Insect score card reflects winners, losers
The drought conditions that have swept through Colorado and the wider region are affecting the animal kingdom in various ways. For longer-lived large animals the exact extent of those effects might not be clear, but for many insects the impacts have been obvious and immediate.
Some of Colorado’s most damaging and beneficial insects — from bark beetles to agricultural pests to biocontrol agents — have responded in rapid and sometimes surprising ways to this year’s exceptionally hot and dry weather, according to local entomologists.
Because of the multitude of factors that can affect a given species’ population, they said, the impacts of one mild winter or early spring can be notoriously hard to judge, but for some insects the effects can already be seen.
“Given that we have tens of thousands of species of insects here, I fully expect (the drought) will enhance some populations and detract from others,” said Bob Hammon, an entomologist with the Colorado State University Extension Office in Grand Junction.
Among those thriving in the drought are bark beetles, who have been able to accelerate their decadelong binge through pine and spruce trees. In farmers’ fields, spider mites, which suck the juice from the leaves of corn and other crops, are among the agricultural pests that thrived following the mild winter and early spring.
“Bad” species are not the only ones that have been soaking up this year’s conditions, though. Tamarisk leaf beetles, bred and released to attack the invasive tamarisk trees choking stream banks, got a head start on their defoliation work after an early spring allowed them to emerge three weeks ahead of schedule.
Even with fast-reproducing insects, though, the effects of a single year might be hard to judge if not looked at in the context of earlier years, such as last year’s exceptionally wet winter.
“For a lot of these things, you have to look at them over multiple years. There are multi-year cycles,” Hammon said.
The full score card of bug winners and losers and their impacts on crop yields, forests and habitat may not be fully known until much later, but it has already been a clear bumper year for some.
For bark beetles, this year’s mild winter and early spring were only a continuation of a trend that started about 15 years ago, said Roy Mask, a U.S. Forest Service entomologist at the agency’s Gunnison office.
More shorter, milder winters have encouraged bark beetles populations to explode throughout western North America.
And as average temperatures warm, beetles have been able to expand their range into altitudes that would have previously been too cold — 2,000 feet up in just the past couple decades, according to some studies.
The clear role of these climatic shifts in the beetles’ spread has led many to point to the red and gray stands of beetle-damaged trees as one of the most obvious signs of climate change.
Research in northern Colorado and southern Wyoming has been able to attribute the mountain pine beetle’s population explosion there to warmer winters, Mask said.
In previous decades, the colder winters that were more typical back then would have killed many more beetles, maintaining a more sustainable equilibrium between the beetles and the trees they attack.
“And since they aren’t dying off, the populations are exploding,” Mask said.
A recent study by University of Colorado-Boulder researchers found that some populations of pine beetles are now reproducing twice a year, rather than once, as used to be the case, since warm temperatures are now allowing them to emerge in the spring in May or June rather than late July.
Some populations of spruce beetles also “seem to be growing quite rapidly,” Mask said, including in the Rio Grande and San Juan national forests.
Despite speculation to the contrary, recent studies have shown that those beetles’ reproductive cycles have not yet accelerated, he said.
But he also said the parasites and predators that would normally keep pine and spruce beetle populations in check just have not been able to keep up with the massive population explosion that has taken place over the past two decades.
They will likely get another generous boost this year.
Mask also pointed to the western tent caterpillar, which chows on aspen leaves in southern Colorado’s forests.
“It hasn’t seen massive population explosions for probably 30 years or more,” he said.
“But last couple years, it has.”
Other species are seeing much shorter-term, temporary explosions in response to this year’s conditions.
Just like in 2002 and 2003, spider mites are exploding in Mesa County, Hammon said.
“They love and hot and dry,” he said.
The microscopic pests can be found on the undersides of leaves, including in corn fields, where they extract the leaves’ moisture, leaving them crispy and dead.
Under severe conditions, he said, they can kill the crops.
But Hammon said almost all local cornfields are treated with miticides for protection, though it can be expensive.
Tomatoes have felt the heat, too, through viruses transmitted by leafhoppers, he said. Leafhopper populations have exploded this year, which Hammon blames on a mild winter in the southwestern U.S., where they overwinter.
For three years in a row in 2002 and following, Hammon lost 100 percent of his roma tomato crop at the Orchard Mesa agricultural research center.
He expects to lose about a quarter of the crop untreated with pesticides this year.
“Every single insect we work on came out early this year,” said Dan Bean, who runs the Palisade Insectary.
The insectary rears and distributes bugs that attack invasive species.
Those species include the oriental fruit moth, which has been affecting peach yields in the Grand Valley since the 1920s.
The moths came out so early this year, the insectary was a bit unprepared, Bean said.
His facility distributes a parasitic wasp to limit the moth population, but the production of those wasps is timed to when the moths typically emerge.
This year, they were a bit later, but the insectary “pushed up production as much as possible,” Bean said.
He also pointed out that because of the head start the moths got from the early, warm spring, they might have time to reproduce an extra time before this year’s winter.
The beneficial insects the insectary works with became active early this year, too.
The insectary has been raising and releasing tamarisk leaf beetles, a natural predator of tamarisk trees in the trees’ native Eurasian range, as a “biocontrol agent” with the hopes the beetle will eventually cut into the invasive plant’s spread.
This year, tamarisk beetles became active three weeks earlier than usual and, since there were almost no cold spells after they emerged, were able to get started earlier than usual at their mission of defoliating tamarisk, Bean said.
He thinks this head start will allow the beetles to make more progress this year than in previous years — though he is quick to note how cautious he is to say anything good at all about a drought.