Armies of Ants

When the California Academy of Sciences announced plans to bring a colony of army ants from Trinidad to Howard Street in San Francisco, people in the neighborhood weren’t convinced this was such a great idea. What if the creatures got out of their supposedly escape-proof quarters and overran the Financial District? Not likely, given the tropical origins of the insects, but I can’t blame these folks for their trepidation. We’ve all seen the movies of army ant hordes swarming through the rainforest, devouring everything in their path, “the Huns and Tartars of the insect world.”

What I didn’t know at the time is that the army ants established a beachhead in California long before there was an Academy of Sciences. According to the Academy’s AntWeb site, the Bay Area has two native army ant species, Neivamyrmex nigrescens and N. californicus. Nigrescens occurs as far north as Sacramento and is widespread in the Southwest and South. Unobtrusive creatures that live and hunt mostly underground or by night, our army ants do what their tropical kin do—but on a smaller scale. Their colonies number in the tens of thousands, not the millions, and they prey on other ants and termites.

Neivamyrmex ants—117 known species, most tropical—have been around for a while. The oldest known fossil army ant is a Neivamyrmex, embedded in 20-million-year-old Dominican amber. Their lineage evolved in South America and dispersed north after the Central American land bridge was in place.

Like the Academy’s tropical Eciton ants, Neivamyrmex ants have no fixed address. Every 17 to 20 days their colonies cycle between a nomadic phase, with a different bivouac every night, and a statary phase when the workers make daily raids from a temporary base. (E. O. Wilson and Bert Ho¬lldobler prefer to call the first phase “migratory” rather than “nomadic,” since army ants, unlike some Asian ant species, don’t travel with their livestock). The shift from statary to nomadic seems to be triggered by a shortage of food in the neighborhood. Working with laboratory colonies of nigrescens in the ’70s, Howard Topoff and John Mirenda found that overfeeding delayed emigration to a new base.

When these ants move, the whole colony comes along—brood (eggs, larvae, pupae) and queen, as well as workers. In most ants, winged queens fly off to found new colonies. But Neivamyrmex queens are wingless. When potential queens emerge from their pupae, workers select one contender as matriarch of a new horde. The colony splits, and the new queen marches away with her prospective mates, her personal retinue of workers, and the rest of her troops.

Lacking functional eyes, Neivamyrmex workers follow chemical trails. Foraging workers lay down pheromones by dragging their abdomens along the substrate. The chemistry of the substance is complex and may involve two components, with one unique to each species. Normally the trails lead to and from bivouac sites and food sources; but the ants sometimes get stuck in a circular pattern and continue milling until they march themselves to death.

Other noses pick up on those trails. A family of snakes, the Typhlopidae, specialize in eating ant brood. There’s one of these secretive, sightless burrowers—the western blind snake, Leptotyphlops humilis—in Southern California, although the behavioral studies I’ve seen involve a close relative, the Texas blind snake, L. dulcis. Texas blind snakes have been observed crawling along with Neivamyrmex nigrescens raiding columns, heading back toward the nest; it’s assumed they were after either the army ants’ brood or the booty brought back from a raid. In laboratory settings, the snakes are able to follow the ants’ pheromone trails.

The ants ignore the reptiles in their midst, leading to speculation that the snakes had acquired the colony odor. Blind snakes may get a secondary benefit from the association, since the ants’ scent appears to repel other snakes—including snake-eating snakes like racers and coachwhips.

No one seems to know whether western blind snakes have a similar relationship with army ants, and it may be too late to find out.

Neivamyrmex ants may be the terrors of the termite nests, but they’re pushovers for the invasive Argentine ant, Linepithema humile. Argentines, established in California since 1905, have displaced native ants wherever they’ve spread. Their supercolonies overwhelm the natives; with more workers, they’re better at finding food, and they outfight natives when foraging parties meet at a food source.

Recent field studies by Andrew Suarez, Ted Case, and Douglas Bolger in San Diego County found that Neivamyrmex army ants were among the native species most sensitive to Argentine takeovers. It’s those earthbound queens that make them vulnerable. Other ant species can disperse by sending out new queens on nuptial flights into Linepithema-free space; but army ants are stuck on the ground, at the mercy of the invaders.

So where does this leave the blind snake? Some ant-eating reptiles, notably horned lizards, find Argentine ants unpalatable and un-nourishing. The western blind snake may also be suffering from the loss of its traditional prey base, including the brood of Neivamyrmex.

Joe Eaton writes about wildlife for the Berkeley Daily Planet.

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