The effect of fire ant (Hymenoptera: Formicidae) venom on ecologically relevant bacteria

Authors: MORRIS, ASHLEY - University Of Florida; Vander Meer, Robert; PEREIRA, ROBERTO - University Of Florida; CHINTA, SATYA - Foresight Science & Technology; BALDWIN, REBECCA - University Of Florida

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2025
Publication Date: N/A
Citation: N/A

Interpretive Summary: The invasive fire ant, Solenopsis invicta, and the Florida-native fire ant, S. geminata, produce chemicals in their venom that have antibacterial and other antimicrobial properties. These chemicals are venom alkaloids colloquially known as the “Solenopsins.” Fire ants have evolved a variety of ways to distribute these antibacterial venom alkaloids into the environment in which they live, including throughout their nest and onto their brood (immatures). A paper was published demonstrating that soybean plants growing in fire ant infested soil did not produce nodules on their roots. This is significant due to the negative impacts this lack of nodules has on soybean health. These nodules are standard for soybean plants, as soybeans have formed a close evolutionary relationship with nodule-forming bacteria to obtain nitrogen from their environment. We hypothesized that the fire ants were distributing venom in the soil, affecting these bacteria and preventing them from forming nodules. We tested this hypothesis by growing 2 species of nitrogen-fixing bacteria and one bacterial pathogen of fire ants. We extracted the venom from both S. invicta and S. geminata, and then evaluated the ability of the venom to inhibit the growth of the 3 bacteria. The results were clear, S. geminata venom inhibited 2 bacterial species to a greater extent than the venom of S. invicta. We quantified the venom and found that more venom was in the S. geminata extract than in the S. invicta extract, which could be the reason for the differences in the extract activities. After increasing the quantity of S. invicta venom to match that of S. geminata, however, S. geminata venom was still the stronger antibiotic towards one nitrogen-fixing bacterium and the pathogenic bacteria! Therefore, we concluded that S. geminata venom has greater activity than an equal amount of S. invicta venom. It turns out that the venom alkaloid components of the two Solenopsis species are unique and likely responsible for the differences in bacterial inhibition. Bacterial growth inhibition by specific alkaloids will be the subject of future research to better understand differential venom inhibition.

Technical Abstract: Fire ants, Solenopsis invicta Buren and S. geminata (Fabricius) have evolved a variety of physiological and chemical defenses against microbe introduction and infection. These mechanisms fit into what is broadly called social immunity. Compounds of most interest are the 2-methyl-6-alkyl or alkenyl piperidine alkaloids found in S. invicta and S. geminata. Solenopsis geminata also has specific pyridines not found in S. invicta. The alkaloids are produced by the poison gland, stored in the venom sac, and released through the sting. These compounds have antibiotic, antifungal, antiparasitic, antiviral, and hemolytic properties. We hypothesize that fire ants alter the microbiome of their environment through the consistent use of these chemicals in and around their nests. In this study, S. invicta and S. geminata extracts were tested in disc-diffusion bioassays against the nitrogen-fixing soybean symbionts Bradyrhizobium japonicum and Br. elkanii and the microbial insecticide Bacillus thuringiensis (Bt). The venom extracts inhibited the growth of all tested microbes, with S. geminata extracts having a greater inhibitory effect than S. invicta.