Climate change could spark insect invasion

Insatiable insects will feast upon plants and crops if temperatures warm and atmospheric carbon dioxide increases, according to a team of researchers from the US.

Ancient clues suggest climate change induced insect feast

The prediction is based on the study of leaf fossils from a period of abrupt global warming around 55.8 million years ago. This event, called the Paleocene-Eocene Thermal Maximum (PETM), is linked to a temporary increase in carbon dioxide in the atmosphere and comparable in size and rate to the current climate changes thought to be brought on by human activity.

Ellen Currano, a member of the geosciences team that carried out the work at Pennsylvania State University, said: “We looked at these time periods to see evidence of insect feeding and to count the types of damage. We looked to see how much damage the insects did and the kinds of leaves on which the damage occurred.”

The team carried out analysis on more than 5,000 fossil leaves that were ravaged by insects dating to before, during, and after the ancient bout of global warming.

“We see a huge increase in the percent of plants that are being attacked,” said Currano.

They found the number of leaves damaged by insects was 15 to 38% during the Paleocene and 33% during the Eocene, but increased to 57% during the intermediate PETM - the large increase in insect consumption corresponding to the time of increased carbon dioxide and temperatures. The researchers also found that the increased feeding occurred towards all plant species and that a more diverse array of insects fed on the leaves.

They reason that there are several explanations for the insect explosion. The warmer temperatures of the PETM allowed insects from the tropics to move north into the more temperate latitudes of the northern hemisphere – a place where they are normally limited by cooler temperatures. In addition, plants become less nutritious as carbon dioxide increases in the atmosphere, because the plants need fewer enzymes to fix carbon in their leaves. Enzymes are rich in nitrogen, which insects need for energy.

“So in order for an insect to get all the nutrients it needs, it’s going to have to eat more, and so therefore the more plant gets consumed,” Currano said.
The team say the new findings sound a cautionary note for the immediate future, as increasing amounts of carbon dioxide are pumped into the air by humans.

The Intergovernmental Panel on Climate Change predicts global temperatures are most likely to rise between 1.8 - 4ËšC by the year 2100. Human-caused emissions of carbon dioxide are “very likely” to blame for the increase, the panel concluded.

Speculating on the implications of this research for the current bout of warming, Currano said: “They think that increases in temperature and carbon dioxide levels during the PETM are good analogs for the future and therefore, that plants may eventually experience higher rates of feeding as humans put more carbon dioxide into the atmosphere.”

The paper appears in the online Proceedings of the National Academy of Sciences.