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Friday, October 22, 2021

The insect invasion

Across the state of California stretches one of the world’s largest invader colonies: Argentine ants.

“In their native range, [Argentine ants] live in colonies about the size of a room. But in their non-native range, they’ve become an agricultural and household pest,” said Rick Grosberg, professor of evolution and ecology at UC Davis. “Now they live in colonies the size of the state of California or central Europe.”

Argentine ants, also called sugar ants, are remarkably adaptable to unfriendly terrain like housing developments and newly-plowed fields, so the area they can inhabit is enormous.

But the ants may have met their match in the students of the Collaborative Learning at the Interface of Mathematics and Biology (CLIMB) program at UC Davis. Under the guidance of Grosberg and Professor Sebastian Schreiber, and with a grant from the National Science Foundation, seven undergraduates spent the past year devising a way to control the invaders. Each year’s CLIMB cohort chooses its own research question, and this year’s group has targeted Argentine ants.

The small, dark ants form a “supercolony” that can decimate native species as it spreads across the globe. Normally, each ant carries a waxy hydrocarbon “tag” on its body that identifies its colony affiliation, letting its nest-mates know it’s one of them and its enemies know it’s unfriendly. Under this territorial system, competing colonies keep each other in check.

But when the first few ants arrived from central Argentina, they seeded a new population with less genetic variation among tags. This narrowing of the genetic pool, known as the bottleneck effect, seems to be the reason that an ant from Davis and an ant from San Diego will still recognize each other as friends.

“They can beat up native ants because of their sheer numbers,” said Ivy Gardner, senior biological sciences major and CLIMB participant.

One problem for researchers has been trying to introduce new tags without the mob of original ants attacking the new ants.

After months of trials and adjustments, the students presented their results in a one-day workshop on Sept. 17. They found that to create the kind of monstrous colony now living in Calif., a bottleneck would have to eliminate 60 percent of the variation in hydrocarbon signals.

The group found that changing environmental factors made only a small bottleneck necessary. In particular, raising the carrying capacity of the land – adding more space, more resources – affected whether the colony stayed united or broke into smaller groups. With a higher carrying capacity, the team’s model predicted it would only require a tiny genetic loss to create the “supercolony.”

As for how the ants migrated originally, genetic clues implicate international sea trade: the ants of the “supercolony” are all genetically similar to those living near Argentina’s Paraná River. In the late 19th century, trading ships would dock there on the way to the southern U.S.

“They would scoop up dirt to use as ballast on ships, and that’s probably how [the ants] got established here,” said Matt Wein, senior genetics major.

For now, a solution eludes the CLIMB team. Some researchers have successfully coaxed ants to fight each other by coating one ant in “foreign” signal hydrocarbons, but applying these waxy substances to whole colonies is problematic.

“Wax is pretty hard to aerosol,” said Wein.

If the “supercolony” develops to fill new territory, the similar hydrocarbon signals might not be the only issue. Finding a solution will involve looking at both ecological and genetic factors. Wein, Gardner and some of their colleagues plan to continue this project independently.

Find more information about the CLIMB program at climb.ucdavis.edu.

Emily Goyins can be reached at science@theaggie.org.

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