Were the wings faded and ragged? If so, then it was probably the natural death of old butterflies. If the wings were brightly colored and mostly intact, it's unlikely that they died naturally. There are numerous possibilities as to why - they might have been poisoned from insecticides, or a predator might have gotten them, although most butterfly predators eat the wings. Preying mantises and spiders in their big webs are the most common predators on adult butterflies, at least here.
MissSherry

Usually when I find just the wings I know it is because of some predators in my garden. I normally find all four wings nearby...and no 'middle parts.' I then find a gigantic spider web...Grrrrr!

The wings looked really good, not faded or ragged. I don't use pesticides, nor does anyone in the community garden where I garden.
I have seen some preying mantis. I'm surprised they can catch butterflies.

ellen

Ellen,

I found three pairs of CS wings in my garden yesterday. Upon closer inspection, I found a praying mantis hiding out in the plant. He immediately went into my freezer to become part of an insect collection a college student is working on. I have tolerated the mantises until this one got too greedy. I don't know if they like Monarchs, but they sure do love BFs.

OKSandy

I found a praying mantid on my milkweed. I just squished his meaty lil behind and he was gone. There are so many spiders, braconids, tachnids, stink bugs, and spined soldier bugs in my yard this year. I can't tell you how many spider webs I've walked thru! And there I am waving my arms and pulling at my hair and clothes - I must look like some zombie from Night of the Living Dead out there! Geez! One thing can be said for our dry summers - we don't have as many predators in the garden!

I would agree - a predator, or even a bird that pulled off the wing. Dragonflies will also catch butterflies, too. I have had TONS of dragonflies hovering over my yard all summer, just like hawks and vultures, they circle around looking for a meal.

Susan

Thanks all for the replies. At least it is not a virus. I do have spiders and mantis. I'll have to get rid of them.

ellen

My guess would be predator too if the wings looked nice yet and there was no spraying around you. We've had a drought this year and I've had loads of predators, maybe because they can't find a juicy meal somewhere else because of those other insects' lack of water. So where do they come but good ole Cathy's butterfly garden, ripe for the pickens. I kill the spiders, but I wouldn't kill praying mantises. I only ever see one ever few years anyway. If I'd find it, I'd relocate it somewhere.
Cathy

I'm not going to kill the mantis. If I see it, and can catch it, I'm going to relocate it.

ellen

If you want to limit the number of mantids in your garden next year, look for their egg cases. They are very distinctive. I find mine on the sides of the house, on my red wood trellises, etc. At first I didn't know what these were, and I kept finding lots of them. Now I know, and I'm going to start trying to "limit" the number of mantids for the garden.

Praying mantids are cannabalistic when they hatch. Though you can get up to several hundred hatchlings from one case, usually only a few survive the feeding frenzy. So save at least one case that you can locate. They are considered beneficials - except when it comes to caterpillars and butterflies. I find them in either different stages of growth, or different species, I don't know for sure how many species there are.

Scroll down the page I linked to for you, and you can see a photo of the egg case. The ones I have are very hard shelled and I'll probably have to smash them with a hammer or something. They kind of remind me of a petrified caterpillar, and in color from tan to brown to reddish brown. These are the ones I find in my yard anyway.

Susan

Here is a link that might be useful: Praying Mantids Egg case

That's interesting, Susan! There is something that looks like the picture in your link of the egg case on a young cherry tree in front of my house - I'll keep an eye on it.
MissSherry

I can't believe anyone would think of killing mantids or even spiders! I love butterflies, and have literally hundreds of them in my gardens. However, I garden organically and it is simply the natural cycle of life for creatures to prey on other creatures in the garden. Things usually balance out if you let them. For example, I find when I have an invasion of aphids that the ladybugs soon appear to take care of them naturally. Relocate the mantids if you must, don't kill.