You say it is a new plant. Was it purchased from a nursery? Some nurseries spray their milkweed stock, often with a systemic, to prevent it from being eaten by caterpillars. Then they sell them as plants for Monarch butterflies...yeah, I know, but some retail outlets do that. When I purchased my first plants, the nursery had a sign saying they were all pesticide-free. I believed them--I bought the one plant that included some fat, happy caterpillars munching away. :)

If the milkweed has been sprayed, it may take a while for the systemic to wear off. How long would depend upon what was used, and when it was applied. If that is what is happening, the feeding caterpillars will continue to die until the systemic is no longer lethal.

Other readers may have some other ideas as to the cause, should this not be applicable to this situation.

Predators is another very strong possibility. Or some may have been mature enough to wander off and pupate. Those systemic pesticides are a horrible thing. People are purchasing plants to encourage pollinators and end up killing them instead. I think plants treated with pesticides should be labeled as such.

Martha

One was a gift, the other was bought at a good nursery. If a systemic pesticide has been used on it, would there be aphids? She says there are aphids on it. How long would it take to wear off? I bought a milkweed at Lowes....I'm suspicious. The caterpillars stay away from it, and there are no aphids on it. The caterpillars go to my other plants. By predators...you mean birds? rodents? lizards?

No, the systemic would take out the aphids, too. I don't know about the predators--my Monarch caterpillars don't tend to get taken. When BT (Bacillus Thuringiensis ) is used for caterpillar control, it doesn't affect most insects. However, BT's effectiveness declines rather quickly, lasting only 12-48 hours or so after application. So I doubt if that could be the cause.

This post was edited by Gyr_Falcon on Fri, Jun 27, 14 at 1:46

If they were big, about 2 inches, they may have wandered off to pupate somewhere.

Sometimes when they molt they go rest somewhere and then molt. That can take a day too.

Predators like wasps, mantises, etc will eat caterpillars.