Braconid Wasps versus Tomato Hornworms

          Ah, tomatoes.  We’ve harvested the first of the season.  How plump, how juicy, how tasty!  With such bounty in the offing, we look down the long days of summer with delight.  But there’s a kink in our path, a stumbling block, a veritable bug in the program, you might say.  Tomato hornworms. Neon green and gaudily stripped and dotted, these voracious destroyers can grow to an enormous size and devour an entire tomato plant in a day or two if not stopped.  What to do?  We go on hornworm hunts.  Dawn and dusk are best when they aren’t hiding under the greenery away from the blazing sun, but hornworms, for all their great size, can be elusive, and the hunt time-consuming.  Fortunately, we have allies.

In sustainable agriculture, we use the most natural methods of pest control that we can.  One of these is to encourage natural predators to take up residence in our fields so that they can eradicate those pests that they find tasty and we’d rather be gone.  A good example are ladybugs whose favorite food are the aphids that suck the juices from plant stems. We are now fortunate that braconid wasps have been making their appearance among the tomatoes.

There are three kinds of parasitism in the natural world:  predators, parasites, and parasitoids.  We’re familiar with predators: foxes, hawks, ladybugs. Usually larger than their prey (on the farming, not the Africa veldt scale!), they eat many individuals over the course of their lives.  We’ve also heard about parasites which live in (or on) a single host their entire life, occasionally debilitating, but rarely killing it.  And then there are the parasitoids.  These are the ‘predators’ that seem most alien to us. A parasitoid spends only a portion of its life in or on a host, using the host for food, and in the process, killing it.  Even the definition can give one the shivers!

Braconid Wasp drawing from Pacific Horticulture

Braconid wasps, small black wasps with transparent wings that are rarely over a 1/2 inch long, are parasitoid.  The adult wasp lays her eggs just under the skin of the tomato hornworm, and while the hornworm is munching along on the tomato leaves, the wasp larvae are eating the worm alive from the inside out!  The larvae once ready to become wasps, burrow out from under the hornworms skin and spin cocoons where they pupate until ready to emerge as full-grown wasps.  Usually, only then, does the tomato hornworm expire.  It’s a long, and to human sensibilities, a gruesome demise, but for the braconids and hornworms, it’s the way nature works.

One of the drawbacks of relying on parasitoids to protect crops is that it is a long-term solution.  It may take a year or two for braconid wasp colonies to have grown in sufficient numbers to adequately control the hornworms, and until then our crops are in danger.  So while we will leave a braconid-infested hornworm alone to suffer its fate, we still pick off and feed the others to the chickens.  In sustainable agriculture, we use a mix of approaches; in the end, we and the wasps will win.

For more on braconid wasps, see: http://www.pacifichorticulture.org/garden-allies/69/2/

 http://aggie-horticulture.tamu.edu/galveston/beneficials/beneficial-04_braconid_wasp_on_hornworm.htm

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