It’s Spider Season. Here’s What An Agricultural Biologist Wants Every Hiker to Know.
I was running on my local trail system one warm September morning when something caught my eye. On the concrete wall of a pedestrian overpass sat a large spider, legs stretched out from its glossy black body. It was probably an inch and a half across from the tip of its back legs to its front. I recognized it without even having to see the red hourglass on its stomach: a mature female western black widow. It was the latest of about a half-dozen widows I had spotted in the two weeks prior—more than I had seen in the rest of the year leading up to it.
Across the U.S. and the rest of the Northern Hemisphere, it’s spider season—the time of year around late summer and early fall when arachnids suddenly seem to be everywhere. For hikers, the change is especially notable. You may have noticed wolf spiders scuttling across the forest floor or picked silk out of your hair after walking face-first one of the many webs festooning the trail. What’s behind the boom?
In a word: love. Or sex, anyway.
“At this time of the summer in Colorado, that’s when the males start wandering and looking for females,” says Jackie Billotte, a research associate at Westminster, Colorado’s Butterfly Pavilion and PhD candidate in agricultural biology at Colorado State University. “Mating becomes the primary goal. So, you’re just bound to run into males that are out looking for a good time, for lack of a better term.”
According to Billotte, there aren’t actually more spiders during spider season: arachnid populations stay fairly steady year-round, with hatchlings of some species overwintering in their egg sacs. It’s just that for most of the year, we don’t notice our eight-legged neighbors.
Spiders, by their nature, don’t spend a lot of time in the open. Most are ambush predators. They either stay in their webs or hunt within a very small range. When males go through their final molt and reach sexual maturity, however, their priorities change. After charging up their palpal bulbs—two swollen reproductive organs mounted on their pedipalps, near their faces—with sperm, they go on the hunt for a partner, prioritizing their reproductive urges over food and safety. Once they’ve found and coupled with a female, the males of some species, like black widows, will damage her web to prevent rival males from accessing her.
After that, their time on earth is limited. While female spiders may survive a few years—and some, like tarantulas, can hang on for decades—Billotte notes that the mature males of most North American species will either end up as prey or succumb to cold temperatures. Others may even end up as food for their mate.
“The Tinder scene for spiders is a little rough,” says Billotte. “The term is sexual cannibalism, and it’s exactly what it sounds like. It’s not an all-the-time thing, and it varies by species…but it does happen, and it makes sense evolutionarily because it’s a free meal.”
We’re also more likely to notice both male and female spiders in late summer and early fall just because they’ve had more time to grow. Newly hatched spiderlings tend to be tiny—sometimes nearly invisible to people. By September, however, they’ve bulked up, aided by access to plentiful food and good weather.
“During the summer, they have more prey availability,” Billotte says. “It’s warmer, so their metabolisms are up, so they can grow faster. It’s just a good condition for them, and that’s when the males are going to go through that last growth spurt.
Should You Be Worried About Spiders on Your Next Hike?
We’ve established that by the beginning of fall, spiders are giant and horny. But that doesn’t mean hikers need to be worried about them.
Billotte says that spider bites are far rarer than most people think. (“People” includes physicians. One 2016 study in the journal Toxicon concluded that nearly 80 percent of spider bite diagnoses by physicians may be wrong.)
While almost all spiders have venom, their bites evolved to capture prey, not harm people. Venom is metabolically expensive to produce, and spiders spend a lot of energy to make it, and can’t afford to waste it. As a result, they generally only bite as a last resort to avoid being crushed. Confronted with a giant mammal thousands of times their size, they’ll usually freeze or run away instead. Here in Colorado, Billotte says, hikers are likely to run into wolf spiders, orb weavers, and roly-poly hunting woodlouse spiders, none of which have a venom powerful enough to harm a human.
Even bites from North America’s two most potent varieties of spider, the recluses and black widows, rarely do real harm. While Billotte says hikers with serious pain or swelling, nausea, or other symptoms after a bite should see a doctor, she says that most bites resolve on their own without treatment. (According to data from America’s Poison Control Centers, there has not been a death from a confirmed black widow bite in the United States since 1983.)
“I would never tell anyone if you get bit and it’s getting red and angry, don’t go to the doctor,” Billotte says. “But are you immediately going to die? No, probably not.”
While it may not feel like it to the spider-averse, getting to see North America’s roughly 4,000 species of spiders hunting, courting, and mating in their natural habitat is an opportunity that Billotte says she hopes hikers of all ages can learn to enjoy.
“I had the privilege to get taken out as a kid and go hiking and camping,” she says. “I had parents that encouraged me to investigate when I was out there, and that became my job. So, you never know. Your kid could be some weird girl…that’s obsessed with bugs, like all of us are.”
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