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| Photo: Alex Wild |
In part one of our blog, the tawny crazy ants (Nylanderia fulva) were overrunning the native species at Estero Llano Grande State Park. Researchers at Brackenridge Field Lab had identified a microsporidian that could possibly control the population, but success rates were still very low.
When the researchers started the experiment at Estero, they would learn even more about the disease. Prior to the first inoculation, they believed the disease came from South America where the ants are from, but they began to see the disease popping up in uninfected populations. This would suggest it was host shifting to the tawny crazy ants from another insect here in North America.
Once they figured this out, the researchers looked at the samples collected prior to the first inoculation at Estero and found the disease in a few other locations outside of the inoculation point. This suggested the establishment of the disease in this population was both from the inoculation and what was going on in the environment naturally. It also suggested the high intensity infections were a result of what the BFL researchers did, and the infection was moving out into the lower background setting.
But how do whole colonies of these ants die from this disease? Two mechanisms may be at work. The colony is organized into castes: workers, queens, eggs, and larvae. Interestingly, queens do not catch the infection. However, there is another way their populations are impacted. Infected workers transfer the disease to developing larvae, most of which become infected workers. The larval gut is uniquely susceptible to infection. Before queens become queens, they are larvae. Infected larvae undergo stress from lack of adequate nutrition. A developing queen needs lots of nutrients in order to become a queen. But when a larva is nutritionally compromised from the infection, they develop into workers. Over time, this would lead to a shortage of queens, and then eventual colony collapse. Additionally, the disease shortens worker life span. In the fall, nests cease to produce new workers and must bridge the gap to spring with only last years workers. Infected workers don’t appear to make it to spring, leaving the colony with insufficient workers to grow.
The experiment at Estero proved successful. In one growing season during 2017, the disease went from being difficult to find to being spread throughout the entire population. Every ant nest had workers infected with the microsporidian in it. This sort of disease outbreak is called an epizootic outbreak. At Estero, the BFL researchers believe this epizootic outbreak was the result of something “bubbling” in the background and then the researchers came in and created a high-intensity infection.
The result on the tawny crazy ants at Estero was dramatic. Populations of the ants were present in spring of 2016 and fall of 2017, and then they started declining into spring of 2019 when the population became rare. They are essentially extinct at Estero now. The last time the researchers visited Estero in fall of 2019, they could not find them.
The inoculation experiment worked. The staff of Estero noticed how native species so beloved to the area were coming back, the lizards and birds were returning. Controlling this invasive species was truly a success story.
Thanks to Ed LeBrun, Research Scientist at BFL, for his assistance with this article.
