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List: Carolina-Leps
Date: Wed, 30 Sep 2020 14:22:19 -0400 (EDT)
From: \Lori M. Carlson\ (via carolinaleps Mailing List) <carolinaleps...>
Subject: Re: Monarch Caterpillars and Milkweed Species

My experience of raising Monarch caterpillars is similar, if not identical, to
Cyndy's experience that she shared with us in her reply. This year I have
taken in at least 100 Monarch caterpillars from 4 different sources as part of
our Caterpillar Support program at Backyard Butterflies. Whatever their
original species of milkweed they easily transitioned to eating whatever I
provided them which was a mixture of A. syriaca, A. tuberosa, A. curassavica,
and Gomphocarpus physocarpus. I've never had a caterpillar refuse to eat what
has been provided.

I've also grown Oxypetalum caeruleum, the blue flower milkweed vine, that
Cyndy mentions. The females would taste-test the plant but not lay eggs on it.

What stands out for me about the use of the words noxious or toxic levels of
cardiac glycoside in milkweeds sounds like it is being used in relation to
humans, livestock, pets, or birds (and other predators) who would eat the
caterpillars, not Monarch caterpillars themselves. If those words are being
used in relation to toxicity to Monarch caterpillars, that sounds like to me a
non-sequitur because Monarch caterpillars are able to metabolize the toxins as
well as sequester them as a defense mechanism. Switching between low to high
or high to low concentrations of toxins is a moot point.

If there is concern about the adult butterfly is not having enough toxin in
their diet to deter predation, it is interesting to note what Loretta provided
about those Monarchs raised on A. syriaca and A. tuberosa producing relatively
non-poisonous adults. These are the favored "go to" natives that many want to
use to support Monarchs. If I'm reading the results of this study correctly
(link provided below) it appears that A. curassavica (the notorious maligned
Tropical Milkweed) results in high levels of sequestered toxins in the
caterpillar which I assume carries over into the adult.

Reference: Milkweed butterfly resistance to plant toxins is linked to
sequestration, not coping with a toxic diet:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4650158/

~ LMCarlson