Signs of Spring 12: Monarch Update!

Photo by D. Sillman

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Monarch butterflies had a good year in eastern North America in 2018-2019.

As I wrote in Signs of Summer 8 (July 25, 2018) the meadows at Harrison Hills Park were loaded with butterfly weed, milkweed and also a great diversity of other flowering plants. In July, Deborah and I found several large (possibly 5th instar) monarch caterpillars on the milkweed plants. Many of our friends throughout Western Pennsylvania also saw clusters of monarch eggs on milkweed and large numbers of adult butterflies brightening up their yards and gardens.

We speculated that the warm weather in spring 2018 in Texas might have given the migrating monarchs a boost (March 2018 in Texas was 5.3 degrees F warmer than average!). It might even have kept them from flying into the unseasonably cold, spring weather in the eastern U.S. The migrating monarchs staying in Texas for longer than usual responded with a population boom! Although limited by the available milkweed, the Texas monarchs multiplied and then, as the weather in April began to moderate, surged northward out across the Eastern  and Midwestern United States reproducing all along the way in the sequentially blooming patches of milkweed.

Female Monarch (photo by K.D.Harrelson (Wikimedia Commons))

In the late summer/early fall these monarchs reached the northern edge of their summer range and turned their migration compasses toward the south and west to head for their overwintering sites in the  coniferous forests in the mountains of the Mexican states of Michoacán and Mexico. These overwintering monarchs live 8 or 9 months (compared to 2 to 5 week life span of the “summer” monarchs) and are the individuals that will push back north into Texas next February and March where they will mate and lay eggs and start the migration cycle all over again!

One of the most accurate ways to assess the monarch butterfly population is to count them when they are in their overwintering forests or to measure the area of the forests that they fill: during the 2018/2019 season 6.05 hectares of forest were filled with overwintering monarchs! This represents a 144% increase over last year’s occupied forest area and is the largest forest area occupied by monarchs since 2006/2007! The eastern monarchs, then, finally had a good year!

Unfortunately, it didn’t last.

Photo by T. Hall, Flickr

This year’s census of the monarch forests was just released. The 2019-2020 overwintering cohort occupied only 2.83 hectares, a 53% drop from last year. Since 2010, the average occupied forest area has been around 3 hectares, so this latest data seems to be a return to the “new normal.” Scientists, though, have calculated that the monarch population needs an overwintering forest area of at least 6 hectares each year to avoid spiraling into extinction by the end of this century.

Why are these eastern monarchs declining so precipitously?  Climate change is altering the distribution and blooming times of the plants that the migrating monarchs rely on for food (nectar) and for egg laying. Milkweed, especially, has been destroyed at a rapid rate by habitat loss due to development and to agricultural activities (over 66 million hectares of milkweed has been lost in the United States alone). The eastern North American monarchs are in great trouble!

Western monarch. USFWS, Public Domain

There is also a western population of monarchs that are separated from the eastern monarchs by the Rocky Mountains. These western monarchs overwinter in the coastal forests of California rather that the mountain forests of Mexico, and as I reported last year (Signs of Spring 1,  March 7, 2019) only 28,000 overwintering monarch butterflies were counted in their 2018-2019 overwintering sites. This represents an 86% decline from the 2017/2018 count and a 99.4% decline from the 4.5 million monarchs that overwintered on the California coast back in the 1980’s. The data from the latest 2019-2020 count was just released: the 30,000 overwintering western monarchs were statistically identical to last year’s count.

The decline of western monarchs is attributable to many factors. There has been a precipitous drop in the total area planted in milkweed throughout the west primarily due to herbicide use and urban development. Also many farms used to leave border areas around their fields which were rich in butterfly-friendly wild plants. Now, though, most western farms cultivate their fields right up to their fence lines eliminating both milkweed and vital sources of nectar. Also, the continuing hot weather and drought, thought to be directly attributable to climate change, have led to extensive wildfires that have destroyed vast areas of forest and wild plants needed by the monarchs for roosting, cover and nectar. The overwintering forests for the monarchs are also being altered by climate change. Winter temperatures in these forests are 2 degrees C warmer than they were just 20 years ago!

Human activities that are meant to help the monarchs may also be hastening the decline of the western monarchs. Many homeowners and landowners have planted tropical milkweed varieties instead of native milkweed in an attempt to provide food and habitat for monarch caterpillars. The tropical milkweeds, though, do not seasonally shed their leaves, and these persisting leaves come into direct contact with wave after wave of monarch butterflies. A variety of diseases are thus spread throughout cohort after cohort of foraging monarchs.

Western monarch on milkweed. USFWS. Public Domain

Western monarchs used to occupy a summer range that extended from the southern California forests all the way up to British Columbia. The northern range of this distribution, though, has greatly retreated: now the western monarch does not even get up into Washington State. Researchers are trying to better document the seasonal movements of these western monarchs in order to develop a plan to save their critical habitats and assist their survival. The western population of monarchs could, according to some experts, be extinct in less than twenty years!

So many people around the country are trying to pitch in to save the monarch! Monarchs, though, all across our country, are still gravely threatened by habitat loss, pesticide and herbicide use, and climate change. Plant some native milkweed! Plant some other, nectar producing native plants! Use fewer herbicides and pesticides! All of Nature will benefit from these steps, and we will continue to help to bring the monarchs back!

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