The presence of heterospecific neighbours can affect the amount of damage caused by insect pests to focal plants. However, how plant neighbours influence herbivore performance is poorly understood, in particular when they are closely related. We tested the independent and interactive effects of tree species identity and tree neighbour type (conspecific vs. heterospecific) on the performance of a major oak pest, the oak processionary moth larvae (OPM, Thaumetopoea processionea L.) fed on Quercus robur L. and Q. petraea Liebl. We performed a factorial greenhouse experiment in which we grew two oak saplings per pot, either from the same species or from both species. We quantified growth and mortality of OPM larvae, leaf phenolic compounds, C:N ratio and bud phenology. OPM larvae performed significantly better on Q. petraea than on Q. robur, regardless of plant neighbour type. Phenolic compounds and C:N, but not phenology, differed between oak species and neighbour species identity. Only bud phenology had a significant effect on OPM performance, which was better when young larvae had access to recently unfolded leaves, regardless of oak species and neighbour identity. Although oak neighbour identity altered the expression of leaf traits, this effect had no measurable consequences on OPM performance. Our results showed that OPM performed better when feeding on Q. petraea than on Q. robur. We also found that oak neighbours altered the expression of leaf traits associated with defence, but that this effect did not cascade onto OPM performance. These findings should be replicated in the field in order to improve forest management strategies against OPM attacks.