaken for brown hummingbirds.
    Adult pix:

    There are also superparasitoids, which parasitise the parasitoids.

    I've got pix of larvae emerging and spinning cocoons on a cabbage looper. Way cool!

    Jane, they might be available commercially, but you need to provide food for the adult wasps, which are usually plants with tiny flowers, such as thyme if you want to foster them in your area.
    Many times, however, biological control happens at the end of the season when there is sufficient prey/food, which is never soon enough for a producer of food.

    Myself, I'm rooting for the Phorid flies they released in FL to pop the heads off of fire ants!

  9. Not to mention that the invading larvae have to protect the host from dying of bacterial or fungal disease:

    Those cocoons will look just the same when the wasps have emerged, only there will be little hatches sawed out the ends.

    There are even parasitoids for aphids if you can imagine something that tiny.

    Oh don't get me started! Oops, too late!


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