1Plant-insect interactions are ubiquitous, and have been studied intensely because of their 2 relevance to damage and pollination in agricultural plants, and to the ecology and evolu-3 tion of biodiversity. Variation within species can affect the outcome of these interactions, 4 such as whether an insect successfully develops on a plant species. Whereas specific genes 5 and chemicals that mediate these interactions have been identified, studies of genome-or 6 metabolome-wide intraspecific variation might be necessary to better explain patterns of 7 host-plant use and adaptation often observed in the wild. Here, we present such a study. 8 Specifically, we assess the consequences of genome-wide genetic variation in the model plant 9 Medicago truncatula for Lycaeides melissa caterpillar growth and survival (i.e., larval perfor-10 mance). Using a rearing experiment and a whole-genome SNP data set (>5 million SNPs), 11 we show that polygenic variation in M. truncatula explains 9-41% of the observed varia-12 tion in caterpillar growth and survival. We detect genetic correlations among caterpillar 13 performance and other plant traits, such as structural defenses and some anonymous chemi-14 cal features; these genetic correlations demonstrate that multiple M. truncatula alleles have 15 pleiotropic effects on plant traits and caterpillar growth or survival (or that there is sub-16 stantial linkage disequilibrium among loci affecting these traits). We further show that a 17 moderate proportion of the genetic effect of M. truncatula alleles on L. melissa performance 18 can be explained by the effect of these alleles on the plant traits we measured, especially 19 leaf toughness. Taken together, our results show that intraspecific genetic variation in M. 20 truncatula has a substantial effect on the successful development of L. melissa caterpillars 21 (i.e., on a plant-insect interaction), and further point toward traits mediating this genetic 22 effect.23 Keywords: plant-insect interactions, herbivory, genomic prediction, quantitative 24 genetics, attenuated total reflectance infrared (ATR-IR) spectroscopy, structural 25 defense 26 approaches have begun to provide a more complete view of how within-species variation 53 affects plant-insect interactions (e.g., Harrison et al., 2018; Nallu et al., 2018). As an ex-54 ample, a recent study of intraspecific variation across 770 traits (including 753 chemical 55 features) in alfalfa showed that among-plant variation in insect herbivore communities was 56 best explained by non-linear interactions among suites of plant traits (Harrison et al., 2018). 57 Such findings highlight the need for quantitative, genome-, phenome-and metabolome-scale 58 analyses of the ecological and evolutionary consequences of intraspecific variation in plant-59 insect systems. In fact, these approaches may be necessary to explain the geographic mosaic 60 of host-plant use and plant-insect co-evolution found in nature (but see, e.g., Berenbaum 61 & Zangerl, 1998), in other words, to address questi...