Tuesday, August 11, 2015

Milkweed Murder Mystery

We really needed the rain we had today, especially the long slow soaking we got last night.  But I was glad for a break in the showers midday, because I wanted to go up to the Nature Center at Moreau Lake State Park to install the August-September wildflower poster.  My friend Sue Pierce came up with this idea a couple of years ago, to create posters featuring what wildflowers park patrons might find on their hikes or paddles each month throughout the growing season.  She and I contributed our photos and she made the posters, which are mounted both inside and outside the Nature Center.  Here's the outdoor one I installed today.

My poster task accomplished, I wandered over to a patch of Common Milkweed near the beach bathhouse to see if I could find the Milkweed Tiger Moth caterpillars I had seen there a couple of weeks ago, chewing away on the milkweed leaves.

I couldn't find a single Milkweed Tiger Moth caterpillar, although I turned over leaf after leaf in my search for them.  But I sure did find a lot of Lady Bugs, including this pair doing their bit for the next generation.

And when I turned over a leaf next to this amorous pair, I was pleased to see a Lady Bug larva, this odd-looking spiky creature chowing down on itsy-bitsy aphids coating the undersides of many milkweed leaves. And this was only the first of many I found in my examination of this milkweed patch.

There was certainly an inexhaustible supply of fodder for Lady Bugs, adults and larvae alike, and not just those minuscule  pale-yellow aphids pictured above.  I also found quite an infestation of the bright-yellow Oleander Aphids on many milkweed stems.  And this congested clump of all-female aphids will be just the beginning of their population, as they spread the colony by producing clones of themselves, and then the clones producing clones,  over and over again, until they have finally exhausted their food supply.

It's a good thing, then, that the Lady Bugs, adults and larvae, help to control this aphid population explosion.  On the back of this leaf, I found a larval Lady Bug actually snagging an aphid in its jaws.  And I also saw a curious thing I have yet to understand.  See that odd oozy lump at the tip of this leaf?  Well, it used to be a fly, but what the heck happened to it?

The fly was stuck to the leaf as of glued there, and its body was erupting with oozing white slime.  Yikes!

Again and again, I found more flies stuck to the undersides of the milkweed leaves.  All were dead, and most were oozing that slime from cracks in their exoskeletons.

This fly seems to have some kind of egg case attached to it.

Yet another dead fly.  One of many more.  Who can explain what plague is attacking them?  Are they dying of some bacterial or viral disease, or are they being parasitized by some other insect?  Does this mass fly death occur only on milkweed, or does it occur elsewhere? This is a phenomenon I have never witnessed before.  I shall have to see if I can find the cause.

Update:  Isn't the internet amazing?  I put some of these dead fly  photos on my Facebook page and promptly got lots of information from truly reliable sources, including a Cornell University professor, Kathie Hodge, who is an expert in fungi.  Professor Hodge suggests that the pathogen was a species of Entomophthora fungus, which uses the fly as a vehicle for spreading its spores, killing the fly in the process.

To read Professor Hodge's detailed account of how this kind of fungus infects a fly, click HERE.


Uta said...

How interesting, I hope you will let us know what you find out about these flys. I will have to check my milkweed.

The Furry Gnome said...

Don't know about the dead flies, but that poster is a great idea.

Anonymous said...

It reminds me of parasitoid wasps. Some of them use flies. http://www.iflscience.com/plants-and-animals/parasitic-wasps-must-lay-their-eggs-right-fly-or-else

suep said...

The poster idea came from seeing the bulletin board at the New England Arboretum, which was on a much larger scale but able to change with the season.
As for those deceased flies ... ewwww ! Cannot be un-seen ... Maybe the fungus killed them, but I'll wager some of them were visited by parasitic wasps too (perhaps taking advantage of their weakness)...?