Red harvester ant (Pogonomyrmex barbatus) preference for cover crop seeds in South Texas

Authors: ELLIOT, LILLY - University Of Texas Rio Grande Valley; RIVERA, DANIELA - University Of Texas Rio Grande Valley; NOVAL, ADRIAN - University Of Texas Rio Grande Valley; CHOUDHURY, ROBIN - University Of Texas Rio Grande Valley; Penn, Hannah

Submitted to: bioRxiv
Publication Type: Other
Publication Acceptance Date: 1/15/2022
Publication Date: N/A
Citation: N/A

Interpretive Summary: Red harvester ants, the main food sources of the Texas horned lizard, are found locally in south Texas. These ants are known to gather seeds from areas around their nests and store the seeds inside their nests until later consumption. Some of these seeds may from weeds or important agricultural commodities and may be also infected with plant fungal pathogens. These pathogens, as they consume the seed themselves, might cause the seed rot within the ants’ storage facility, decreasing storage time limits. Conversely, these pathogens may make the seeds easier for the ants to access and consume by breaking down the seedcoat. We studied how head mold, a type of fungal pathogen, found sorghum seeds from local agricultural fields changed ant preferences for sorghum seeds. We simultaneously offered seeds with and without head mold to 20 ant colonies with no prior experience with sorghum; then we monitored how many seeds of each type were collected by the colonies after 1, 2, 4, and 24 hours. We found that red harvester ants did not have any preference for infected or uninfected seeds, taking both equally over time. Given this non-preference, ants were assumed to be storing infected seeds next to uninfected seeds within their colonies. However, the risk that stored pathogen-infected seeds will pose as a source of future seed infection, especially in agricultural fields where ant colonies are commonly disturbed through tillage, needs to be further examined.

Technical Abstract: Harvester ants are known to selectively forage seeds, potentially impacting nearby plant community composition. In agricultural areas, harvester ants may be viewed as pests by foraging on crop seeds or as beneficials by preferentially removing weed seeds. However, little work has been done on harvester ant preferences for cover crop seeds. Local observations suggest that ants may take cover crop seeds, but no studies have evaluated ant agricultural impacts or seed preferences in the Lower Rio Grande Valley (LRGV). We examined red harvester ant (Pogonomyrmex barbatus Smith) preferences for commonly used cover crop seeds in the LRGV (vetch, oat, fescue, sunn hemp, and radish with wheatgrass as a control) and a seed inoculation treatment meant to increase root nodulation. We tested seed sets using choice tests housed in seed depots located within the foraging range of ant colonies with no prior exposure to the selected seeds. Of the evaluated cover crop seeds, wheatgrass and oat were the first to be removed entirely from the depot, with vetch remaining after 24 h. When we inoculated the two most preferred seeds to determine if there was a preference for non-inoculated seeds, we found no difference between inoculated and non-inoculated seeds. These data indicate that harvester ant foraging preferences can inform grower management recommendations regarding the risks of using certain cover crops in fields with known harvester ant presence.