Bob Koch, an entomologist with the University of Minnesota Extension, says the harmful variety of stink bug is finding ways to live through harsh winters. (The Minnesota Star Tribune)

The brown marmorated stink bug has been marching westward across the United States for decades. It was first spotted in Minnesota in 2010 and its numbers increased substantially in the state in 2015 and 2016.

The bug smells like cilantro — or a dirty sock, depending on the nose of the beholder. But the real problem the bug poses is to agriculture.

Brown marmorated stink bugs are a threat to fruits, vegetables and crops. They are particularly damaging to apples. They feed on them by puncturing an apple's skin, leaving brown stains that make the fruit unmarketable.

In an effort to better track the invasive bug, scientists at the University of Minnesota Extension are asking people to download an app developed by the U and Purdue University with funding from the Minnesota Invasive Terrestrial Plants and Pests Center.

The app includes high-resolution photos that help people identify the bugs. They look a lot like the brown stink bug, a noninvasive species that is less of a concern to entomologists.

"The whole reason behind the app is because we're trying to figure out where this new stink bug occurs in Minnesota," said Bob Koch, an entomologist at the U Extension. "People can distinguish this stink bug from other species and they can report it."

Reports from the public feed into a larger regional monitoring system. After reporting the sighting, the spotter is encouraged to kill the bugs.

The first brown marmorated stink bug was spotted, coincidentally, in a Minnesota Department of Agriculture laboratory. The bug had hitched a ride in on a shipment of lab supplies.

In other states, the bugs started to show up in homes and then spread to cause crop damage. They caused millions of dollars in crop losses on the East and West coasts. In Minnesota, they're starting to live through winter by slipping into people's homes.

"If it plays out like it did in those states, within several years we'll probably start seeing crop damage," Koch said.

One neighborhood near Interstate 35 in Wyoming, Minn., has a population of the bugs that is successfully reproducing.

The risk of massive crop damage from the bugs is not imminent, Koch said, but farmers and gardeners should be watchful. The bugs like corn and soybeans.

"They are starting to show up in apple orchards and soybean fields, and it's something that they should be scouting for because the numbers are starting to increase," Koch said. "The invasion is just beginning."

The Twin Cities metro area, with its heat-trapping density and asphalt, is the epicenter of the brown marmorated stink bug invasion.

Hundreds of the bugs are housed in tents in a closet at the U. There, entomologists are studying tiny wasps from Asia that attack the stink bug's eggs, and the effectiveness of insecticides on the bugs. They're also looking at how increasing temperatures in the state will affect the bug's population.

"The warmer summers we have, we can potentially get into more generations per year," said Bill Hutchison, another entomologist at the U Extension.

While most Minnesota winters will be too harsh for the bugs, Hutchison and his colleague, Byju Govindan, recently discovered the insect can produce two new generations per year in Minnesota. The finding indicates the bug is adapting to Minnesota's climate, and more adults may overwinter in homes and heated structures. In coming years, more crops will be at risk.

Hundreds of thousands of the bugs enter homes in winters on the East Coast, and that pattern is starting to emerge in the Twin Cities.

The app, called Midwest Stinkbug Assistant, is free for Apple and Android users. About 320 people across five states have downloaded it so far.

"The more hands and eyes we have out there beyond what our lab crews can do and what the MDA can do, the better," Hutchison said. "The term citizen science comes to mind, and an app is a really nice way to do this."

Most of the new sightings of the bugs are coming from people who see them in their homes, and Hutchison said wryly that it would be nice if 1 million Minnesotans would download the app.

"The more downloads the better," he said.

Adam Belz • 612-673-4405

1006637263 WHITEBOARD.3 (The Minnesota Star Tribune)

An app developed at the University of Minnesota and Purdue University helps gardeners and farmers identify the invasive pest. (The Minnesota Star Tribune)