The Rocky Mountain apollo butterfly, Parnassius smintheus, and its host-plant Sedum lanceolatum, are endemic to open alpine meadows threatened by the encroachment of trees. I explore variability in interactions between P. smintheus and S. lanceolatum relative to the treeline-delimited meadow edge, and consider the consequences of continued tree encroachment for these and other species facing similar threats. First, I demonstrate that S. lanceolatum distribution and quality vary relative to the meadow edge, with plants near the treeline being both more abundant and more nutritious than those elsewhere in the meadow. Next, I show that this variation influences both oviposition and larval feeding by P. smintheus in unexpected ways: females actively oviposit in response to both the abundance and quality of hosts yet show no strong attraction to the meadow edge, while the spatial patterns of host-plants and herbivory upon those host-plants is decoupled (i.e., not ideal) only near the treeline, despite the abundance and apparent suitability of hosts there. I also show that, because larval P. smintheus can actively respond to the distribution of their hosts, the spatial pattern of herbivory is likely the product of choice, not chance. Finally, I explore how previous stress, including P. smintheus herbivory and flowering history, affect the growth of S. lanceolatum relative to the treeline, showing that while flowering is more stressful to S. lanceolatum overall than herbivory, herbivory may lead to compensatory growth away from the treeline. I conclude that P. smintheus-S. lanceolatum interactions vary spatially, that abundant host-plant resources near the meadow edge may in fact not be available to larvae, and that the extent of actually usable larval habitat may therefore differ from that of apparently suitable habitat. Overall, I propose that a synthetic assessment of habitat for both adults and larvae will give a clearer sense of likely butterfly responses to environmental change and, consequently, aid conservation of Lepidoptera.