Save the monarch butterflies

It is hard to imagine that the most commonly encountered butterfly species of my childhood (one that in the 1990s may have numbered more than a billion individuals) may soon be considered quite rare. Monarch butterflies (Danaus plexippus) make one of the most extraordinary North American migrations, taking four life cycles to complete the journey to and from their wintering habitats in Mexico, California and Florida. Due to a combination of winter habitat loss, a decline in milkweed plants needed as larval hosts and an invasive plant that is toxic to monarch caterpillars, the Xerces Society for invertebrate conservation now reports that perhaps just 33 million monarchs remain, a staggering 90 percent decline across North America in the last 20 years.Except for butterflies that follow the coastline to southern Florida and perhaps farther down, into the Caribbean, the rest of the monarch butterflies east of the Rockies migrate to a single forest type in the mountains above Mexico City. Oyamel fir forests are Mexico’s most endangered forest type, occurring at high elevation on just 12 mountaintops. Global climate change is now affecting this forest type, which evolved in cooler, wetter conditions. Monarchs overwinter here because the wet, cool weather followed by evaporation during radiational cooling at night enables them to withstand freezing conditions. A warmer and drier climate will leave them vulnerable because they may not be cool enough to slow their metabolisms during their dormant period.The loss of native milkweed in North America, due to conversion of the old fields and wet meadows where milkweed occurs to development and agricultural use,deprived monarch larvae of a vital food source. Monarch caterpillars feed on milkweed and incorporate a toxin from these plants that is similar to digitalis and poisonous to most vertebrates. Two invasive vines called black swallow-wort (Cynanchum louiseae syn.Vincetoxicum nigrum) and pale swallow-wort (Cynanchum rossicum syn. Vincetoxicum rossicum) are not only significant threats to native plants and extremely difficult to eradicate, they are also toxic to monarch caterpillars. Butterflies sometimes oviposit on these plants and their broods fail when they ingest the toxic leaves. Pale swallow-wort has a particular affinity for calcareous soils and would thrive if it became established in the sweet soils of our marble valleys. The Invasive Plant Atlas of New England has two records of black swallow-wort from Lakeville and one of pale swallowort from near the Barkhampsted reservoir. Early detection is vital for the control of these two invasive speciesMonarch conservation requires action on all of these threats across the life cycle of this species. In our region, the most significant things we can do is promote and protect milkweed stands as larval hosts, and prevent the introduction and spread of invasive swallow-wort species. It would be a great tragedy if we were to witness the loss of another species that was once as common as the now-extinct passenger pigeon without doing what is in our power to prevent it.Tim Abbott is program director of Housatonic Valley Association’s Litchfield Hills Greenprint. His blog is at greensleeves.typepad.com.