Tarantula tunnels under Colorado highways could promote mating, stop squishing

Insect advocates trying to prevent the squishing of tarantulas as they seek mates on southeastern Colorado plains have asked the Colorado Department of Transportation to create safe crossing tunnels under several highways.

Hundreds of male tarantulas perish under vehicle tires this time of year as they trek from their foot-deep dens, crawling distances of up to one mile to find females.

Tiger salamanders, box turtles and other prairie species would benefit, too, proponents say, if the concrete tunnels could be installed soon.

CDOT officials haven’t committed. They’ve questioned whether the protective wildlife fencing that guides pronghorn, deer, bears and elk into highway crossings built around Colorado would work for softball-sized spiders. Wildlife overpasses can cost up to $2 million. A culvert-type tunnel under a highway would be much cheaper.

“How would you funnel tarantulas?” CDOT spokeswoman Michelle Peulen said.

CDOT’s environmental team tasked with assessing road harm to wildlife is participating in discussions with biologists from the Butterfly Pavilion, an insect zoo northwest of Denver – exploring research and habitat improvement possibilities, Peulen said. U.S. Fish and Wildlife Service spokesman Joe Szuszwalak said federal scientists “are happy to help.” However, if a species isn’t officially designated as threatened or endangered (the brown tarantula in Colorado is not) “we’re not going to be the lead agency on it.”

Bug populations worldwide are declining and scientists for years have warned of a looming “insect apocalypse” that threatens a collapse of ecosystems and the ability to grow food. A United Nations assessment in 2019 warned that half a million insect species face extinction, and recent studies have estimated 40% of insect species worldwide are declining.

In southeastern Colorado, tarantula populations (east of Interstate 25 and mostly south of Highway 50) play essential roles knocking back pests, including beetles, grasshoppers, cicadas and caterpillars.

Tarantulas also are celebrated and promoted in La Junta and other towns trying to draw tourists.

The annual mating treks made by thousands of male tarantulas are mostly one-way trips. When they reach and inseminate females, the females typically eat them – needing protein for nourishing sacks of 70 to 200 baby spiders.

A few males able to escape, breaking a pattern of self-sacrifice, usually die within a couple of months due to colder weather, said Rich Reading, vice president of science and conservation for the Butterfly Pavilion and a member of the Colorado Wildlife Commission.

A “very low fence,” bent at the top, easily could funnel tarantulas toward tunnels under highways consisting of concrete culverts already widely in use, said Reading, who reckoned research exploring where crossings would make the most sense would cost less than $60,000.

Target roadways: U.S. Highway 385 (Lamar to Campo), state Highway 109 (Rocky Ford to Kim), and Highway 101 (Toonerville to Las Animas).

“Tarantulas, because they are a top predator in the invertebrate world, are really important. They help control insect populations. They fit into the food chain. They eat grasshoppers,” Reading said. “We want to protect everything. We want to make sure we conserve small things, as well as big things. Underpasses also would provide opportunities for reptiles and amphibians to cross roads without getting smashed.”

Tarantula surveys in Colorado have found healthy densities, as many as 70 spiders within a 2.4-acre area. “We’re trying to understand why we have density in some places and not in others,” Reading said.  And Butterfly Pavilion teams are raising funds and considering installation of cameras where tarantulas cross roads.

A Colorado State University researcher has been studying the vegetation, soil and how brown tarantulas navigate remaining prairie habitat. They live alone and hunt mostly at night, popping up from their dens and pouncing on beetles and other bugs, piercing them with fangs and injecting venom. Tarantulas’ fate depends on what happens in August and September as mature males disperse from dens, limited to terrain they can reach by crawling.

That means roads and cars create challenges.

Beyond vehicle tires, tarantulas face natural predators — tarantula hawk wasps (the state insect of New Mexico). First, these dart down and sting tarantulas, injecting a paralyzing venom. Then, with tarantulas’ eight legs and front pinchers immobilized for months, the wasps drag them down into their own dens. They lay eggs on them and, as the larvae grow, tiny wasps devour the tarantulas.

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