By Sarah Kempke
"Mrs. Molenaar, over here!" Laura Molenaar and her group of young monarch larva monitors spread out over a tangled berm beside the Glacial Lakes State Trail in New London. They're combing the milkweed plants and poking through damp grass and goldenrod in search of monarch butterfly caterpillars and eggs. Molenaar, a teacher at New London/Spicer Middle School, peers at an object curled inside a milkweed leaf.
Monarch Butterfly Biology and Conservation Web Sites
"What is it?" asks Libby, the inquisitive 10-year-old who is holding the leaf open to show a cocoonlike mass inside.
"You tell me what you think it is," Molenaar says. She gives Libby a hand lens and watches as the girl leans in for a closer look.
Libby focuses on the leaf. "There are little spiders coming out!" she cries, staring intently through the magnifier.
At 7:30 a.m. on this Wednesday in late August, nine students have gathered at Molenaar's home, as they have every week since early June. They're eager to collect their weekly monarch density data for the Monarch Larva Monitoring Project, a citizen-science initiative run by Karen Oberhauser, monarch butterfly biologist at the University of Minnesota. The students vary in age, from 9-year-olds Evan and Christopher, to 16-year-old Emily. All share a passion for monarchs and curiosity about the ecology of the Minnesota milkweed ecosystem.
Scientists have known since 1975 that the monarch butterflies (Danaus plexippus) from the eastern half of the United States and southeastern Canadian provinces retreat to forested mountains in central Mexico for winter. With the help of citizen scientists like Molenaar's crew, they are following the migratory routes and learning how the abundance of these common butterflies depends on the conditions in both their winter and summer habitats.
Down the road from the Glacial Lakes trail, at Sibley State Park, naturalist Dick Clayton and volunteer Mike Meyer monitor monarchs. The park offers seasonal programs on monarch biology for campers. Monarch larvae are displayed in a terrarium, so visitors can observe the monarch life cycle.
Over the course of two weeks, visitors can see the tiny eggs hatch and the larvae grow. Each larva sheds skin four times to move through five larval instars before shedding the final caterpillar skin. The final shed reveals a brilliant green pupa, which hangs from the side or lid of the terrarium. Within the jewel-like pupa, the insect spends up to two weeks undergoing metamorphosis, transforming from an earth-bound creature into a winged butterfly.
Outside the visitors center, Meyer and Clayton kneel in a patch of prairie restoration area, searching for signs of monarchs. They often stop to answer questions from curious visitors.
"Monarchs really draw people in," says Meyer. "It's exciting to watch their eyes light up when they see a monarch caterpillar." A milkweed patch can be as diverse as an ocean tide pool. Stands of tall, wide-leaved milkweed plants protrude from grassy fields, like islands of deep-green vegetation abuzz with insects. Large purple blooms attract skippers, monarchs, and other nectaring insects. Brilliant red milkweed bugs and milkweed beetles carry out entire life cycles -- egg to adult -- on a single plant.
Tiny green, black, and yellow aphids also thrive on milkweed, sucking juices from the leaves and stems. Ants eat honeydew secreted by the aphids and protect the aphids from ladybug larvae and other predators. Of all the inhabitants of the milkweed patch, the most charismatic -- Clayton, Meyer, Molenaar, and the student monitors all agree -- is the monarch butterfly.
Monarch larvae feed exclusively on the dozens of milkweed plant species that grow in North America. Milkweed contains sticky sap and chemical deterrents called cardenolides that discourage many herbivores from eating the plant. Monarchs have evolved the ability to process the cardenolides and store them in their own tissues so that they also become unpalatable to predators.
Common milkweed (Asclepias syriaca) grows prodigiously in Minnesota, thriving in disturbed lands, including farms, gardens, parks, roadsides, and railroad rights of way. Monarchs migrate to Minnesota in summer to breed, taking full advantage of this abundant larval food source. They return to Mexico in late summer to avoid harsh northern winters.
Beginning in mid-March, monarchs fly north from Mexico to lay their eggs on newly sprouted spring milkweed in the southern United States, from Texas to Florida and Kansas to Virginia. A month later, this spring generation emerges as adults. They fly north, seeking newly sprouted milkweed plants. Members of this second generation reach Minnesota and lay eggs here in late May and early June. Their eggs develop into a third generation of monarchs that begin to emerge in early July. They lay a fourth generation of eggs in July and August.
Most other Minnesota butterflies can survive the harsh winters. Some, like the tiger swallowtail, winter as a pupa. Others, like the viceroy, spend the winter as a larva tucked into a shriveled plant leaf. Still others winter as eggs or adults. However, no stage of monarchs can survive long periods of freezing temperatures.
When the days begin to cool and shorten in late August and early September, the season's last adult monarchs emerge from their emerald green and gold chrysalides. Cued by the shortening day length, decreasing temperatures, and senescing milkweed, they emerge with undeveloped reproductive organs, in a state called diapause. Instead of mating and laying eggs as earlier generations did, these monarchs save their energy to migrate nearly 2,000 miles to high-altitude forests of central Mexico. There they will cluster together by the millions on oyamel fir trees, waiting for spring to signal their return to North America when milkweeds sprout once again.
Oberhauser and graduate student Michelle Prysby developed the Monarch Larva Monitoring Project in 1996. The project relies on volunteers to keep fingers on the pulse of this magnificent migrating population.
"Michelle was interested in the movement of monarchs as they expanded through their summer breeding range," says Oberhauser. "But she was also interested in education and getting the public excited about monarch conservation."
Oberhauser and Prysby recruited volunteers throughout the United States, asking nature centers, schools, and individuals to record the presence of monarchs in a local milkweed patch throughout the summer. During the past decade, volunteers have monitored more than 600 sites throughout the nation, including 100 in Minnesota.
Today, volunteers from Texas to Ontario use standardized methods to collect data. They monitor their patch each week, looking carefully at each milkweed plant to find and record every last monarch. The time required varies with the size of site. For large sites, volunteers may take a random sample of the milkweed plants and inspect each sampled plant to determine average monarch densities.
Volunteers search for white, pinhead-sized eggs on the undersides of leaves and look for larvae munching on foliage. Trained to discern between the five larval instars, volunteers record the presence of each. They also enter daily weather and milkweed conditions on the project's online database. Scientists can then study this information to determine what makes monarch populations thrive or perish.
By summer's end, data collected by volunteers provide a good picture of the season's monarch distribution and abundance. Over the past decade, they have documented shifts in monarch numbers and threats to the population. For instance, after millions of monarchs perished during severe storms in Mexico in January 2002, volunteers documented one of the lowest summer populations ever. Volunteers have since observed a return to average numbers, including a healthy crop of monarchs in Minnesota last summer.
Project data have led to other discoveries. For example, volunteers documented egg laying in the southern United States during fall migration. This discovery led to speculation by Oberhauser and others that a changing climate and propagated garden milkweed could be changing the migratory pattern for some monarchs, causing them to break diapause.
"It is very likely that monarchs that break diapause in the fall have essentially given up the opportunity to migrate," says Oberhauser. While their offspring might still head south for the ancestral wintering grounds, their late departure from the United States might threaten their survival.
Rebecca Batalden, a doctoral student in Oberhauser's lab, is using project data to evaluate how climate change could affect the migrating population of monarch butterflies. She compares monarch presence with weather conditions at monitoring sites to determine the temperature and precipitation levels -- the climate niche -- where monarchs currently thrive.
A computer model predicts where these monarch climate niches will occur as the climate changes. Results of this study suggest that under current warming trends, late summer in central Minnesota could be too hot for monarchs to survive here by 2050. They could still live in northern Minnesota.
Many volunteers have witnessed the loss of their particular monitoring sites as fields give way to parking lots and other development. Monarchs have also lost habitat with the advent of herbicide-tolerant crops, which farmers may spray with herbicides that kill milkweed plants scattered in the field.
Luckily for monarchs, their migratory behavior and dependence on the milkweed -- a prolific plant -- just might save them from the threats of habitat loss and climate change. Milkweed does a good job of spreading into new areas. And monarchs are strong flyers. They can likely fly long distances to reach a suitable climate, even if that climate recedes farther north.
After monitoring their second site for the day at Stoney Ridge Farm, Molenaar's young monitors take a moment to reflect on their experience as citizen scientists.
"It is fun to do work for an actual program," says Elliot, "and to know that it will be used for real science."
"We need to protect monarch habitat in Mexico, so that we can keep monarchs in Minnesota," Emily says.
"And cut down on cars and go to electric cars," chimes in Eric, referring to carbon dioxide emissions warming the climate.
Though the citizen scientists did not find any monarch eggs or larvae here on this day, they know that documenting the absence of monarchs is also important because it gives a picture of the conditions when monarchs are not present. They also know that, as the milkweeds begin to yellow and fade, the monarchs have started their great southward journey -- continuing this magnificent migratory cycle.