Relationship between Density of Aphid Host Plants and an Associated Aphid-tending Ant (Formica altipetens)

Abstract

-Formica altipetens (Hymenoptera: Formicidae) tends aphids on sage (Artemisia spp.) and forbs (e.g., Lupinus sericeus, Geum trifiorum and Eriogonum umbellatum) at a grazed site in southwestern Montana. Numbers of ant mounds and aphid host plants (sage and forbs) were counted in 5-m quadrats within a 2-ha study area. There were significant negative correlations between the number of ant mounds and aphid host plants. Soil texture and topsoil depth were uniform across the site. Mound and sage density were not correlated to a measure of pocket gopher burrowing. Forb density was negatively correlated to pocket gopher burrowing, but a partial correlation analysis showed that there was a significant negative correlation between mound density and forb density. Ant nest activity was not related to sage density, but sage growth is greater where ant mounds are absent. These patterns suggest that aphid host plants are not a limiting resource for ants and that ants may limit plant growth.