Insect herbivores exploit plant cues to discern host and non-host plants. Studies of visual plant cues have focused on colour despite the inherent polarization sensitivity of insect photoreceptors and the information carried by polarization of foliar reflectance, most notably the degree of linear polarization (DoLP; 0–100%). TheDoLPof foliar reflection was hypothesized to be a host plant cue for insects but was never experimentally tested. Here, we show that cabbage white butterflies,Pieris rapae(Pieridae), exploit theDoLPof foliar reflections to discriminate among plants. In experiments with paired digital plant images,P. rapaefemales preferred images of the host plant cabbage with a lowDoLP(31%) characteristic of cabbage foliage over images of a non-host potato plant with a higherDoLP(50%). By reversing theDoLPof these images, we were able to shift the butterflies' preference for the cabbage host plant image to the potato non-host plant image, indicating that theDoLPhad a greater effect on foraging decisions than the differential colour, intensity, or shape of the two plant images. Although previously not recognized, theDoLPof foliar reflection is an essential plant cue that may commonly be exploited by foraging insect herbivores.