Historical Observations of Insect Migration
In October 1950, ornithologists Elizabeth and David Lack documented substantial insect migration through the Puerto de Bujaruelo mountain pass in the Pyrenees. They observed hundreds of butterflies and thousands of dragonflies and tiny flies per hour. This event marked Europe's first recorded instance of fly migration, with widespread scientific follow-up occurring decades later.
Atlantic Crossing by Painted Lady Butterflies
Spanish entomologist Gerard Talavera initiated a decade-long investigation in 2013 after observing painted lady butterflies, not native to South America, in French Guiana.
The study, published in Nature Communications in 2024, provided the first direct evidence of any insect crossing the Atlantic Ocean. Researchers suggest such crossings may occur frequently.
Previously, in 2014, evidence emerged of painted lady butterflies crossing the Sahara Desert as part of their annual migration. Painted lady butterflies cannot survive cooler climates and migrate in response to environmental cues such as day length, temperature, and food availability. They utilize winds to reach breeding grounds where abundant vegetation allows females to lay over 1,000 eggs. Subsequent generations may also migrate seasonally.
Bogong Moths and Celestial Navigation
Bogong moths in Australia have been known to migrate for thousands of years, with Indigenous Australians incorporating them into cultural festivals. Their swarms can be large enough to be mistaken for rain clouds by meteorologists.
By the 1970s, scientists established that nocturnal migratory birds used stars for navigation. Subsequent research demonstrated that bogong moths also use the starry sky as a compass for hundreds of kilometers, even with a brain significantly smaller than a grain of rice. On cloudy nights, they can utilize Earth’s magnetic field for navigation. However, light pollution from urban areas, such as Canberra, poses a disorientation risk.
Bogong moth populations have experienced significant declines, with a 99.5% reduction during a severe 2017 drought. This decline is linked to drought, habitat loss from agricultural expansion, and pesticide use, impacting species like the mountain pygmy possum, which relies on moths for food. Recent years have shown a partial recovery, but future droughts are expected to worsen, presenting an uncertain future for the species.
Insect Tracking Methodologies
Tracking insects is challenging due to their small size and vast numbers. Scientists have developed various methods, including following them in aircraft and dusting them with fluorescent powder for later recapture.
Dr. Jason Chapman, a behavioural ecologist from the University of Exeter, utilized radar and a blimp to quantify insect migration. His team used two radars to scan a kilometer of sky and a net suspended from a 5-meter blimp to collect insects.
This research, conducted over seven months, found that over 3 trillion insects migrate annually over southern England, revealing insect migration as a previously understudied mass phenomenon.
Roughly 5% of the UK's 24,000 insect species are migratory, with some, like ladybirds, lacewings, and aphids, being highly abundant. Insect migrations often span multiple generations, with no single individual completing the entire journey.
Renewed Research and Ecological Importance
Seventy years after the initial observations, scientists returned to the Puerto de Bujaruelo pass, recording 17 million insects migrating through annually. Approximately 90% of these are pollinators, contributing to plant genetic diversity by moving pollen over long distances. Migratory insects like drone hoverflies also decompose waste and recycle nutrients. Other species, such as hoverflies, consume agricultural pests and serve as a food source for migratory birds.
Global Insect Decline
Global insect populations are declining due to climate change, habitat loss from industrial farming, and pesticide use. A German study reported a 97% decline in aphid-eating migratory hoverflies over the past 50 years.
While insect migrations remained largely unstudied for centuries, current research is revealing their significance even as populations diminish.