The repellent properties of volatile organic compounds (VOCs) emitted by several companion plant species (CPs) were tested on the rosy apple aphid, Dysaphis plantaginea (Passerini) (Hemiptera: Aphididae), in a new dynamic tubular olfactometer connected to a dynamic plant chamber. This device allows a rapid and direct measurement of the repellency of whole plants on an aphid population. The VOC production, correlated to the number of plants in the chamber, was determined, and the VOC concentration in the emitted blend was evaluated to establish a causal link with the aphid response. Three species showed strong repellency properties to D. plantaginea: peppermint, Mentha piperita L., lavandin, Lavandula x intermedia Emeric ex Loisel., and rosemary, Salvia rosmarinus Schleid. (all Lamiaceae). In addition, tests on various rosemary chemotypes showed that the repellency could vary within the chemotype. The developed device revealed a classification of the tested CPs according to the analyzed VOCs and a link between the repellency index (RI) and the presence of certain VOCs. Thus, eucalyptol, beta-ocimene, allo-ocimene, piperitone, l-menthone-1, and l-menthone-2 were associated with the highest RIs, although not all of them were identified as repellents. Thus, the repellent effect of a CP results primarily from the produced olfactory mixture in which the presence and quantity of particular VOCs would modulate the repellent efficiency.