Answer: Many gardeners will keep their poinsettias in containers and set them outside for the summer, then bring them indoors in the fall so they can keep them from year to year. In a brief search I found that poinsettia flowers are a nectar source for butterflies -- including monarchs -- during the winter months in warm climates such as Florida or New Zealand. I did not find any reference indicating that the larval stage feeds on the plants, however. I hope this answers your question.
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