Thanks for your question. The development of red veins in milkweeds (Asclepias syriaca) is a normal event. This redness is due to the presence of a purple pigment in them that occurs with normal aging and in response to environmental signals such as shorter day lengths and cooling temps. There is absolutely no evidence that once these veins become red, the plant repels monarchs and is lethal for them.

If you are interested in a bit more information, read on.

During their long evolutionary history, milkweeds started to produce chemical substances known as cardenolides. These compounds inhibit the metabolism of energy sources in many species. If consumed in sufficient amounts, they can lead to cardiac arrest in humans. It is hypothesized that milkweeds evolved this as a defense mechanism against being eaten by herbivores.

But, during their evolutionary history monarch butterflies developed systems to counteract the adverse effects of cardenolides. Rather than being killed, monarchs were able to safely store these compounds within themselves. In fact, the concentration of cardenolides within monarchs is far greater than within the milkweed plant. If something ate a monarch, it would be an extremely distasteful experience for that predator.

So, the same compound that is a defense mechanism in milkweeds is also a defense mechanism in monarchs. Rather remarkable when you think about it!

See the following for further information about these and related points.

https://extension.umn.edu/garden-and-home/yard-and-garden/gardening-in-minnesota/common-milkweed

https://www.sbnature.org/collections-research/invertebrates/entomology/butterflyfaq

https://www.amnh.org/explore/news-blogs/aposematism-warning-signals

Thanks for the interesting question and for using our forum.