There are also a couple more reasons that this could happen. I posted some information from the FAQ section.

Here is a link that might be useful: Why does a Monarch caterpillar or chrysalis turn black?

This is one of the hardest questions to answer as there are so many things that can go wrong. There are lots of diseases that affect butterflies, bacterial, fungal, viral, and other. Without actually taking a sample and examining it there is no definitive answer, with the exception of an exiting parasitoid. Bacterial infections are often times secondary to other infections (just like in humans). If the pupa was damaged while it hardened, it can leave it susceptible to bacterial infections. Ants will eat tasty chrysalises. I have seen hackberry chrysalises cleaned out by ants in the matter of a couple hours but other chrysalises left alone. I suspect they try monarch chrysalises (and others) but the caterpillars sequestering of distasteful chemicals protects them. However, if an ant manages to pierce the pupal shell, it could let in bacteria or fungi.

Also, because the pupal shell on monarchs are so thin in comparison to others, I think we see when things go wrong sooner. We had problems with chaclid wasps this year. I the monarch pupae we saw them turn brown and I assumed it was a bacterial infection till I found one with the tell tale exit hole. In the swallowtails, we never saw anything till the wasps came out.

If you can, keep watching it. What continues to happen to it gives clues. If it starts weeping, it is probably a bacterial or viral infection. If it just dries out, probably OE or some other microsporidia. If you see an exit hole, a parasitoid got it.

Keep up the great citizen scientist work!
-Elisabeth

Ants would not have a problem with the toxins. Many other insects eat Monarchs, too. I've even seen a slug eating a Monarch pupae. Someone told me they knew of a ladybug that was eating on a Monarch pupae. Those Japanese Ladybugs are big predators.

Just curious, why do you think ants would not have a problem with the toxins?
-Elisabeth

Wow. A really sad but nicely documented demise Bob. The pics are wonderful and valuable. Thanks for sharing. I have a 4 ft. high tent full of Monarch cats right now, and I am dreading the most certain demise of some of them.

Other invertebrates do not have a problem with the toxins. Preying Mantis eat them up. Spiders, assassin bugs,... Ants eat the honeydew from aphids who suck the juices out of milkweeds. I do not think ants would have a problem at all with the toxins.

Runmede;
I will disagree scientifically with you :). There are lots of compounds that invertebrates sequester with the hope of discouraging other invertebrates. In fact, many of the compounds Lep larvae sequester are used to deter and fight parasitoids as well as the other wasps. The abstract below is for Chrysomela beetles and outlines how they sequester salicin, the compound found in willows that viceroys sequester as well. The beetles sequester the compound and it is found in the eggs to prevent being eaten by ants. Saliin is toxic to ants. So, I suspect that many other compounds may be used by Leps to prevent ants as well. Also, some monarchs will have little to no appreciable amounts of sequestered compounds depending on the species of Asclepias they feed on. I have seen all kinds of things eat different stages of butterflies. However, there are times that one will be left alone and I think there are reasons.

Cheers,
Elisabeth

ABSTRACT. Eggs and neonate larvae of chrysomelid beetles (sub-tribes Chrysomelina and Phyllodectina) were investigated for the presence of defensive substances.

The two isoxazolinone glucosides (compounds 1 and 2), characteristic of the adult defence secretion, were detected in the eggs of all studied species. Compound 2, containing a nitropropionate, is always present in concentrations (above 10-2 M), which are highly deterrent to the ant Myrmica rubra. This compound is not at all or only slightly toxic to ants at 10-2 M. Compound 1, devoid of nitropropionate, is a minor constituent, and is neither deterrent nor toxic to ants.

The five Chrysomela species studied and Phratora vitellinae also sequester salicin in their eggs in amounts highly deterrent and toxic to ants. A single Chrysomela egg often contains enough salicin to kill an ant. While the isoxazolinones are discarded with the egg shells, salicin is used by neonate larvae as a precursor for the production of salicylaldehyde in the thoracic defence glands, already functional at hatching. No salicin could be detected in the eggs of those species whose larvae produce cyclopentanoid monoterpenes, even if they feed on Salicaceae. No larva of any species seems to be able to produce detectable amounts of monoterpenes at birth. A very early defence, possible only in those species using salicin as the precursor for their defensive secretion, could be highly advantageous in protecting the clustered larvae during the long process of hatching and in avoiding cannibalism between siblings.

Only trace amounts of oleic acid were found in the eggs of Gastrophysa viridula, in contrast to previous reports on its presence in large quantities in the American G. cyanea.

"Spiders, mites, ambush bugs, ants, lacewings, wasps and stinkbugs all eat monarch eggs or larvae."

Here is a link that might be useful: Monarch Facts

I'm referring to Monarchs when I said that other invertebrates eat them.

Mona