The monarch butterfly is the Bambi of the insect world in North America, a favorite among children and adults because of its colorful wings and its migratory pattern, which takes it as far north as Canada and south into Mexico and California where it overwinters.
For decades, monarch populations have fluctuated, as this graph from the Word Wildlife Fund shows. Reasons for the fluctuation include weather patterns — extremely cold and wet winters are especially harmful — as well as deforestation in the areas where they gather to overwinter, and loss of habitat in the U.S.
Monarch caterpillars feed on one plant only — the milkweed. However, there’s been noticeably less milkweed in corn and soybean fields recently as farmers have used modern techniques to keep these and other weeds out of their fields. In order to counteract this trend, many conservationist groups have encouraged people to grow milkweed in their backyards and gardens, and they have even distributed milkweed seeds for this purpose.
Unfortunately, these well-intentioned efforts may have backfired, according to a new article by scientists from the University of Georgia published in the Proceedings of the Royal Society B. There are more than 100 different species of milkweed, and they are not all created equally. The species that was distributed most by conservationists — tropical milkweed (Asclepias curassavica) — is one that does not deteriorate in cold weather. Instead, it continues to thrive, producing new leaves and flowers in the fall and winter, which is causing monarchs to stay where the food is instead of continuing their migration south for the winter.
“Tropical milkweed provides monarch larval food throughout the year, and reports of monarchs breeding during the winter — rather than migrating or overwintering — have become common in the southern U.S.,” the authors wrote. “These behaviors are almost exclusively restricted to sites where tropical milkweed is present.”
To make matters worse, the researchers found that monarchs that did not migrate were more likely to be infected by the protozoan Ophryocystis elektroscirrha (OE), which causes the butterflies to suffer from wing deformities, smaller body size, reduced flight performance, and shorter adult lifespans.
“Shifts towards year-round breeding on tropical milkweed, resulting in high rates of OE infection, could pose an additional emerging threat to the long-term viability of migratory monarchs,” they wrote. “Transitioning from migratory to non-migratory behaviors coupled with a shift to year-round breeding on introduced host plants dramatically increases the prevalence of a debilitating parasite for North American monarchs.”
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