Monday, October 31, 2011

Hornworm vs. Braconid Wasp

Pupating braconid wasp larvae on a tobacco hornworm

One of my coolest finds this summer was this hornworm covered in braconid wasp cocoons.  My first reaction was "oh... poor caterpillar!".  I have since learned that this is often a welcomed sight, especially by gardeners, since the hornworm can be a terrible pest in the garden.  Because there are seven stripes on this subject, I believe this is a tobacco hornworm as opposed to a tomato hornworm. The two hornworms are very similar.  It is a bit hard to tell because of all the cocoons in the way!    For more on hornworms, check out this great site:
The dark spots you see are the exit wounds where the larvae exited the caterpillar so they could pupate.

What you are seeing here is the last stage of development for the braconid wasp.  The wasps are pupating in the cocoons, and will soon emerge as adult wasps.  The wasps themselves are not actually very big ~ no more than 1/2 an inch in length.  What I found amazing, is how they got to this stage.  Lets start at the begininng...

The braconid wasp is a parasitoid.  This means that it spends a significant portion of its life attached to, or inside of, a host organism.  The host, in the end, usually dies.  The braconid wasp has many different hosts it likes to feed upon.  These include aphids, bark beetles, squash bugs, stink bugs and caterpillars like the one seen here. 

To keep it simple, here is how the lifecycle of the wasp goes:

An adult female lays her eggs inside a host organism by piercing its skin with her long sting-like ovipostor.  Her ovipositor injects the eggs inside the host, where, once hatched,  they will eat the host's viscera (internal organs) while developing.  The host (amazingly!) remains alive during this process. 

Click the pictures for a larger view!
When the wasp larvae are ready for the next stage of development, they eat their way out of the caterpillar. They then spin themselves into the tiny white cocoons you see hanging on the outside of the hornworm.  Here they will pupate until they are ready to emerge as adult cocoons.  The caterpillar soon dies after this. 

It is a rather gruesome life cycle.  So I still think "oh... poor caterpillar!", but knowing that the hornworm here is the pest, and the wasp is the beneficial insect eases my mind a bit. 

I just hope that in my next life, I don't come back as a hornworm!

To find out more about this fascinating process, check out the following sites:

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