Climate change and monarchs
Monarch butterflies (Danaus plexippus) are being threatened by the early arrival of spring in Texas, according to an article from Yale Climate Connections. As climate change has made spring come earlier, monarchs have begun migrating before the milkweed plants in the rest of the country have grown enough to support the eggs and caterpillars.
Human interactions with the environment beyond climate change have also had a negative effect on milkweed. Pesticide and herbicide use has led to decreased milkweed populations across the U.S. Additionally, recent efforts to plant more milkweed have included planting tropical milkweed (Asclepias curassavica). Tropical milkweed is non-native to the U.S. and blooms much longer than native species. So, monarchs can be ‘tricked’ into staying at locations longer than they would naturally, slowing their migration to the point where they may not make it to warmer weather in time.
Tropical milkweed can also pose another threat. It can carry a parasite which leaves the butterflies severely weakened and therefore less likely to survive their migrations.
Climate change and milkweed
Climate change can also affect the relationship between monarchs and milkweeds’ toxicity. According to an article from Science Daily, a recent study by Bret Elderd, Matthew Faldyn, and Mark Hunter in Ecology, climate change is increasing the amount of toxins in tropical milkweed.
Monarchs have evolved to resist the toxins (called carndenolides) present in the leaves of all milkweed species. Living on the toxic plant and absorbing the toxins into their body protects monarchs from predators. However, monarchs are only resistant up to a specific level of cardenolides. Too much and it becomes toxic to them as well.
The study found that monarchs that had been feeding on tropical milkweed had better survival rates than those who fed on other species. Tropical milkweed has nearly the maximum amount of cardenolides that the caterpillars can survive, which protects them the most from predators. Unfortunately, when temperatures rise, milkweed produces more cardenolides.
The study found that tropical milkweed was more sensitive to these changes compared to native milkweeds like swamp milkweed (A. incarnata). With tropical milkweed’s already high amount of cardenolides, the increase in temperatures can push the amounts into levels toxic to the caterpillars, resulting in weaker adult butterflies.
Elder and Faldyn used small greenhouse structures in their experiment, but predicted that the temperature predictions for the area (Baton Rouge, Louisiana) within 40 years would reach these effects. Heat waves could push the plants to these levels even sooner for short amounts of time.
Some Wisconsin native species include common milkweed (A. syriaca), swamp milkweed (A. incarnata) and showy milkweed (A.speciosa). Common milkweed needs well-drained, open areas like prairies. Wetter gardens can try swamp milkweed if it is shady, or showy milkweed if it is moist soil but open and sunny.
Still, it is clear that just planting native milkweeds will not address all of the issues monarchs are facing. Climate change threatens the butterflies directly as well, as is clear from Chip Taylor’s studies reported in the Yale Climate Connections article. So, fighting climate change is absolutely necessary for the protection of the most famous of butterfly species.