Hungry Caterpillars Can Alter Carbon Emissions on a Huge Scale

A study led by the University of Cambridge found that periodic large-scale outbreaks of leaf-eating caterpillars can improve the water quality of nearby lakes, but also increase the lake’s carbon dioxide emissions.

Caterpillar outbreaks of the invasive gypsy moth, Lymantria dispar dispar and forest tent caterpillar moth, Malacasoma dystria occur in temperate forests at least once every five years. Insects chewed so many leaves, and it turned out that fewer leaves and increased insect feces would disrupt the nutrient cycle between the land and nearby lakes, especially carbon and nitrogen.

Nitrogen-rich insect feces, called feces, can be washed into lake water to act as fertilizer for microorganisms, and then release carbon dioxide into the atmosphere during metabolism. Researchers say that in the year of an outbreak, a large amount of excrement will benefit the growth of bacteria that produce greenhouse gases in the lake, while sacrificing the algae that remove carbon dioxide from the atmosphere.

Forest Tent Caterpillars on Trees
Forest tent caterpillars on trees in Sudbury, Ontario. Credit: John Gunn

“These insects are basically small machines that turn carbon-rich leaves into nitrogen-rich manure. The manure falls into the lake instead of falling leaves, which significantly changes the chemical composition of the water; we believe this will increase the lake’s ability to become a greenhouse The extent of the source of the gas,” said Professor Andrew Tanentzap from the Department of Plant Science at the University of Cambridge, who is the senior author of the paper.

With climate change, the range is expected to expand northwards, and the insect population will grow further. This puts boreal forests at greater risk of defoliation outbreaks in the future, which may cause more carbon dioxide to be released from nearby lakes.

Forest Tent Caterpillars
Forest tent caterpillars on trees in Sudbury, Ontario. Credit: John Gunn

This shift to the north is also worrying because there are more freshwater lakes further north. Climate change is also expected to benefit the deciduous broad-leaved trees around the lake, amplifying the effects of insects.

The study found that in the years of insect outbreaks, the leaf area of the forest decreased by an average of 22%. At the same time, compared with years without germination, the dissolved nitrogen in nearby lakes increased by 112% and the dissolved carbon decreased by 27%. When the lake basin contains a higher proportion of deciduous broad-leaved trees (such as oak and maple), the impact is greatest, and caterpillars prefer these trees than coniferous trees such as pine trees.

To obtain their results, the researchers combined 32-year insect outbreak surveys and lake water chemistry government data from 12 lake basins in Ontario, Canada, and satellite remote sensing data on forest types and coverage. Leaf area per month. The results were published today in the journal Nature Communications.

Forest Tent Caterpillars in Sudbury, Ontario
Forest tent caterpillars on trees in Sudbury, Ontario. Credit: John Gunn

This is considered to be the most extensive research ever conducted on how insect outbreaks affect the carbon and nitrogen dynamics of freshwater. The scale of previous studies was too small to draw a broader generalization.

A previous 26-year study of 266 lakes across the northern hemisphere showed that carbon accumulates naturally in these lakes, a process called browning. This trend is due to a variety of factors, including climate change and recovery from historical acid rain and logging activities. Comparison of the new results with these data shows that the outbreak of leaf-eating caterpillars can effectively offset the year-long carbon accumulation in nearby lakes and significantly improve water quality.

Forest Tent Caterpillars on Trees in Sudbury, Ontario
Forest tent caterpillars on trees in Sudbury, Ontario. Credit: John Gunn

In the years when there is no outbreak of leaf-eating insects, the carbon and nitrogen entering the lake generally come from litter and decaying litter, and the amount reaches its peak in autumn. In the few years of the outbreak, studies have found that the dissolved carbon of nearby freshwater lakes has decreased by an average of 27%.

“When the trees around the lake are mostly deciduous, the outbreak of leaf-eating insects can reduce the dissolved carbon in the lake water by nearly a third. Surprisingly, these insects can have such a significant impact on water quality,” Cambridge Said Sam Woodman, a researcher at the University’s Department of Plant Science and the first author of the report.

He added: “From a water quality point of view, they are a good thing, but from a climate point of view, they are bad, but they are completely ignored in the climate model.”

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