The monarch butterfly population has been in serious decline for years now, something many gardeners know. To compensate for habitat loss, gardeners have been encouraged to plant more milkweeds, the insect's larval host plant. But this has led to unforeseen negative consequences, especially in warm winter regions of the U.S.
Native milkweeds can be hard-to-impossible to find, so the tropical, showy "scarlet" milkweed has become ubiquitous in garden centers across the country. This plant, Asclepias curassavica, is native to the American tropics and has spread to pantropical regions worldwide. It has become invasive in some areas, and threatens to become a pest in South Florida.
Many monarch butterflies harbor a protozoan, Ophryocystis elektroscirrha (OE), that can weaken the adult, prevent the pupa from emerging from the chrysalis, or deform the wings. Monarchs visiting milkweeds deposit spores when they visit milkweeds. Normally, migration culls weakened individuals, and the OE spores die when the plants die back in winter. The plants grow back in spring and summer with fresh, uninfected leaves. But in areas with warm winters tropical milkweed grows all year, thus maintaining high levels of OE spores. Areas of Georgia, coastal Carolinas, Florida and the Gulf Coast have become hotspots of infection.
|Monarch on Scarlet Milkweed|
Apart from the immediate threat to individual monarchs, year-round milkweed is also, probably more ominously, threatening the migration itself. The presence of the milkweed affects the butterfly's hormonal balance, and works as a trigger to make it reproduce. So monarchs that find themselves in areas with warm winters don't migrate, and a year-round population gets established. With increasing warming trends this area of permanent, sickly individuals will only increase.
Migration plays a critical role in maintaining a robust gene pool, for it culls badly infected individuals, which simply don't survive the trip. But migration may play other vital roles as well, in ways we haven't discovered.
Some organizations like the Xerces Society and the Florida Native Plant Society actively campaign against the use of tropical milkweeds. Some people, though, citing the drastic declines in the monarch population, feel that keeping the numbers up is of primary importance.
Weaning gardeners away from tropical milkweed is going to be a monumental project, especially since it was promoted so aggressively as a solution to monarch population decline.
Monarch on Asclepias curassavica
In and of itself, I'm not particularly heartbroken over the loss of scarlet milkweed in our yard. Due to neglect, they've sort of died out this spring anyway. It is a water hog, and the stems quickly get leggy and woody. It also is a magnet for aphids and spider mites, which would make any self-respecting female monarch look for greener pastures.
|Asclepias incarnata, "Swamp Milkweed," a Native|