Effect of herbivore identity and developmental stage on volatile emission of Populua nigra . Four treatments include Lymantria dispar (2nd instar), L. dispar (5th instar), Laothoe populi (5th instar), and a mixture of L. dispar (5th instar) and L. populi (5th instar). Box-plots showing the same letter are not statistically significant from one another after a Tukey test performed on the fitted values after applying a GLS model, excluding the effect of the feeding intensity. P values are given in Table 1. Plots showing no letters indicate that there was no effect of the treatment on volatile emission.