The Fly and the Caterpillar: Not a Love Story

As I was doing a quick look-see in the vegetable garden yesterday, I heard a fly buzzing.  Why a buzzing fly caught my attention, I really can't tell you, but it did.  Perhaps it was because the noise was coming from the top of a tomato plant, which isn't normally fly country.  Anyway, I stopped to see what was going on.

At first I didn't see anything, then a movement attracted my attention.  Was that a smallish horn worm? 

I kept looking, finally able to see the outline of the caterpillar against the underside of one of the tomato leaves tucked in a ways.  Suddenly I saw a quick, dark movement and the caterpillar thrashed wildly.  What was going on???

As I watched, it became obvious that a fly was attacking the hornworm.  Every time the fly circled around and came near, the hornworm would move.... Trying to defend itself?  Trying to attack the fly?  Trying to keep the fly from landing?  I couldn't really see what was going on, but I took a series of photos, hoping to capture something with my lens that my eyes couldn't see.

Here are the best shots from the encounter...

The hornworm when I first saw it.

The first view of the fly.

Another "fly by" by the fly.  (Look carefully - you can see the fly's legs above the caterpillar;  the body of the fly is mostly hidden by the tomato stem.)

The fly comes back for another go-around.

And another attack - Fly attack #4...

This one lasted a little longer and the fly seemed to land on the caterpillar's body, despite the hornworm's thrashing and attempts to defend itself.

Fly attack #5.....  (You can just see the eyes of the fly coming into focus to the left of and slightly below the caterpillar.)

Fly attack #6...

Now there's fluid on the back of the caterpillar;  the fly has definitely made contact.

Still the fly comes back again....

But this seems to be the end of the attack.  A drop of "blood" on the back of the caterpillar remains, but the buzzing fly is gone.

According to my camera's time stamp, all of these photos were taken within a 2 minute window.  I don't know how long the attack had been going on when I chanced upon it.  Still, overall, the action didn't last that long.

I am assuming that the fly was a female and that she laid at least one egg in the caterpillar.  Judging from the fly's hairy abdomen that I could see in the photos, this was probably a tachinid fly.  If that's the case, the hornworm will go back to eating as if nothing has happened, but when it goes to pupate, the fly larva that has grown inside it will take over and kill the developing moth.  The hornworm pupa will actually only produce one or more fly pupae, which will, in their turn, produce more tachinid flies to carry on the cycle.

I knew that tachinid flies were parasitoids of caterpillars, but this is the first time I've actually witnessed the interaction between the caterpillar and the fly.  I see many, many tachinid flies in the garden, especially in the fall when they are feeding at my asters, so I knew this sort of event had to be occurring.  Adult tachinid flies are pollen and nectar feeders...and good pollinators.  Hornworms, while I think they are cool and interesting, are pests on tomato plants.  This is natural predator control at its most basic.

Life is not particularly gentle, but it tends to balance itself when allowed to do so.  Sometimes the variety of feeding strategies truly boggles my mind.   What predatory insects have you seen in YOUR garden lately?