Hungry caterpillars an underappreciateddriver of carbon emissions3 November 2021

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

A study led by the University of Cambridge hasfound that periodic mass outbreaks of leaf-munching caterpillars can improve the waterquality of nearby lakes—but may also increase thelakes' carbon dioxide emissions.

Outbreaks of caterpillars of invasive gypsy moths, Lymantria dispar dispar, and forest tent caterpillarmoths, Malacasoma disstria occur at least everyfive years in temperate forests. The insects munchthrough so many leaves that the resultingdecrease in leaf-fall and increase in insectexcrement has been found to alter the cycling ofnutrients, particularly carbon and nitrogen,between land and nearby lakes on a huge scale.

Nitrogen-rich insect excrement, called frass, canwash into lake water and act as fertilizer formicrobes, which then release carbon dioxide intothe atmosphere as they metabolize. Theresearchers suggest that in outbreak years thelarge quantities of frass will favor the growth ofgreenhouse gas-producing bacteria in lakes at theexpense of algae that remove CO2 from theatmosphere.

"These insects are basically little machines thatconvert carbon-rich leaves into nitrogen-rich poo.The poo drops into lakes instead of the leaves, andthis significantly changes the water chemistry—wethink it will increase the extent to which lakes aresources of greenhouse gases," said ProfessorAndrew Tanentzap in the University of Cambridge'sDepartment of Plant Sciences, senior author of thepaper.

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

Northwards range expansion and increased insectpopulation growth is anticipated as the climatechanges. This puts northern forests at increasedrisk of defoliator outbreaks in the future, potentiallycausing greater quantities of CO2 to be releasedfrom nearby lakes.

This northwards shift is also concerning becausethere are more freshwater lakes further north. Andclimate change is also expected to favorbroadleaved deciduous trees around the lakes,which will amplify the effect of the insects.

The study found that in years with insect outbreaks,the leaf area of forests was reduced by an average

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of 22%. At the same time, nearby lakes contained112% more dissolved nitrogen and 27% lessdissolved carbon compared to non-outbreak years.The effects were greatest when lake catchmentscontained higher proportions of deciduousbroadleaved trees, such as oaks and maples,which the caterpillars favor over coniferous treeslike pines.

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

To get their results, researchers combined 32 yearsof government data from insect outbreak surveysand lake water chemistry in 12 lake catchmentsacross Ontario, Canada, and satellite remotesensing data on forest type and monthly leaf areacover. The results are published today in thejournal Nature Communications.

This is believed to be the most extensive study everundertaken into how insect outbreaks impactfreshwater carbon and nitrogen dynamics. Previousstudies have been so small that it has been difficultto extract wider generalities.

A previous 26-year study of 266 lakes across thenorthern hemisphere has shown that carbon isnaturally accumulating in these lake waters, in aprocess called browning. The trend is attributed toa variety of factors including climate change, andrecovery from historical acid rain and loggingactivities. Comparing the new results to this datashowed that an outbreak of leaf-munchingcaterpillars can effectively offset an entire year's

worth of carbon accumulation in nearbylakes—significantly improving water quality.

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

In years without outbreaks of leaf-eating insects,carbon and nitrogen entering lakes usually comesfrom decaying leaf and needle litter, and peaks inquantity in autumn. In outbreak years, the studyfound that nearby freshwater lakes contained anaverage of 27% less dissolved carbon.

"Outbreaks of leaf-eating insects can reduce thecarbon dissolved in lake water by almost a thirdwhen the trees around the lake are mainlydeciduous. It's just amazing that these insects canhave such a pronounced effect on water quality,"said Sam Woodman, a researcher in the Universityof Cambridge's Department of Plant Sciences andfirst author of the report.

He added: "From a water quality perspectivethey're a good thing, but from a climate perspectivethey're pretty bad—yet they've been completelyoverlooked in climate models."

More information: Forest defoliator outbreaksalter nutrient cycling in northern waters, NatureCommunications (2021). DOI:10.1038/s41467-021-26666-1

Provided by University of Cambridge

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APA citation: Hungry caterpillars an underappreciated driver of carbon emissions (2021, November 3)retrieved 14 January 2022 from https://phys.org/news/2021-11-hungry-caterpillars-underappreciated-driver-carbon.html

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