Extent and impacts of winter breeding in the North American monarch butterfly.

Since the 1960s, scientists have observed the North American monarch butterfly (Danaus plexippus) continuing reproductive activities past the fall migration and into the winter months when the climate is mild. Recent work suggests that small populations of winter breeding monarchs are present in western and southeastern USA, as well as northwestern Mexico, with new winter breeding populations forming in areas where non-native milkweeds are planted. The year-round presence of milkweed plants and temperatures suitable for immature monarch development are vital factors allowing for winter breeding.

Host plant specificity of the monarch butterfly Danaus plexippus: A systematic review and meta-analysis.

The preference-performance hypothesis explains host specificity in phytophagous insects, positing that host plants chosen by adults confer the greatest larval fitness. However, adults sometimes oviposit on plants supporting low larval success because the components of host specificity (adult preference, plant palatability, and larval survival) are non-binary and not necessarily correlated. Palatability (willingness to eat) is governed by chemical cues and physical barriers such as trichomes, while survival (ability to complete development) depends upon nutrition and toxicity.