Nearly a billion monarch butterflies from the eastern United States and Canada migrate to the mountains of Mexico each year, and this year’s migration is already underway. This has long been known as one of the longest insect migrations in the world, undertaken so that monarchs can tap rich food reserves unavailable in frozen New England. New evidence suggests another important benefit of the migration: getting rid of hitchhikers.
Some monarch butterflies carry unwelcome hitchhikers, a tiny protozoan parasite with a big name: Ophryocystis elektroscirrha. The parasite is transmitted from female monarchs to their offspring when the butterflies scatter dormant parasite spores on milkweed plants while laying eggs. The caterpillars eat the spores, which germinate in their gut. The parasites penetrate the gut wall and migrate to subcutaneous tissues, where they undergo two phases of vegetative reproduction. The parasite then initiates sexual reproduction when the monarch caterpillar forms a chrysalis to metamorphose into an adult butterfly. An infected chrysalis has dark spots or blotches on it that can be seen about three days before the butterfly emerges. Infected butterflies emerge covered with microscopic spores on their eyes, antennae, wing veins, and body scales.
Once a butterfly is infected, it can’t be cured. Monarchs that are severely infected will often have difficulty emerging from the chrysalis and can be too weak to fully expand their wings. Heavily infected individuals lose weight at a faster rate than healthy ones, especially if they do not have unlimited access to water or nectar. They are often smaller than healthy butterflies and have a shortened adult lifespan. But many parasitized monarchs show no obvious signs of deformity and can be nearly impossible to distinguish from healthy butterflies without looking directly for parasite spores under a microscope.
Heavily infected monarchs account for less than 8 percent of the eastern population. The western population migrates a shorter distance, to coastal California, and has about a 30 percent infection rate. A non-migratory population in southern Florida has a whopping 85 percent heavy infection rate. Monarchs on many islands in the Caribbean are thought to be nonmigratory and would be expected to have infection rates similar to those found in south Florida.
I recently examined nonmigratory monarchs in the Dominican Republic mountains with my colleagues at the Vermont Institute of Natural Science (VINS). Captured monarchs were gently removed from the net while another set of hands dabbed a small piece of clear tape lightly on the lower abdomen to collect a few scales before releasing the monarch. The tape was mounted on a glass slide and examined under a microscope.
Contrary to our expectations, only 20 percent of the Dominican Republic monarchs were heavily infected. Perhaps this monarch population migrates seasonally up and down the mountains, reducing the parasites’ prevalence.
Sonia Altizer and Catherine Bradley at Emory University recently discovered why long distance monarchs have fewer hitchhikers. The two biologists built a treadmill for butterflies – a flightmill. They used a lightweight carbon rod about a yard long and a quarter-inch in diameter. The rod was mounted on a stainless steel pivot to provide nearly frictionless movement. Monarchs raised in the laboratory specifically for the study were attached to one end of the rod using ultra-light steel wires. Counter weights were placed at the other end of the rod. Tethered butterflies were able to fly continuous circles. An infrared light beam recorded the time elapsed during each flight lap for infected and healthy monarchs.
The infected monarchs looked the same, weighed the same, and had the same survival rate to adulthood, but the flightmill showed their weaker condition. Infected monarchs flew more slowly, tired faster, and had to expend more energy flying than healthy butterflies. These results, published in the March issue of Ecology Letters, suggest that the long and difficult migration kills the heavily infected butterflies. Only lightly infected or uninfected monarchs arrive in Mexico to continue the species.
“We know that several species of birds, insects, and other animals undergo two-way migrations of several thousand miles or longer,” said Altizer in a recent Emory press release. “These journeys can be thought of as animals essentially running a marathon every fall and spring. So if animals are infected with parasites, this would be like a distance runner trying to run a marathon with the flu. In this case, parasitized animals will drop out of the race, and across the whole population, prevalence of disease will decline.”
“The results of our study add one more reason to protect monarch migration east of the Rockies,” added Altizer. “If this migration collapses due to climate warming, habitat loss, pesticide use or other reasons, we probably won’t lose monarchs as a species, but we’d be left with remnant, non-migratory populations that are heavily infected with parasites.”