Tuesday, September 4, 2007

Pile on the Bully!

There is not much worse for a tomato garden than a visit from a tomato hornworm (Manduca quinquemaculata). Except for, perhaps, a visit from an unsupervised nephew. Even one of these big, green, meaty caterpillars can decimate a tomato plant almost as quickly and thoroughly as a five-year-old boy with a shovel -- a child you assume, oh-so-naively, is under close watch of his parents, while you are inside the house putting on coffee for the very people who are allowing their child to destroy your yard.

Five-year-old nephews eventually go home with their inattentive parents, but the tomato hornworm is very difficult to eradicate, because it blends in with the tomato foliage, and does most of its work at dawn and dusk, while you are inside doing other things. Dammit.

Glen and I found three caterpillars on our plants earlier this summer, one Saturday morning. They caused extensive damage, stripping the leaves from the plant, chewing through the branches, and even biting some of the young, green fruit (they seldom bite reddening tomatoes). We felt violated, angry. We knew we needed to take quick action, so we peeled them off the plant -- and what yucky work that is: they are STRONG, and have nasty horns on their rear ends (hence their name). I hate to kill things, even larvae of rather bland moths. But our garden is important to us, and so measures were taken to ensure those particular worms would not return to our tomato plants.

This is the tomato hornworm:

It becomes the Five-Spotted Hawk Moth, if allowed to live:


By August, our damaged tomato plants had bounced back and began producing some wonderful fruit for us. And then, this Saturday, without warning, we found two more hornworms. However, this time, they were covered with white lumps. I had heard about this in my Master Gardener class, and so, I ran for the camera:



The bully of the tomato patch had finally met its match: the Braconid wasp (Apanteles congregatus). The mother wasp finds a juicy tomato hornworm, and injects her eggs under the caterpillar's flesh. Over the next few days, the baby wasps feast on the guts of the caterpillar, and eventually migrate outside, affixing themselves atop the caterpillar, where they pupate. And the pupae eventually emerge as adults to begin the process all over again. Ah, the circle of life!

If you see a hornworm in this sorry state, you may think it's a good idea to just kill it, along with all of the wasp pupae, because, after all, who wants wasps? We do! So don't intervene (except maybe to pull the caterpillar off your tomato plant; even though it will soon be with Elvis, it can do some more damage to your plants). These wasps are not interested in humans, and won't sting us. They have one purpose, and that is to find more hornworms and eat them from the inside out.

During the summer, there are several different types of beneficial wasps that will make you reconsider your relationship with these insects. In addition to the Braconid wasp (a variety of which will also parasitize aphids, another dastardly enemy of your garden), there's also the Cicada Killer (Sphecius speciosus), a frightfully large wasp -- but one who is a gentle giant and not interested in you. At all. In fact, the males don't even have stingers. Their sole mission is to find adult cicadas, a late summer garden pest. The female digs a tunnel in your yard, about the diameter of a pinky finger. Unlike other wasps, who are social, and tend to sting to defend the nest, this is a solitary creature uses her sting only to paralyze cicadas. And once she does, she drags the cicada/s into her den, and lays her eggs all over it/them. The eggs hatch in a couple of days, and the cicada serves as food for the youngsters. Yumm-o!

Gruesome? Totally. Nature can be cruel, but it's not without checks and balances. Many backyard bullies, like real life human bullies, will run roughshod over our plants for awhile, but eventually, they get too big and slow. And then, the army of beneficials descend and turn things around. Natural controls rule!




More information:
Mercer County Horticulturist Barbara J. Bromley's fact sheet on beneficial insects
Mercer County Horticulturist Barbara J. Bromley's fact sheet on attracting beneficial insects
Wikipedia entry on the Tomato Hornworm
Wikipedia entry on the Braconid Wasp
Wikipedia entry on the Cicada Killer Wasp




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