Abstract Aphids are widespread pests that impact natural and cultivated plant communities. Aphids can induce stress responses in plants that lead to changes in volatile organic compound (VOC) emissions, which in turn can affect atmospheric chemistry. Among other things, the oxidation of VOCs can lead to the formation of secondary organic aerosol (SOA). Field measurements during an aphid infestation of a Scots pine forest in Switzerland showed that the aphid stress had contrasting effects on SOA mass concentrations measured at 10 and 350 m downwind of the forest. At the elevated downwind site, the biogenic SOA mass concentration increased by a factor of 2. This is consistent with laboratory experiments that show increased SOA yields from the oxidation of aphid-induced pine VOC emissions. In contrast, at the downwind site close to the forest canopy, the organic mass concentration decreased, which we suggest is due to changes in the mixing layer height and the forest–atmosphere exchange processes. Therefore, the environmental impact of aphid stress on SOA mass concentration can be positive (increased SOA mass) or negative (decreased SOA mass) depending on the location and scale.