Solenopsis invicta Buren, the red imported fire ant (RIFA), is a destructive invasive ant species that has spread around the world. Due to the devastating effects this ant can have on nonnative environments, researchers are constantly searching for new clues on how to better control it. 

Two common methods of control for pest ants are insecticidal baits and contact insecticides. Since ant behavior can differ wildly between species, understanding how ants respond to different products can provide clues that researchers can use to improve the effectiveness and application methods of these products in the field. The authors of this research paper set out to do exactly that by evaluating the behavioral responses of RIFA workers to surfaces treated with various insecticides.  Specifically, they looked at beta-cypermethrin, thiamethoxam, fipronil, indoxacarb, chlorfenapyr, spinetoram, rotenone, avermectin, and chlorantraniliprole.

In previous studies, the authors observed RIFA workers covering wet, sticky, or repellent surfaces with soil particles to avoid direct contact with the treated surface. Their goal in this study was to evaluate if foraging RIFA workers would respond similarly when exposed to various insecticidal active ingredients. And, if workers did cover a treated surface with soil, would that be enough to reduce the effectiveness of the treatments. This isn’t all they looked at in this study, but I’ll only be focusing on these experiments in this review. 

In a series of laboratory and field trials, the researchers observed RIFA workers foraging for food that was placed on a treated surface. During the experiments, they measured 1) the number of foragers that would travel on the treated surfaces and 2) how much sand (if any) was brought onto treated surfaces. What they found was that surfaces treated with beta-cypermethrin or rotenone significantly reduced the number of foraging ants. And, RIFA workers transported significantly more particles onto surfaces treated with fipronil (at 50, 500, and 5000 µg/mL), rotenone (5000 µg/mL), or avermectin (5000 µg/mL) when compared to controls. Lastly, they found that when surfaces treated with fipronil or rotenone were covered by particles, RIFA workers had a higher survival rate.  

There were a few key takeaways from this study for me. First, the RIFA has an interesting behavioral avoidance strategy that can reduce the efficacy of some contact insecticides, and that this particle-covering behavior appeared to be concentration-dependent. Meaning that the workers were only observed moving particles onto the treated surface when higher concentrations of active ingredients were used. This to me would suggest that even non-repellents such as fipronil may be detectable by RIFA workers and their efficacy could be impacted at higher rates. Further reinforcing the point that, when targeting ants, the product you apply and how you apply it truly matters in the success of your control program. Ants are notoriously difficult to manage, and don’t need our help making that task any harder than it already is. 

Lastly, we still have more to learn about the behavior of the pests we manage, even on an organism as well studied as the red imported fire ant.  

Article by Mike Bentley, PhD, BCE

References

Wen, C., Shen, L., Chen, J. et al. Red imported fire ants cover the insecticide-treated surfaces with particles to reduce contact toxicity. J Pest Sci 95, 1135–1150 (2022). https://doi.org/10.1007/s10340-021-01474-0

Image Credits

Red Imported Fire Ant – Solenopsis invicta, Givhans Ferry State Park, Ridgeville, South Carolina. Photo by Judy Gallagher, Flickr.com

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