Turn Off Porch Light To Aid Caterpillars And Safeguard Backyard Ecosystems

Moderate levels of artificial light at night – like the fixture illuminating your backyard – bring more caterpillar predators and reduce the chance that these lepidoptera larvae grow up to become moths and serve as food for larger prey.

This ecological impact was demonstrated in a new Cornell University study published in the Proceedings of the Royal Society B: Biological Sciences.

The scientists placed more than 550 lifelike caterpillar replicas made of soft clay in a forest, setting to ascertain how the mockups were attacked and hunted by predators compared to a control group.

“We measured predation rates on the clay caterpillars – which look like the real thing,” said John Deitsch, who conducted the research as his undergraduate honors thesis in the nearly pitch-dark Hubbard Brook Experimental Forest, in the White Mountains of New Hampshire. “Predators left marks on the clay. Predation rates on clay caterpillars and the abundance of arthropod predators were significantly higher on the artificial light at night treatment plots. This suggests an increase of mortality pressure on caterpillars.”

Deitsch and Sara Kaiser, research ecologist and director of the Hubbard Brook Field Ornithology Program at the Cornell Lab of Ornithology, co-authored the research, “Artificial Light at Night Increases Top-Down Pressure on Caterpillars: Experimental Evidence From a Light-Naive Forest.”

The caterpillar models are made from green, extruded clay to mimic the color and size of Noctuidae (owlet moths) and Notodontidae (prominent moths) caterpillars. The soft clay easily allows for imprints so that the scientists can determine if predators – like arthropods, insects, or birds – landed on the model or tried to take a bite.

While effects of artificial light at night have often been studied on adult insects (such as moths), the larvae (caterpillars) have seen little research.

Of the 552 clay caterpillars deployed and glued to leaves to look authentic, 521 models were recovered and 249 (47.8%) showed predatory marks from arthropods, during the summer-long nighttime study.

Further, the research found that caterpillar predation rates were 27% higher on experimental plots – compared to the control areas in the same forest – that had 10 to 15 lux (about the brightness of a streetlight), which is an illumination measurement for LED lighting.

Given the global ubiquity of artificial light at night, increased threat to caterpillars is yet another ecological problem for lepidoptera, in addition to habitat loss, agricultural-chemical pollutants, invasive species and climate change, according to the paper.

Caterpillars are the most vulnerable at that larval stage. “They are eating leaves and growing in order to mature to the next stage,” said Kaiser.

“When you turn on a porch light, you suddenly see a bunch of insects outside the door,” Kaiser said. “But when you draw in those arthropod predators by adding light, then what is the impact on developing larvae? Top-down pressure – the possibility of being eaten by something.”