My guess is that birds are eating the caterpillars and your plants probably have some aphids, which secrete a sweet substance that attracts ants.

Birds avoid milkweed caterpillars. There are insects that will feed on them, though. Including some ant species. Ladybugs will eat small caterpillars and eggs, for example.

Actually, it depends on what milkweed it is feeding on because some milkweed are less toxic than others. Common milkweed isn't that toxic. Birds do eat the caterpillars, it is a learned behavior. First, the birds have to eat a caterpillar that is toxic enough and then they learn not to eat them. The same thing goes for adults.

List of Predators:
Birds
Reptiles
Amphibians
Insects (not problem with the toxins....assassin bugs, preying mantis, wasp, ladybugs, etc.
This list is in no way complete.

Here is a link that might be useful: Monarch Watch Predation

I saw a hummingbird follow right behind a female monarch - eating the eggs right after that monarch laid them!!!

Bet that hummer can tell a female from a male, no problem.

Ants most definately eat butterfly eggs and 1st instar caterpillars (See below a section of an abstract). In fact, many plants, such as Senna, have what are called "extra floral nectaries". These are areas (usually little protruberances) that secrete nectar to attract ants. The ants in turn protect the plant from herbivores.

There are lots of other things that eat eggs as well, ladybugs, tree crickets, lacewing nymphs.

In addition, small wasps called chaclids, can lay their eggs inside butterfly eggs. This is why we often collect eggs and bring them to a safe place to raise.

By the way, I have see a lot of chrysalises eaten by ants as well. They leave a nice neat hole, a little smaller than a pencil eraser.

-Elisabeth

Abstract
I investigated variation in the effect of ants at extrafloral nectaries on the plant reproductive success among three populations of Cassia fasciculata (Leguminosae) in northern Florida. Results show that the possession of extrafloral nectaries can lead to increased reproduction. Density of ants and removal of experimentally placed Drosophila larvae were far higher on C. fasciculata than on nonnectariferous background plants. Ants preyed on eggs and small larvae of lepidopteran herbivores. C. fasciculata plants from which ants were excluded had more larvae of the major seed pod predator Phoebis sennae (Pieridae; Lepidoptera), suffered higher pod predation (at one site), and had lower reproductive output (at two sites) than did control plants to which ants had access. Effects of extrafloral nectaries on reproduction of C. fasciculata varied among the three populations, due apparently to differences in density of ants and potential herbivores.

That's very interesting Elisabeth! I find it incredibly wild what goes on out there, to stay alive. Amazing!

For years I've been letting milkweed grow on my tiny garden out front. Love the flowers & scent. For the last 2-3 years we have been getting big caterpillars but no chrylis. Is their anything I cando to safeguard them?

Reddragonlady you could collect the eggs or caterpillars and bring them inside and raise them. I prefer to collect eggs, but will also bring in a few caterpillars of various sizes, and raise them in containers. If left outside most of the larvae will probably be preyed upon, whereas inside most end up pupating and eclosing successfully.

I have the same problem. Eggs disappear and no caterpillars more that 3mm long. Have come to the conclusion it is ants as they constantly patrol my plants. I think they are Argentinian sugar ants. They seem to be taking over from all other ants here in the Coromandel, NZ.