Tree canopy cover and elevation affect the distribution of red harvester ant nests in a peri-urban setting

Authors: ELLIOTT, LILLY - University Of Texas Rio Grande Valley; MARTINEZ, ISABEL - University Of Texas Rio Grande Valley; PEREIRA, ENGIL - University Of Texas Rio Grande Valley; CHOUDHURY, ROBIN - University Of Texas Rio Grande Valley; Penn, Hannah

Submitted to: Insect Conservation and Diversity
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/7/2023
Publication Date: 4/5/2023
Citation: Elliott, L., Martinez, I., Pereira, E., Choudhury, R., Penn, H. 2023. Tree canopy cover and elevation affect the distribution of red harvester ant nests in a peri-urban setting. Insect Conservation and Diversity. pp. 1-11. https://doi.org/10.1093/ee/nvad025.
DOI: https://doi.org/10.1093/ee/nvad025

Interpretive Summary: Harvester ants are seed-eating insects that live in dry climates and are known to nest in natural areas, agricultural lands, and suburban areas. Though farmers have developed methods to remove harvester ant nests from fields, no prior work on nest locations within urban areas has been done, especially in South Texas. Harvester ants are considered pests in urban/suburban areas due to their painful stings and the fact that these ants remove all plants surrounding their nests, leaving bare soil. This plant removal destroys the lawn and gardens of areas where the ants live, causing many homeowners and land managers to kill ant colonies using insecticides. This study was done to evaluate what areas harvester ants prefer to nest in within a suburban/urban area with particular focus on the amount of paved surfaces and irrigation levels that may change soil moisture. All harvest ant colonies near the University of Texas Rio Grande Valley Edinburg, TX campus were mapped, with special focus on those found within the sports fields. Nest placement was compared with surrounding landscaping, elevation, and soil moisture from 22 samples collected throughout the mapped area using GIS software. The data from this study will allow researchers to determine the specific conditions harvester ant colonies prefer and could help citizens take proper measures to reduce the likelihood of colony establishment on their property without the use of insecticides.

Technical Abstract: 1. With an increase in human population, regional land use in south Texas has shifted acreage from grassland and shrublands to a peri-urban matrix in the past 30 years. 2. Despite the shift from natural areas to anthropogenically modified habitats, native red harvester ants (Pogonomyrmex barbatus) have maintained nest sites within parts of these matrices. 3. To determine which environmental variables in human-dominated landscapes may play a role in red harvester ant nest site selection, we mapped observed nests during 2020 and 2021. We then evaluated nest presence and absence relative to elevation, percentage of surrounding impervious surfaces, distance to roadways, and tree canopy cover (with NDVI). For a sub-sample of the study site, we also measured soil moisture and estimated potential foraging area per colony with Voronoi tessellation. 4. Nests were clustered together near high human-use areas such as athletic fields, lawns, sidewalks, and railroad tracks. Nests were more likely to be found in areas with higher elevation and lower tree canopy cover, with no impact of impervious surfaces. In fact, many nests were observed immediately adjacent to roadways and parking lots. While we found no differences in soil moisture in areas with and without nests at the sub-site, estimated foraging area per nest decreased from 2020 to 2021, corresponding with an increase in rainfall. 5. Red harvester ants are highly adept at nesting in disturbed, urbanized matrices, but still appear to be subject to environmental constraints like shading, potential flood risk (elevation), and access to food resources (foraging area).