Rising temperatures have helped mountain pine beetles (a species of bark beetle) to invade high-elevation trees that are not strong enough to stop them, finds a new study.

Insects like beetles kill and feed on large number of trees. In turn, trees use chemicals to defend and ward off insects.

But this new study by a team of scientists from the University of Wisconsin-Madison has found that the warming temperatures have resulted in mountain pine beetles increasingly invading high-elevation forests, like the whitebark pine forests of the northern Rocky Mountains, and attacking them. These trees in high elevations have not evolved any defenses to protect themselves.

"Warming temperatures have allowed tree-killing beetles to thrive in areas that were historically too cold for them most years," Ken Raffa, a UW-Madison professor of entomology and a senior author of the new report, said in a statement. "The tree species at these high elevations never evolved strong defenses."

The attack by mountain pine beetles is posing a major threat to the forest ecosystems of the whitebark pine forests. They offer better habitats for grizzly bears and play a significant role in managing hydrology of the mountain west by shading snow and regulating the flow of melt water.

The mountain pine beetle's historic host is the lodgepole pine that is found in lower elevations. Earlier beetles attacked those pine trees that were either old or weak. As temperatures increased, there has been a population boom among the insects. There has been a significant rise in the infestation of mountain pine beetles.

According to the researchers, lodgepole pine trees co-evolved with bark beetles. Hence, the trees have developed a defense system such as using strong chemicals and toxic compounds to keep off insects.

Despite having a strong defense system, the mountain pine beetles prefer feeding on lodgepole pine trees suggesting that they have not yet adjusted its host preference to whitebark pine. This could mean that in mixed stands where both lodgepole pine and whitebark pine co-exist, the whitebark pine might persist at least in the short term.

The findings of the study appear in the latest issue of Proceedings of the National Academy of Sciences (PNAS).