Fly, white butterflies, out to sea,
Frail pale wings for the winds to try,
Small white wings that we scarce can see
Fly.
Some fly light as a laugh of glee,
Some fly soft as a low long sigh:
All to the haven where each would be
Fly.
—Algernon Charles Swinburne, “Envoy”
Summer isn’t over yet—it doesn’t officially end until September 22, when the autumnal equinox occurs—but fall migrations are already underway. An estimated 191,800 birds migrated through Los Angeles County on the night this article was written, up from 82,200 the night before.
On the West Coast, the annual fall bird migration begins as early as July for some species, and doesn’t end until midwinter for others. Billions of birds travel the Pacific Flyway each year. Some birds are long-distance travelers making the trek from the Arctic to the Antarctic. Others move much shorter distances: from Northern to Southern California, or from the mountains to the coast. For some species of bird, the Santa Monica Mountains and coast offer a place to pause on the journey; for others, this is the destination. For some, the coastline may serve like a map to aid navigation, but they will not stop to rest on the way.
Birds aren’t the only ones on the wing. A surprising number of insects also migrate in the fall, traveling away from the cold weather. Western monarchs are already traveling south from the Pacific Northwest and west from the Rockies to winter on the California coast. The Eastern species undertakes a journey from as far north as Canada all the way down to Mexico—a 3000-mile journey. No individual monarch completes that journey; instead, it takes multiple generations to complete the round trip migration.
Before this species’ dramatic recent decline, millions of monarchs filled the sky and covered entire groves of trees on the California coast with vivid orange and black, like autumn leaves. The Xerces Society, which has monitored the western population since the 1990s, counted just 9,119 butterflies in 2025, the second lowest number of overwintering butterflies recorded since the count began in 1997. This year may be equally bad for this iconic species, especially in the Topanga area, where many of the trees that provide crucial shelter for this species were destroyed in the Palisades Fire.
Monarchs are the most famous example of a migratory insect, but they aren’t the only one. Like the monarch, the painted lady butterfly, Vanessa cardui, also makes a multi-generation migration that takes them from as far north as Alaska and Canada all the way to their winter home in the deserts of Western Mexico. Some painted ladies even cross the Sierra Nevada range.
The European population of this species spends the summers as far north as the Arctic Circle. It crosses the Mediterranean and the Sahara Desert to winter in sub-Saharan Africa. Each butterfly weighs less than one gram—about the weight of a single paperclip. Despite their small size, painted ladies are remarkably fast flyers and have been observed traveling at high altitudes, reportedly as high as a record 22,000 feet.
Here in California, painted lady butterflies experience unusually large migration events in 2005 and 2019, thanks to abundant rain at just the right time, but even in a drought year this small butterfly travels amazing distances.
There are many other butterflies that undertake shorter migrations. While the local population of red admirals, Vanessa atalanta, does not appear to migrate, admirals in higher elevations are known to move towards the coast to avoid freezing weather.
The mourning cloak, Nymphalis antiopa, is another species that makes the journey down from the mountains in the fall, but this butterfly has also been observed overwintering at high elevations.
Butterfly researcher Art Shapiro, who has monitored butterfly populations for more than 50 years and maintains the UC Davis butterfly website butterfly.ucdavis.edu writes that, “Some adults overwinter even at Donner. They emerge just after snowmelt, which often coincides (late May-June) with when immigrants from the new brood arrive there from downslope!”
It’s unclear why some migrate and others do not.
The Common buckeye is a year-round resident in the Santa Monica Mountains and much of coastal California. Further north, buckeyes have been observed following a migration pattern similar to the mourning cloaks and red admirals—from higher elevations to lower ones in autumn, and back again in spring. However, on the East Coast, buckeyes undertake a much more substantial journey, traveling from New England to as far south as Florida in the fall.
English poet Algernon Swinburne’s butterflies with their frail, pale wings, were probably members of the cabbage white butterfly family, possibly Pieris rapae, now one of the most ubiquitous butterflies on earth. This butterfly and several closely related species have been documented migrating from England to mainland Europe and back again in the spring, as well as traveling in large numbers through the Pyrenees to the Mediterranean. They’ve even been documented migrating across a large swath of Australia, where they are a non-native species. Frail, pale wings, powered by an incredible determination.
In California, as in Australia, the cabbage white is an introduced species that thrives on invasive mustard and vegetable crops in the mustard and cabbage family, but here, it doesn’t seem to feel the need to migrate—mild temperatures and a year-round supply of food offer ideal habitat.
Migratory butterflies are joined by a host of other insects. Many dragonfly species migrate in a similar multigenerational pattern. Like butterflies, they can travel great distances. The globe skimmer dragonfly, Pantala flavescens, holds the record for the longest migration of any known insect, with a multigenerational roundtrip migration of more than 11,000 miles, across the Indian Ocean and back again. Not even the monarch butterfly can compete.
Huge numbers of common green darners, Anax junius, travel the Eastern and Central flyways of America, heading south in the autumn with many of the bird species that eat them. A 2018 study led by Ecologist Michael T. Hallworth and published in the Royal Society Publication Biology Letters, used “a combination of stable-hydrogen isotope analysis of 852 wing samples from eight countries spanning 140 years, combined with 21 years of citizen science data,” to track the full annual cycle of the species. “We demonstrate that darners undertake complex long-distance annual migrations governed largely by temperature that involve at least three generations,” the study concludes.
The migration along the West Coast of the US is smaller, but still impressive. Large and powerful for an insect, but small and frail by any other metric, the Western population is known to chart a course over the ocean, possibly using the coastline as a guide. They travel at high elevations alongside the migratory bird species that prey on them.
The blue dasher (Pachydiplax longipennis), one of the most common North American dragonflies, is now also thought to migrate. Large numbers of these bright blue insects have been observed heading south along the East Coast in the fall. A smaller but still significant number travel along the Pacific Flyway, although it remains unclear whether they are migrating with the seasons or in search of new habitat as summer progresses and the fresh water this species depends on dries up.
There are smaller insects that also migrate. Milkweed bugs, Oncopeltus fasciatus, like the monarchs they share their host plant with, travel long distances to escape from the cold. The convergent ladybug, Hippodamia convergens, lives up to its name by converging sometimes in huge numbers. But unlike the butterflies that move downslope in the autumn, ladybugs head into the mountains where they gather together to enter a dormant state—diapause—for the winter. In the spring they disperse back down from the mountains to mate and lay eggs.
Not all migrant insects are welcome. Large and powerful flyers like locusts and grasshoppers can form great swarms, travel impressive distances in search of food, and decimate that food source once they find it. The six-spotted leafhopper, Macrosteles quadrilineatus, is one of a number of migratory pest species that can spread disease and harm crops and native vegetation. This insect, barely 4 mm long, has wings but depends on the wind to carry it on its migration.
There are even migratory aphids. Schizaphis graminum, the greenbug aphid, also uses the wind to travel. Aphids are so small they can be classed as “aerial plankton”—part of a huge biomass of bacteria, spores, and other tiny lifeforms carried on the wind. They do not navigate to a specific destination in the way butterflies and dragonflies do, but they still migrate, sometimes traveling long distances.
We still know relatively little about insect migrations. We are only beginning to explore and understand how and why insects migrate. We still don’t know why some migrate and others don’t or how they transmit the knowledge of where to go and how to get there from one generation to the next in a journey that requires multiple generations to complete. Yet, for millions of years, migratory insects have taken to the sky on epic journeys, “all to the haven where each would be.”
