On warm nights in northern Israel, researchers have observed an unusual nocturnal phenomenon: large numbers of woodlice emerging from the soil and forming coordinated circular movements under streetlights. Scientists say the behavior may be linked to artificial lighting at night and its effects on nocturnal wildlife.
Woodlice—also known as isopods or “pill bugs”—are small land-dwelling crustaceans related to crabs and shrimp. Found in gardens and natural habitats worldwide, they typically live under stones, logs and leaf litter to retain moisture and avoid drying out. They play an important ecological role by breaking down decaying plant material and recycling nutrients back into the soil.
The unusual behavior was first reported in the Golan Heights by amateur naturalist Eviatar Itzkovich, who observed dense, rotating aggregations of the animals on summer nights. The reports prompted researchers to investigate environmental factors that might be influencing the movement.
A study from the Hebrew University of Jerusalem, published in the peer-reviewed journal Ecology and Evolution, examined the phenomenon in the species Armadillo sordidus. Until recently, little was known about this species, which typically remains hidden under stones and damp soil.
The research team, led by PhD student Idan Sheizaf under Professor Ariel Chipman, tested a range of environmental conditions, including magnetic fields and different types of light. Strong magnets placed near the aggregations had no measurable effect. Ultraviolet light attracted only a small number of individuals and did not produce coordinated circular movement.
White light, however, consistently triggered the behavior. When researchers placed a vertically oriented lamp above the ground, the animals formed a circular boundary around the illuminated area and began following its edge. As more individuals joined, the movement developed into a continuous rotating “mill” involving more than 5,000 animals.
According to the researchers, the effect appears to be driven not simply by brightness but by geometry: The vertical beam of light creates a ring-shaped illuminated boundary that the animals follow instinctively. Once enough individuals are present, their movement becomes self-reinforcing, sustaining the circular pattern.
Field observations also indicated the behavior is not related to mating, as many individuals observed were females, including egg-carrying specimens.
The researchers also noted that predators such as centipedes took advantage of the concentrated activity.
“While collective movement is common in the animal kingdom, seeing it in this form in isopods was entirely unexpected,” said Sheizaf. “It appears that the geometry of our modern world—specifically the circular pools of light created by streetlights—interacts with the animals’ natural behavior to produce this phenomenon.”
The researchers caution that artificial light at night may have broader ecological effects beyond disorientation, potentially altering movement patterns in ways that keep animals exposed and away from shelter.
