My doctorate research mostly focused on the invasive ant, Nylanderia fulva, also known as the tawny crazy ant (TCA). This species is native to South America where it isn’t widely considered a major pest in its native range. But, in the southeastern US where this species has been introduced, the absence of predators and lack of other natural factors to keep populations in check have allowed TCA colonies to dominate in some areas, reaching plague-like numbers where workers literally blanket the ground.

 

 

Once colonies reach these population numbers, they become nearly impossible to control with conventional methods. Dead and dying workers can even pile up in the thousands and completely cover treated areas, becoming a sort of corpse-bridge for other foragers to safely cross without ever coming in contact with a treated surface.

The overarching goal of my research was to investigate how the TCA was able to gain a competitive advantage over ant species and potential predators to become such a dominant force in a non-native environment. The hope was that this information could be used to improve our control efforts by gaining insight into the seasonal ecology of this invasive ant.  

One surprising behavior being observed in the field at the time was that TCA workers were outcompeting and displacing a larger and more aggressive invasive ant in the southeastern US, the red imported fire ant (RIFA). What made this such an incredible feat is that the RIFA is widely considered one of the most aggressive and damaging invasive ants in the world. This was pretty remarkable because, up until this point, there really weren’t any other ants (invasive or native) that could put up much of a fight against the RIFA.

 

 

Armed with some interesting field observations, anecdotal evidence of aggressive displacement, and some background history on this newer invasive ant, I set out to explore the factors that could contribute to the TCA’s competitive success in the field. To do this, I laid out a series of experiments that focused on different aspects of competitive advantage such as body size and thermal tolerance. Or, an organisms ability to tolerate hot and cold temperatures. One of my first experiments, and the focus of this blog, was to investigate how a smaller ant species like the tawny crazy ant was able to find success against its larger competitor. To do this, I looked at how both species handled the cold and the heat.

Both the TCA and RIFA are from South America where temperatures are generally warmer than what they experience in many parts of the Southeastern US. Being able to handle the cold or heat better than your competitors is a major advantage in the ant world. The hot and cold temperature limits of an organism are known in ecology as the critical thermal minimia (CT_min) and critical thermal maxima (CT_max). If you are more cold-tolerant than other ant species in your area, then you can start looking for food sooner in the morning or later into the colder months when temperatures may be too cold for your rivals to endure. And, the same applies for tolerance to hotter temperatures. My goal was to test the cold and heat tolerance of both species to determine who had the advantage here. My hypothesis was that the RIFA, the larger bodied ant, would be more tolerant to both temperature extremes.

To find out if temperature tolerance could be what was giving the TCA a competitive advantage, I exposed workers of both species to increasing or decreasing temperatures and recorded when workers of each species would reach their limit. By that, I mean I recorded when the ants would collapse and not move. Importantly, I also tested their temperature tolerance at three different rates of temperature change, meaning that I changed how fast or slow the temperature would change. This gave me a more accurate picture of not only what the thermal limits of each species were, but also how well they tolerated different rates of temperature change. This allowed me to better estimate how each species may respond to more ecologically relevant conditions that would be closer to what we see in nature without actually running experiments outside.

What I found wasn’t exactly surprising. The TCA had a narrower thermal tolerance range than the RIFA, meaning that RIFA workers were able to withstand both colder and hotter temperatures than the TCA. This wasn’t surprising because RIFA workers are larger on average than TCA workers.

So, the outcome of the study didn’t point to TCA being a better competitor for food at any thermal extremes. Later studies completed by other researchers working on this species determined that the TCA actually has the ability to detoxify the RIFA’s venom when workers fight. While there is still a wide gap in knowledge surrounding the TCA and why it’s so successful in the US, the data we have so far points to sheer numbers, high reproductive rates, and escape from natural predators as keys to success. Though, our data on temperature tolerance does help to shed more light on the seasonality of this species,  giving PMPs more information on when control measures should begin each year to help gain the upper hand on this species.  

Article by Mike Bentley, PhD, BCE

References

M. T. Bentley, D. A. Hahn, F. M. Oi, The Thermal Breadth of Nylanderia fulva (Hymenoptera: Formicidae) Is Narrower Than That of Solenopsis invicta at Three Thermal Ramping Rates: 1.0, 0.12, and 0.06°C min ⁻¹, Environmental Entomology, Volume 45, Issue 4, August 2016, Pages 1058–1062, https://doi.org/10.1093/ee/nvw050

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