The secret compass that guides moths on their journeys

An Invisible Highway in the Night Sky

To the human eye, the night seems peaceful. Yet, high above our heads, an unsuspected spectacle is unfolding: celestial highways traveled by billions of insects. Every year, vast populations of moths travel hundreds, even thousands of kilometers, safely sheltered by the darkness. They cross entire continents, carried by seasonal winds, and manage to maintain a precise course as the landscape passes by, unseen, beneath their wings.

How do these tiny nocturnal navigators accomplish such feats? Without sunlight or clear visual landmarks, navigating over such long distances should be impossible. Yet that is exactly what they do, reaching their breeding grounds before their offspring begin the return journey south. New research has finally shed light on the tools that allow them to stay on course in the dark.

The study’s findings suggest that certain nocturnal insects, including moths, combine two types of information for their migrations. They appear to rely on both visual cues and the Earth’s magnetic field. Together, these two signals seem to form a remarkably effective guidance system.

A Global Agricultural Challenge

These migrations are no trivial matter. In North America as well as in Eurasia, billions of moths fly north each spring to reproduce. Later in the year, the next generation retraces its path in the opposite direction, heading toward warmer regions in the south. But not all of these travelers are harmless. Many of them are among the worst agricultural pests on the planet.

Their larvae feed on essential crops such as corn, wheat, and rice, causing considerable economic damage in many parts of the world. This is what motivated the researchers. Yi-Bo Ma, co-first author of the study and a master’s student at Nanjing Agricultural University in China, emphasizes the importance of this work. “Some of the most abundant species involved in these migrations are the world’s most destructive agricultural pests, which makes a full understanding of their migratory patterns essential,” he explains.

The researcher adds: “Although it is believed that many of these species use the Earth’s magnetic field—particularly at night when navigation is more difficult—the sensory basis for this navigation has not yet been studied.”

In the cockpit of a butterfly flight simulator

To unravel this mystery, the scientists focused on a specific species: the fall armyworm, a moth known for its devastating impact. Native to the Americas, it has spread rapidly across Africa and Asia in recent years, and its ability to migrate long distances is key to its invasive power. The team wanted to understand how it maintains its course while flying at night.

To do this, they designed an ingenious device—a sort of virtual flight simulator. Each insect was delicately secured, preventing it from moving forward but allowing it to rotate freely, just as it would during actual flight. The setup was placed at the center of a system of coils capable of precisely controlling the magnetic field surrounding the moth.

A very simple visual cue was projected onto the simulator’s wall: a black triangle standing out against a dark horizon. This cue provided the insects with a visual reference point while the researchers manipulated the magnetic field. “Although this setup is a significant simplification compared to natural flight conditions, it provided a controlled environment for isolating the contributions of geomagnetic and visual cues—a step toward understanding how they function in more realistic contexts,” explains Yi-Bo Ma.

When the Internal Compass and Vision Conflict

The experiment took place in several phases, during which the researchers recorded the butterflies’ orientation during five-minute flight periods. Initially, the visual cue and the direction of the simulated magnetic field matched what the insects would encounter during their migration season. Under these conditions—whether for spring or fall migration—the butterflies consistently oriented themselves toward the visual cue. This provided evidence that they knew how to use this signal for navigation.

The scientists then introduced a disruption. They rotated the horizontal component of the magnetic field by 180 degrees, creating a direct conflict between the magnetic direction and the visual cue. Despite this contradiction, the butterflies continued to orient themselves toward the visual marker throughout the five-minute test. This result suggested that the visual cue took precedence over the magnetic signal. But that was only the beginning.

Over time, an interesting phenomenon occurred: the group’s collective orientation disappeared. The moths were no longer pointing in the same direction. “This delayed response was consistent with similar results from studies on the Bogong moth and suggests that moths need time to process conflicts between signals,” explains Gui-Jun Wan, co-author of the study and associate professor at Nanjing Agricultural University. He adds: “The absence of visual cues led to a significant loss of flight stability in the moths, which likely explains the disorganization of their orientation.”

An Innate, Integrated Navigation System

The team continued the experiment by rotating the visual cue before realigning it with the magnetic field. Each time the two signals aligned again, the butterflies quickly regained a coherent and stable group orientation. This observation was confirmed in butterflies raised in the laboratory under lighting conditions that simulated seasonal cycles. This indicates that their navigation system is innate, genetically programmed, rather than learned from the environment.

Migration is one of the most fascinating behaviors in the animal kingdom. It is known that birds and sea turtles often rely on the Earth’s magnetic field for their long journeys. This research demonstrates that insects may use a similar strategy, although they also rely heavily on visual cues. The results reveal that the fall soldier fly needs both types of information to stay on course.

Visual cues appear to be particularly crucial: they stabilize flight and allow the insect to correctly interpret magnetic signals. These findings, published in the journal eLife, highlight the importance of integrating multiple cues for successful navigation. They pave the way for future research to determine whether other species of migratory butterflies share similar mechanisms. Understanding how these insects navigate could, in the long run, help scientists predict their movements and limit the damage they inflict on crops worldwide.

Source: earth.com

The secret compass that guides moths on their journeys