Immature monarch temperature tolerance: Measuring cold hardiness through supercooling points and lower lethal temperatures

Background/Question/Methods

Eastern North American monarchs are well known for their long-distance annual migration from the eastern US and southern Canada to high elevation overwintering sites in Mexico, a journey that can cover over 4000 km. However, climate change models indicate these sites may not be suitable for monarchs within as little as 40 years. Citizen science reports have shown some monarchs are now both present and breeding in Texas and other Gulf Coast states throughout the winter. For the most part, they are using non-native tropical milkweed (Asclepias curassavica). These areas experience freezing temperatures and thus immature monarchs may be exposed to lethally cold temperatures. To test immature monarch cold tolerance, I measured both the supercooling point (SCP) and lower lethal temperature (LT₅₀) for different developmental stages.

Results/Conclusions

Median SCPs for immature monarchs range from -26.1° to -9.6° C, with eggs having the coldest SCP and third instar larvae having the warmest SCP. Larvae appear to be freeze intolerant, with the LT₅₀s at temperatures near each stage’s respective median SCP. However, eggs seem to be chill-intolerant, with an LT₅₀ occurring at a temperature much higher than their median SCP. While most monarchs tested were summer monarchs lab-reared in southern US winter conditions, the results were further verified by testing wild immature monarchs raised on tropical milkweed in Texas. These findings can help inform future modeling and conservation efforts for monarchs throughout their life cycle, in particular guiding host plant restoration and winter habitat management in the southern US.