What Are Braconid Wasps?

Braconid wasp cocoons on a hornworm caterpillar.
Braconid wasp cocoons on a hornworm caterpillar. Flickr user (a href="https://www.flickr.com/photos/onthespiral/">wormwould (CC license)
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What Are Braconid Wasps?

Braconid wasp cocoons on a hornworm caterpillar.
Braconid wasp cocoons on a hornworm caterpillar. Flickr user wormwould (CC license)

Ask a gardener which pest she hates the most, and she's likely to respond without hesitation, "Hornworms!" These freakishly large caterpillars can devour an entire tomato crop overnight. But nothing thrills a gardener more than finding a hornworm covered in little white cases, like the one pictured here. Just when hope is almost lost, the braconid wasps arrive to save the day. What are braconid wasps?

Braconid wasps are Mother Nature's way of keeping pests like hornworms under control. These parasitic wasps disrupt their host insect's development, effectively stopping the pest in its tracks. Braconid wasps are parasitoids, meaning they eventually kill their hosts.

Although we're probably most familiar with the larger braconid wasps that live on hornworms, there are actually thousands of braconid wasp species throughout the world, each infecting and killing certain types of host insects. There are braconids that kill aphids, braconids that kill beetles, braconids that kill flies, and of course, braconids that kill moths and butterflies.

The Braconid Wasp Life Cycle

It's difficult to describe the braconid wasp life cycle, because each braconid wasp species develops in association with its host insect's life cycle. Very generally, the braconid life cycle begins when the female wasp deposits her eggs in the host insect, and the braconid larvae emerge and develop within the host insect's body. When the wasp larvae are ready to pupate, they may do so in or on the host insect (which is well on its way to dying if it hasn't succumbed to the parasitoids already.) The new generation of adult braconid wasps emerges from their cocoons and begins the life cycle again.

The Braconid Wasp and Hornworm Life Cycle

Braconid wasps that kill hornworms are larval parasitoids. The female braconid wasp deposits her eggs inside the hornworm caterpillar's body. As the wasp larvae develop and feed inside the caterpillar. When they're ready to pupate, the braconid wasp larvae chew their way out of their host, and spin silk cocoons on the caterpillar's exoskeleton. The tiny adult wasps emerge from these cocoons a short time later.

The affected caterpillar may continue to live as the braconid wasps are developing inside its body, but it will die before it can pupate. So while the current generation of caterpillars may have already munched your tomato plants down to the stems, they won't survive to become reproductive adults.

Misconceptions About Hornworm Parasites

And while we're talking about these hornworm parasitoids, let's clear up a few misconceptions about them:

"Those white things on the hornworm are parasite eggs."

No, they aren't. The braconid wasp injects her eggs into the caterpillar's body, under the skin, where you can't see them. Those white things on the hornworm's body are actually cocoons, the pupal stage of the braconid wasp. And if you watch them closely, you might get to see the tiny adult wasps emerging and flying away.

"The wasps hatch from those cocoons and attack the hornworm."

Wrong again. The adult wasps emerge from their cocoons, fly off and mate, and then the females look for new hornworm hosts in which to deposit its eggs. The hornworm "attack" is perpetrated by the wasp larvae that hatch from eggs inside the caterpillar's body. The damage to that caterpillar occurred well before those white cocoons were spun on its skin.

How Braconid Wasps Kill Their Hosts

Braconid wasps use a remarkable weapon to disable the defenses of their host insects – a virus. These parasitic wasps coevolved with polydnaviruses, which they carry and inject into the host insects along with their eggs. The polydnaviruses have no negative affects on the braconid wasps, and reside within cells in the wasp ovary.

When the braconid wasp deposits eggs in a host insect, she also injects the polydnavirus. The virus is activated in the host insect, and immediately goes to work disabling the host's defenses against intruders (the intruders being the braconid wasp eggs). Without the virus running interference, the wasp eggs would quickly be destroyed by the host insect's immune response. The polydnavirus allows the wasp eggs to survive, and the wasp larvae to hatch and begin feeding inside the host insect.

 

Sources:

  • Bugs Rule! An Introduction to the World of Insects, by Whitney Cranshaw and Richard Redak
  • Family Braconidae – Braconid Wasps, Bugguide.net. Accessed online August 17, 2015.
  • "Viral DNA delivers wasp's sting," by Richard Kwok, Nature, February 12, 2009. Accessed online August 17, 2015.
  • Braconid Wasp Cocoon, Illinois Nature History Survey, University of Illinois at Urbana-Champlain. Accessed online August 17, 2015.