Friday, September 21, 2012

Tomato killer meets its match

Photo: Wikipedia Commons

Ah, the tomato. A much beloved fruit, although many enthusiasts of Lycopersicon esculentus might give it a second thought if they knew that it is spawn of the nightshade family (Solanaceae), which contains many very poisonous species. But there's obviously no issues with eating the ripe plump succulent fruit, or our world's population would not be what it is. You'll not want to graze on the foliage, however, as it contains small amounts of various toxins, true to the tomato's nightshade heritage.

There is one very prominent tomato foliage feeder, much to the dismay of hard-working tomato growers. It is the larva of one of our flashiest moths, the Carolina sphinx, Manduca sexta. Its caterpillars are also quite showy, although their good looks don't often win them a pass from peeved tomatoists. Nor from a very deadly parasitoid wasp.

One of my co-workers mentioned that she had plenty of tobacco hornworms noshing on her tomato crop, and that many were victimized by a very interesting and oft-seen hornworm predator. I convinced her to cup one up and bring it into the office so I could capture some images, and she most kindly did just that.

NOMENCLATURE NOTE: Sometimes caterpillar common names are confusing, and this is one of those cases. Manduca sexta is an example of a moth that has different names for the adult (Carolina sphinx) and the larva (tobacco hornworm). In this case, it is doubly confusing as there is also a tomato hornworm, Manduca quinquemaculata, larva of the five-spotted hawk moth. It is also a nightshade specialist, but is far less commonly found on tomatoes than is the tobacco hornworm. The common names of these two species should probably switched for clarity's sake.

In any event, you probably couldn't help but to notice all of those little whitish tubes bristling from the hornworm caterpillar, and if you grow tomatoes, you've probably seen this with your own eyes. These tiny sacs are the cocoons of a parasitoid braconid wasp, Cotesia congregata.

 Photo: Beatriz Moisset/Wikipedia Commons

The wasps are tiny indeed, as can be seen in this image. Note how neatly the tips of the cocoons rupture when the adult wasp pushes its way out. Female Cotesia wasps seek out tobacco hornworms, and when a victim is located, she'll inject numerous eggs into its tissue with a needlelike ovipositor. Along with the eggs comes symbiotic viruses that hamper the caterpillar's defense systems. The wasp eggs hatch in about two days, and the little grublets begin devouring the nonvital soft parts of the caterpillar's innards.

About two weeks after the wasp grubs hatch, they'll be mature and then burst through the hornworm's skin and form the strange little cottony cocoons that have given many a gardener pause. A week or so later, the adult wasps will emerge from the cocoons, find other Cotesia wasps of the opposite sex, mate, and begin the cycle anew. By this point in the cycle, the caterpillar will have perished, its inner workings too ravished by the wasp grubs for it to survive.

Nature is chock full of predators on every level, and the hornworm/Cotesia wasp dynamic is an especially obvious example of a predator-prey relationship that would normally go unnoticed. I recently wrote about two similar but far less seen parasitoids HERE. Were it not for the legions of predators engaging and killing their legions of victims, ecosystems would go wacky. Even though some of the tactics employed by insect predators - especially, perhaps, the parasitoids - seem grisly, they are also fascinating studies.

In this case, we have a plant - the tomato - that has evolved chemical toxicity that is effective enough to exclude most caterpillars from eating its foliage. Some tubular warriors will always manage to battle through the plant's chemical defense systems, and in this case the hornworm is the conqueror. It wins two prizes, as not only does the caterpillar get to nosh on tomato foliage free of competing herbivores, it also sequesters the nightshade's toxins and thus becomes unpalatable to most predators. Nothing is enemy-free, however - enter the parasitoid Cotesia congregata wasp. But the wasp undoubtedly has its enemies as well. It may well be victimized by even more devious hyperparasitoids - other wasps whose larvae consume those of the parasitoid. Even if that doesn't happen, the adult wasps are vulnerable to scores of predatory insects, songbirds, and others.

A bit of hornworm management advice. Most gardeners don't want their tomato crop depleted by ravenous caterpillars, so if you see caterpillars bristling with wasp cocoons, leave some be. They'll hatch more wasps and in this case the wasp is the gardener's friend. Better yet, set aside a small patch of tomatoes and allow the hornworm caterpillars to have at them. With luck, a few will make it all of the way to adulthood, and then your yard will be graced by one of our coolest moths, as SEEN HERE.

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2 comments:

Barb Padgett said...

No offense, Jim, but ICK! :)

Bethany said...

I've been pulling dozens of these guys off my tomato plants for the last two weeks. No wasp eggs though!