Ants farming aphids for honeydew on a wild senna stem, Senna hebecarpa. The ants appear to be red-banded carpenter ant, Camponotus chromaiodes, based on a quick internet search. An even quicker search suggests the aphids may be black bean aphid, Aphis fabae. Taken with 60mm AF f/2.8 Micro-Nikkor.

WILD SENNA, Senna hebecarpa, flowers