The annual monarch butterfly migration will soon be underway this fall.

These beautiful butterflies will be slurping up flower nectar to fuel their migration to their small refuge and overwintering site in Mexico. But how big will this migration be?

An early warm spring interrupted with a late frost may be the Monarch butterflies’ worst nightmare. At the spring meeting of the Kansas Academy of Sciences, Dr. Orley “Chip” Taylor described a newly recognized factor in the population size of this unique North American butterfly.

Entomologist Taylor has been watching the populations of this large orange butterfly for years. He heads Monarch Watch at the University of Kansas, a group that coordinates the efforts of civilians and young students to tag monarch butterflies across the United States and Canada.

Monarch butterflies are not cold hardy and cannot survive our freezing winters. However, they are one of very few insect species that can feed on milkweed. And milkweed is cold hardy, with milkweed species growing all the way into Canada. Therefore the monarch butterflies fly north each year to take advantage of this milkweed food source. But they must migrate back to Mexico each winter.

Each spring, the adult butterflies that have overwintered begin flying north and laying eggs on milkweed. These generations of monarchs stairstep their way until some reach Canada. With the approach of winter, they must fly south again to overwinter in Mexico.

Taylor has assembled historical data, showing that while monarch populations fluctuate each year, they have been shrinking over time. There is good reason to associate this overall decline with the increased use of herbicides that have wiped out milkweeds. Milkweeds were a common weed in soybean and other crop fields until modern herbicide-resistant crops were developed.

Although monarch population graphs show an overall decline, there are still some years where the monarch population dropped dramatically. By going back to the weather data, it became clear to Taylor that these desperate years for monarchs were years where the winter was mild and spring came early. The monarch butterflies rapidly moved north, often further than they normally migrated in those early months. Then came a late freeze that killed off adult monarch butterflies, their eggs and young caterpillars that had begun feeding on the young milkweed.

Such an early and mild spring, followed by a late severe freeze, took a severe toll on the monarch population that year and likely depressed the following year’s numbers as well. This unique weather pattern appears to account for the most severe dips in the monarch population across the last decades. And the availability of flowers to produce nectar to fuel their fall trip back to Mexico also is a factor.

While these specific annual weather patterns explain the really bad drops in monarch butterflies, they do not change the big picture, where the overall decline is related to loss of milkweeds in the critical Midwest and Great Plains regions. Monarch Watch also participates in distributing milkweed plants and promoting butterfly gardens.

Understanding the details affecting the life history of the monarch butterfly is important for making wider decisions in farming practices, roadside mowing, and even assigning protected status. In 2014, the US Fish and Wildlife Service was petitioned to add the monarch butterfly for protection under the Endangered Species Act. This triggers that agency to conduct an assessment to determine if the monarch needs protection. The FWS is still conducting this assessment using the Species Status Assessment framework. Unlike small rare butterflies that may depend on a single rare flower only found on a California mountaintop, the monarch butterfly is dependent on larger numbers of butterflies during migration in order to sustain its migrating population. There is also the possibility that the migrating population could collapse while other monarchs that do not migrate in Mexico continue to live on.

Meanwhile, many states in the monarch migratory path have formed research groups to study and advise how they can improve monarch survival in their state, and avoid ESA listing. The monarch butterfly is a “celebrity species” that attracts the attention of citizens concerned with preserving nature. This means that there are many online websites with information on the monarch butterfly — some of it accurate and some inaccurate. One strategy is to limit searches to state agency links and to Monarch Watch, and to the links posted by those sites. The complex interactions of long term herbicides and annual weather differences makes simple explanations of the monarch migration usually wrong.

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