Why should we care if a green energy spells doom for certain insects? It matters since the bugs are a key part of an important food chain, say scientists.
Solar panels can lure aquatic insects to their doom by mimicking the appearance of water.
Large solar arrays in the desert could have particularly far-reaching effects on insects and the animals that feed on them.
Applying white strips to the panel to reduce the size of the polarizing patches reduces the effect.
Solar panels might be a cornerstone of green energy, but new research shows that they may have a dark side: the panels are a mesmerizing, lethal attraction to certain aquatic insects, drawing them to the panels' surface and trapping them there until they die.
"It's like these organisms become dazzled to death," said Bruce Robertson of Michigan State University's W.K. Kellogg Biological Station in Hickory Corners who led the study. "It's like going to the most amazing 3D movie you've ever seen and you can't leave. They just fly and fly and fly over these surfaces, and they get exhausted and die."
The potential insect losses are important, Robertson said, because the insects provide food for fish and other aquatic species. "Aquatic insects are a fundamentally important base of the aquatic food web," he said.
The panels' attractiveness comes from the way the panels reflect light, which is similar to the way that water reflects light -- only "better" in the eyes of the insects.
Many aquatic insects have specialized vision that can see polarized light, helping the insects find water.
When sunlight hits water, light waves that are perpendicular to the water surface are absorbed, while light waves parallel to the surface are reflected. The resulting beam of light leaving the surface is polarized, meaning that the light waves are aligned in one direction.
Many aquatic insects including mayflies and caddis flies have evolved the ability to detect polarized light, presumably as a way to seek the water sources they need for food or breeding.
But solar panels are even more alluring than water to the insects. "The solar panels attract far more insects than even water because they are polarizing 100 percent whereas water only polarizes 60 percent," Robertson said.
He and colleagues compared how many insects hovered near different surfaces, including solar panels and non-polarizing surfaces. The team found that the solar panels were the most attractive to the insects they studied. They published their findings in Conservation Biology.
"As solar panels become an increasingly large part of our energy accounting, the ecological traps associated with solar panels have the potential to become radically important in wildlife conservation," he said.
"Given what we know about polarized light and about insects and about other species that respond to polarized light, this makes a whole lot of sense," said light pollution expert Travis Longcore of the University of Southern California in Los Angeles.
One area of particular concern is the desert, where large solar installations are planned or underway. "There's an explosion of interest in large scale development of solar, especially in the Mojave desert," Robertson said.
The results could be dramatic.
"When you think about the desert, there's almost no more rare resource than water. You might be attracting enormous numbers of organisms from miles around," he added. "If you attract desert insects, they might attract birds or bats or desert reptiles that would try to feed on them. It's hard to know if that's a good thing or a bad thing or just a thing."
Longcore points out that insect attraction is one of many concerns conservationists have about big solar projects that are near or in natural lands.
"There are other impacts on habitats: changing the microclimate, providing perches for predators, and increasing the density of crows and ravens that eat the eggs of desert turtles," Longcore said. Crows and ravens move in wherever there are human disturbances, he said.
"This is yet another reason that we should be looking at implementing solar projects in cities and in other developed areas like in parking lots and on roofs," Longcore concluded.
The good news from Robertson's study is that the lure of solar panels was dramatically reduced when white borders or grids of white strips crisscrossed the panels, dividing the panel into smaller segments. This reduced the attractiveness by 10 to 26-fold, the study found, effectively eliminating the effect.
"Many solar panels are already designed this way," Robertson said.