1. The presence of predators is known to suppress prey populations not only by consumptive but also by non‐consumptive effects, as it stresses the prey inducing costly changes to behaviour and physiology. However, there is recent evidence that disturbance from non‐predacious, non‐competing commensals can also negatively affect herbivore performance. 2. Populations of cherry‐oat aphids ( Rhopalosiphum padi L.) were initiated with adult aphids in mesocosms containing seedling wheat grass. Following aphid establishment, fruit flies ( Drosophila melanogaster Meigen) were added at a relatively high density to half of the mesocosms and the aphids were left for another 5 days to reproduce. The experiment was performed over two blocks at 24 ± 4 °C. It was hypothesized that a relatively high density of commensals would stress the aphids and reduce their fitness, causing lower population growth and resulting in lower population sizes. 3. Aphid numbers were significantly lower in mesocosms with commensal flies after 5 days of fly presence across the two experimental blocks, documenting that fly disturbance suppresses aphid fitness and population growth. 4. The negative effect of fruit flies on aphid population growth must have come from the disturbance that flies imposed on the aphids in their search for food, indicating that the flies stressed the aphids. Thus, our study indicates that commensals may stress herbivores that do not distinguish between enemies and other active species in their environment, adding in the overall herbivore top‐down control through fitness costs.