The foraging range of Argentine ants is difficult to infer from visual observation of ant trails because of their diffuse and fluid colony structure. We studied the movement of ant food resources in California vineyards by providing ants with a source of protein-labelled sugar water. After a six day period, ants were collected at various distances from the sugar source and tested for the protein marker using ELISA. As distance from the sugar source increased, the percentage of ants marked with the protein decreased. The labelled food moved across distances much greater than average inter-nest distances, reflecting the diffuse structure of Argentine ant colonies.