Researchers suspected something was different in the pine beetle population.
Jeff Mitton and his team from the University of Colorado (Boulder) found a complete new generation of beetles living from June through August. Before this, the bugs weren't believed to be flying this early.
"Although we view this as extraordinary because of the visual impact and impact that it has on the landscape, many insects are showing similar responses," Mitton said.
Beetles can lay around 60 eggs in a year, so an extra generation would spawn a very large number of grandchildren in one summer. With closely-packed pine stands, the beetle armies swell.
"But it's an exponential increase in how many survive and therefore a whole lot more trees are getting hit," Mitton said.
Pines use resin to push the beetles out of their bored holes. Just a few beetles cannot overtake a big tree, but…
"The beetle wins when enough beetles attack a single tree that the resin pressure goes to zero," Mitton said.
Drought dehydrates the pines and keeps them from making defensive resin. The Black Hills are still in a drought.
"Springtime's coming earlier, fall's coming later, the summers are longer, but they're not necessarily wetter. And so the trees are seeing a longer, and the way they measure it, a drier year, because they have the same amount of water and they are trying to grow for a longer time now," Mitton said.
If groves become too dense, plants have to compete for water even more and beetle families tend to cluster in these areas anyways. Earlier research from colder Canadian zones found beetles that once took around three years to grow can now do so in one.