2017-10-05 / Nature

Tracking the Explosive Butterfly Migration

By Charles Avenengo

Hundreds of Monarch butterflies were seen, earlier this week, on Ocean Trail at Sachuest Point. 
(Photo by Carmen Rugel) Hundreds of Monarch butterflies were seen, earlier this week, on Ocean Trail at Sachuest Point. (Photo by Carmen Rugel) When members of the Fifth Ward Bird Club searched for birds at Brenton Point State Park on Oct. 1, they found the pickings slim. Many of the bird migrants had pushed on to the south.

However, the birdwatchers noticed a profusion of butterflies. The park was filled primarily with Monarchs and Painted Ladies that were in migration.

Wildlife observers have noticed large numbers of Monarchs this autumn. This comes as good news, because the decline of our most familiar butterfly has been well documented in recent years. This decrease has been attributed to a loss of the butterflies’ wintering habitat in the Mexican mountains due to logging, and in the Midwestern Corn Belt, where the increased planting of genetically modified corn has led to greater use of herbicides that eliminate milkweed, which is the Monarch’s primary food source.

Painted Lady. 
(Photo by Bob Weaver) Painted Lady. (Photo by Bob Weaver) But observers are encouraged by the current explosion.

The migration of butterflies ranks up there as among the most spectacular on earth. Birds and whales have equally impressive annual migrations that also span thousands of miles. However, compared to these animals, butterflies are delicate and only weigh a half-gram or so. Armed with a brain not even the size of a pinhead, onlookers wonder how they can pull off this remarkable feat.

Any Monarch butterfly currently seen will attempt to fly to one of 11 and 14 small patches of pine woods in the mountains of México. There, they will remain over winter. In these small patches, sometimes only one acre, the butterflies congregate so densely that their collective weight bends over tree branches. These Mexican wintering roosts were unknown to outsiders and weren’t discovered by biologists until the 1970s.

After the winter dormancy, the Monarchs begin their northward trek. En route, they will mate and the female will lay its eggs on milkweed plants. Then they will die. A second generation is produced, and they too will breed and die. The third generation of the season is born, and it is this generation that is currently working their way south to a place they have never seen. That is a mind-boggling proposition.

One reason for the current explosion has been attributed to an increased awareness of the Monarch’s plight. This has resulted in more gardeners planting milkweed. Additionally, Mexican authorities, realizing the tourism potential, have been able to significantly curb logging at wintering roosts.

The Painted Lady butterfly is also currently attracting attention.

When observed initially, their upper side can be confused with Monarchs. However, when seen side-by-side, the Painted Ladies are noticeably smaller.

Painted Ladies are also called Cosmopolitans because they are the most widespread butterfly on earth, inhabiting all continents except Australia and Antarctica.

There was an early and large arrival of Painted Ladies last spring. As a result, the butterflies produced two generations instead of one, creating the massive population that is currently migrating south.

Painted Ladies are more intensely studied in Europe than in the U.S. There, collective efforts between researchers and citizens have recently determined that they migrate 6,000 miles from Europe to Africa. Additionally, a study in England revealed that, although Painted Ladies are observed in gardens nectaring on plants to refuel during migration, the bulk of the species migrates thousands of feet in the air and are unable to be seen from the ground.

Painted Ladies have been clocked at 30 mph and can cover 100 miles a day. However, their migrations are erratic. The current explosion of millions of Painted Ladies is more of an anomaly than the norm. Researchers in the U.S. are trying to unravel the reasons for this explosion. And while tracking the movements of Painted Ladies has become more organized by researchers in recent years, there are still many unanswered questions. What is known is that they are working their way south to our warmer states and to the Mexican deserts, where like the Monarchs, they too will spend the winter.

Painted Ladies aren’t as particular as Monarchs. While the Monarch uses only milkweed as it's host plant, over 300 types of plants worldwide have been documented to be used by Painted Ladies. This might help explain their enormous numbers.

So, get out there to observe the last of the season’s migrating butterflies, because like the warm weather, they won’t be around much longer.

Naturalist Charles Avenengo has been chasing Aquidneck Island wildlife for more than 40 years.

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