Monday, January 6, 2020

Cattail Caterpillars on Indian Grass in January

Acronicta insularis, Cattail Caterpillar,
larva of the Cattail Moth.  The larvae do
eat grasses as well as cattails.
 It was a strange sight to find Cattail Caterpillars on the dry stems of Indian Grass during my walk in the Creek Field in the early evening of January 6, 2020.  I spotted at least a dozen and used a flash to get a photo of one.

I went back the next morning to get photos by daylight.   

I found one resting on--what?  The sheath of the grass leaf?  Webbing?  A combination of the two?

I found another just emerging from between the sheath and the stem.

Except it wasn't "emerging."  

My first thought--that these healthy-looking caterpillars were alive--was just plain wrong.  When I posted the photos to, I received this reply:

These are all cadavers of Acronicta insularis, having been mined out some time ago (late summer or fall?) by the parasitoid mummy wasp Aleiodes stigmator (which has since eclosed, the exit holes not so obvious in your photos).  

Well, now I didn't have to worry about what they were going to eat. 

But if they had been parasitized by wasps, where were the exit holes?

I took several home where I could magnify the little "mummies."

I looked at them from different angles.

From the top:


                                            From the side:

  From the side of the head:

I couldn't find anything that looked like exit holes.  So then I started wondering what else could have killed these caterpillars.     

However, a Google search for Aleiodes stigmator turned up a publication that brought me right back to the wasp track:

Scott R. Shaw 
Professor of Entomology and Curator, University of Wyoming Insect Museum Department of Renewable Resources (3354) 
University of Wyoming 1000 East University Avenue Laramie, Wyoming 82071, USA

This paper explained that the absence of exit holes probably means that the wasp larvae are still inside, as some wasps overwinter inside their mummies.    Dr. Shaw's essay recommended keeping the mummified caterpillars in a closed container at outside temperatures in a shed or garage and photographing the wasps when they do emerge.   

I had better follow his instructions immediately--lest the wasps hatch from the mummies lying on my desk and fly around the house! 

Many thanks to and to Dr. Shaw and all the experts who share their knowledge so generously and help people like me learn more about the invertebrates in our neighborhoods!   

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