The faeces of hungry caterpillars have been shown to act as fertilizer for aquatic microbes, which in turn release carbon dioxide into the atmosphere.

Major outbreaks of hungry caterpillars occur every five years in temperate forests. The caterpillars eat their way through countless tasty leaves. Their poop then ends up in nearby lakes. And then something remarkable happens. Researchers have found in a new study that these hungry caterpillars significantly improve water quality in nearby lakes. But, on the other hand, in this way they contribute a hitherto overlooked contribution to climate change.

Frass

When the caterpillars feast on countless leaves, the droppings – also known as frass – regularly fall into nearby bodies of water. “The caterpillars are actually small machines that convert carbon-rich leaves into nitrogen-rich poop,” explains researcher Andrew Tanentzap. “Then, instead of the leaves, these droppings fall into the water. And this changes the chemistry of the water significantly.”

Hungry caterpillars on a tree in Ontario, Canada. Image: John Gunn

On the one hand, this is good news. The researchers show in their study that the poo significantly improves the water quality of the lakes. In years without caterpillars plunging onto the carbon-rich leaves, rotting leaves and detached needles whirl into the water. In this way, carbon accumulates in these lakes. Hungry caterpillars, however, can offset this buildup of carbon, significantly improving water quality. For example, it appears that freshwater lakes contain an average of 27 percent less dissolved carbon during outbreak years. “Outbreaks of leaf-eating caterpillars can reduce carbon dissolved in water by nearly a third,” said study researcher Sam Woodman. “It’s just amazing that these insects have such a big impact on water quality.”

Reverse

But there is a downside. And that is perhaps just as important. When the faeces of the hungry caterpillars end up in the water, it acts as fertilizer for microbes. They then release carbon dioxide into the atmosphere during their metabolic process. The researchers now suggest that the large amounts of feces that end up in the water during major outbreak years could promote the growth of greenhouse gas-producing bacteria in the lakes. And that is at the expense of the algae that actually remove CO2 from the atmosphere. It means that the caterpillars can thus increase carbon dioxide emissions. “We think it will increase the extent to which lakes are sources of greenhouse gas emissions,” Tanentzapc warns.

Climate change

In addition, as the climate changes, scientists expect the caterpillar’s range to expand northward. They also suspect that climate change will only increase the number of caterpillars. This may lead to more caterpillar outbreaks in northern forests. And this means that larger amounts of CO2 may also be released from nearby lakes. This northward shift is also worrying, as further north we find more freshwater lakes for the caterpillars to drop their droppings into. Finally, climate change will cause more deciduous trees to grow around the lakes, which could further amplify the caterpillars’ effect.

Climate models

So it seems that small, inconspicuous caterpillars can make a significant contribution to climate change. A pretty surprising discovery. “They are still completely overlooked in climate models,” Woodman said. This means that from now on the effect of the caterpillars must be included in climate projections, in order to paint a more accurate picture of the future.

All in all, the story of caterpillar Never Enough is twofold. “From a water quality perspective, they’re a good thing,” Woodman says. “But from a climate perspective, they’re actually pretty bad.”