MISSOULA, Mont., Sept. 8 (UPI) -- In Montana and other parts of the Northwest, hobo spiders are migrating inside as cooler weather arrives. The influx can be an unpleasant late-summer surprise for unprepared homeowners.
To curb the invasion, one man in Missoula, Montana, set up spider traps near cracks and crevices inside his home. The results were frightening. As his photos revealed, dozens of hobo spiders -- sometimes called "aggressive house spiders" -- ended up trapped in the cardboard contraptions.
Josh McCloud, owner of HoldFast EnviroPest Solutions, an organic extermination service in Missoula, told local ABC-affiliate KTMF-TV the spiders often show up in large numbers.
"They can just blow up in population, and I find that every home I walk into there's evidence that there has been or there are hobo spiders there, so they are very common," said McCloud.
The spider has no natural predators, and McCloud believes recent warmer-than-unusual winters have allowed local populations to grow.
"We had a 60-degree day in February, and we had some really warm winter, and I think that kind of contributed, that gives them a larger growing season," McCloud said. "They tend to hunker down during the winter, you don't really see a lot of activity."
Winter, unfortunately, is a few months away. As autumn brings cooler temperatures, the spiders will continue to show up indoors in the Northwest.
"About the end of July until about first of September that's their mating season anyways, so they're out there being a little more aggressive and that's when they are looking for their mate, their food," Heath Edwards, who runs a pest control business in Idaho, told an ABC-affiliate KIDK-TV. "Then September comes, it gets a little colder so they start migrating in the house just to keep warm."
The CDC has previously listed the hobo spider as one of three poisonous spiders -- along with the black widow and brown recluse -- in the United States. But many biologists question the evidence linking the spider's bite with instances of necrosis, the death of cells around a bite. The spider is considered harmless in Europe, and comparisons of "venom" from the two spider populations have revealed so significant differences.
A 2014 study by arachnologists at the University of California, Riverside, found no evidence that hobo spider bites have caused necrosis. Study authors concluded the fear surrounding hobo spiders had been overblown by false reports and confused diagnoses.
Poisonous or not, experts say those who want to avoid a surprise encounter with a hobo spider should keep their living space clean and devoid of clutter. The spiders especially love piles of laundry.