Destroying Crazy Ant Nest Structure Makes Them Vulnerable to Pathogens
Research initiated at a UT field station keeps progressing, in good news for the war on this invasive species.
Tawny crazy ant workers tend larvae. Credit: Edward LeBrun/University of Texas at Austin.
Invasive tawny crazy ants have been wreaking havoc across the U.S. Gulf Coast from Florida to Texas, disrupting ecosystems and causing headaches for homeowners. Now scientists at The University of Texas at Austin have devised a reliable way to introduce a natural pathogen in colonies, so that local tawny crazy ant populations collapse and other native species can recover.
In a recent paper in the Journal of Animal Ecology, the researchers describe key insights they gleaned from laboratory studies that helped them refine their methods.
More than a decade ago, researchers at UT’s Invasive Species Lab discovered that some tawny crazy ants in Florida were infected with a natural pathogen called a microsporidian. This pathogen reproduces inside the cells of tawny crazy ants, and can only transmit when infected, adult worker ants care for developing larvae. It also leaves native ants and other arthropods unharmed, making it an excellent candidate biocontrol agent.
Tawny crazy ants form supercolonies with all infestations in the southeastern US sharing the same genetic supercolony identity. Crazy ants from anywhere in the region will accept workers from anywhere else into their nest as colony mates. In principle, transmitting this disease between infestations should be as simple as introducing live, infected workers from a distant, infected local supercolony into an uninfected local supercolony. However, the researchers faced a puzzle. In practice, their efforts to introduce the disease into uninfected crazy ant populations in nature frequently failed, while, in the laboratory the technique always worked.
Ants employ social immune behaviors to prevent soil borne, insect killing, externally infecting fungi from spreading among nestmates. LeBrun and his fellow researchers hypothesized that crazy ants were using similar behaviors to prevent infected workers from reaching the core of the colony and caring for larvae. They suspected that the complexity of natural nest architecture was the key difference in transmission between the laboratory and field nests.
Nest space organizes ant colonies, allowing them to segregate workers specialized on separate tasks—like brood care, or corpse removal, or foraging—into separate areas. This spatial segregation of workers might allow social immune behaviors to quarantine diseased individuals at the periphery and prevent disease migration to the colony core.
A simple test proved the concept. Colonies occupying multi-chambered nests were able to prevent infection from reaching the developing brood while those in standard single chambered nest boxes were not.
“This is the first demonstration that in ant societies nest spatial structure is of primary importance in allowing social immune behaviors to prevent diseases from reaching the colony core, where the queen and brood live,” said Edward LeBrun, a research scientist in UT Austin’s Department of Integrative Biology who led the research. It provides a demonstration of the efficacy of “architectural immunity.”
Diagram of the nest array and the simple nest box. In the array, chambers are ordered according to increasing distance from the queen chamber. Tubes were considered part of the chamber that preceded them. Credit: University of Texas at Austin.
Further work revealed the behavioral mechanisms involved. Infected and uninfected crazy ants introduced into the colony behave differently. Infected crazy ants introduced near the queen migrate to the periphery of the nest, assuming corpse removal and foraging tasks while uninfected individuals remain near the colony core. Further, infected individuals self-isolate, avoiding groups of other ants. Aggressive interactions also occur between infected and uninfected crazy ants. Potentially, uninfected ants recognize the infection. These apparently self-sacrificing behaviors may be partially policed by the workers at the colony core. In addition, infected workers preferentially remove the dead bodies of other infected ants preventing uninfected ants from contacting infectious agents.
Many of these behaviors had previously been seen in ants defending against externally infecting fungi. However, this is the first demonstration that supercolonial, invasive ants also use these behaviors and that they are useful against a broad suite of pathogens. Like the types of quarantine and social distancing policies human societies use to limit pandemics, ants have a conserved suite of behaviors they employ to defend against pathogens generally.
The insight from this work changed the way that the group attempts to introduce the pathogen into uninfected crazy ant infestations in nature. Previously infected ants were introduced into trails of ants feeding at food resources or directly into nests but with care not to damage the host nest. Now they do the reverse.
“The way we do it now is we destroy the nest—we just tear it up—and then we introduce infected ants,” LeBrun said. “So then all the ants are mixed up and they’ve got to go move house and find a new environment. And they all have to do that together, both infected and uninfected ants, and so it helps get past that period of both self-isolation and only doing tasks near where they were introduced.”
“These results, and others not in the paper, have allowed us to reach the point where we can very reliably introduce this pathogen into uninfected local supercolonies in the field,” LeBrun said.
The study’s other authors are Alejandro Fernandez Santillana, Lowell Sekula, Cameron Macones and Lawrence Gilbert.
Work was funded by the Lee and Ramona Bass Foundation, the US Airforce in co-operation with the US Army Corps of Engineers and the US Fish and Wildlife Service, Travis County Natural Resources Division and Austin Water Wildlands Conservation Division.
