Many temperate deciduous forests suffer an attack from gypsy moths around once every five to 10 years. During such an outbreak the gypsy-moth eggs hatch in spring, with the larvae emerging just as the leaves start to bud. Oak leaves are their favourite, but any broadleaf will do, and on average they manage to munch their way through 40% of the forest before the larvae enter their pupal stage around July.

In order to assess what kind of impact these moth outbreaks might have on carbon storage, David Medvigy from Princeton University, US, and his colleagues carried out a series of model simulations. Using recent observations from the Silas Little Experimental Forest, an oak and pine forest located in Pemberton Township in the Pine Barrens of southern New Jersey, US, they investigated the effect of moth outbreaks on forest carbon dynamics.

The results showed that gypsy-moth outbreaks cause a huge dent in a forest's ability to store carbon. "Other things being equal, we find that an oak-pine forest in the New Jersey pinelands that was unaffected by the gypsy moth would store about 50% more above-ground carbon than a similar forest that was defoliated every five years," said Medvigy, whose findings are published in Environmental Research Letters (ERL) as part of the ERL Focus on Extreme Events and the Carbon Cycle.

What is more, moth outbreaks may make forests more susceptible to drought, with moth-ravaged trees less able to withstand water scarcity. And the gypsy moth's preference for oak may be reshaping forest diversity, giving pine trees a competitive edge in oak-pine forests.

For North America these moths are a major problem. Originally indigenous to Europe, the gypsy moth was introduced to North America in 1869, with the aim of developing the silkworm industry. Some of the moths were released accidentally and within 10 years they had become a major pest. Today the moths decimate millions of acres of woodland every year and cost the economy millions of dollars in damages.

Worse still, climate change may make these epidemics more widespread. "In general, as the climate warms, the range of the gypsy moth has the potential to shift northward, both in North America and in Europe," Medvigy told environmentalresearchweb. Increases in rainfall could help give the moths a foothold in the western US, and warmer temperatures may put areas of high elevation (such as the Rocky Mountains) within reach.

This study only looked at gypsy-moth data, but other insect pests may also be having a significant impact on forest carbon storage. Previous research has indicated that mountain-pine-beetle outbreaks in British Columbia, Canada, can convert forests from being carbon sinks to being carbon sources.

As a result there is no guarantee that forests will continue to mop up our carbon-dioxide emissions in the future. "Insect outbreaks are very important," said Medvigy. "Changes in the frequency and/or intensity of insect outbreaks will change the balance of carbon between the land and the atmosphere. Consequently, predictions of future climate can be improved by considering the impacts of insect attacks."