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When can temperatures be dangerously cold?
"January and early February is the coldest time of the season. This is a really critical time for the monarchs," says Dr. Lincoln Brower. "We know temperatures below -8°C will kill about 50% of the monarchs. However, if the butterflies are also WET, it will kill close to 80-90% of the monarchs."

Graphic © Dr. Lincoln P. Brower, Sweet Briar College