Considered one of the world’s smallest butterflies and the tiniest in North America, the Western pygmy-blue (Brephidium exilis) has a wingspan between 1.2 and 2 centimeters and can easily rest on a fingernail. Other butterflies under the Polyommatinae subfamily, also known as blues or gossamer-winged butterflies, may be smaller. Some researchers even argue that some species of moth are even tinier.
But despite the Western pygmy blue’s teeny size, it is resilient and can thrive in areas not normally found if it finds enough resources to support its life cycle. Butterflies are opportunists and will thrive in a habitat with necessary food sources for caterpillars. While some species are strictly limited to a host plant, others can migrate and will feed on various types of flowers. For this reason, there are several worldwide populations of the Western pygmy-blue.
When John Calhoun saw an image of a tiny coppery butterfly on iNaturalist, he was astonished. Calhoun is a research associate with the McGuire Center for Lepidopteran Biodiversity at the Florida Museum of Natural History and the butterfly, which he identified as a female B. exilis, was well outside its typical habitat.
Its sighting, reported last year, was the first time it was seen in Tampa, Florida. And when further looking at the data on iNaturalist, Calhoun found another entry of the insect from three months earlier, in April 2022.
Western pygmy blues are usually found in deserts, salt marshes and the wastelands of the Southwest. As a caterpillar, the insect feasts on white goosefoot, tumbleweed and saltbush species found in central California, southern Nevada and New Mexico to central Arizona. As adults, it will feed on flower nectar.
The butterflies are also seen in more southern regions like Baja, California, Mexico and Venezuela. The farthest East in North America the Western pygmy-blue that had ever been spotted before, was in Alabama and some parts of Texas.
“A couple of individuals, including myself, ultimately identified it as a Western pygmy blue which was mind-blowing because that wasn’t even on the radar and something that would likely show up in Florida,” says Calhoun.
Almost three decades ago, Western-pygmy blues were also spotted in Saudi Arabia and the United Arab Emirates and, since then, have spread to Kuwait. Two years ago, the butterflies were spotted in the gardens of a hotel along the Dead Sea.
Experts suspect they made their way across the planet through a desert plant someone brought over from the U.S. The plant may have had eggs that later hatched and populated the country.
“Saudi Arabia has many environments similar to some of the saline and alkaline environments that it likes here [in the U.S]. And there are almost certainly plants in that area that its caterpillars can feed on that are either native to that area or maybe native to the U.S. and were introduced there. […] It’s kind of in the right place there because those habitats are similar,” says Kevin Burls, a conservation biologist with the Xerces Society not involved with the study.
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The Western pygmy-blue’s tiny but mighty wings allow the insect to migrate North from its normal range in the West. “So, they make a decision to go up, get into the high winds up higher, and get carried long distances. And that’s likely the case for some of those observations of the Western pygmy blue in Arkansas, eastern Texas, and Louisiana as it gets further afield. I think it’s unclear,” says Burls.
Calhoun thinks it’s more likely that the pygmy blues in Florida may have been pushed over ‘naturally’ and not brought into the state by humans. Calhoun and Ron Smith, authors of a paper published in the Southern Lepidopterists’ News that details the butterfly in Tampa, suspect the insect was swept into the area by a weather front.
“They were probably distributed naturally or through a weather system that came through the northern Gulf. And, you know, just basically swept them up […] and they just kind of dropped down if they find a suitable environment,” Calhoun says.
Female Western Pygmy-blue photographed in Ft. De Soto Park in Pinellas Co., Florida (Credit: R. Sanchez)
Calhoun explains that the butterflies most likely did not migrate because researchers did not find them around the Florida panhandle, especially in coastal areas that have enough plants for food. This suggests the insects caught a ride on winds that brought them to the Tampa Bay area.
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A few factors contributing to the butterflies’ success are their ability to establish new populations with available habitats and feed on non-native plants like tumbleweed. Other butterflies belonging to the Lycaenidae family, like the federally listed Karner blue butterfly caterpillars, rely on one host plant for survival, lupine.
“Whereas for the Big Western pygmy blue, it’s just got individuals moving all the time. In the lowest warmest environments, it can breed all year round. And then as the spring and summer warm up in higher and mostly northerly locations, it spreads to those areas, fills in those habitats as they become available, and then dies off in the winter,” says Burls.
It is unknown if the Western pygmy-blue will remain in Florida permanently or if this was a fluke that will eventually fade away. Clouds of tiny butterflies flutter around patches of crested saltbush and sea blite on Florida’s coast, but the established populations may be temporary.
“It’s [the Western-pygmy blue] like a western snowbird that decided to move to Florida, and time will tell if they decide to stay,” Calhoun says.