Abstract <p>Butterflies select host plants, which are an essential but often limiting resource, based on a variety of features, including characteristics of host plant distribution and patch size. Since selection is assumed to translate to fitness benefits, assessing characteristics butterflies use to select host plants at reintroduction sites could reveal catalysts for post-release success. To understand whether endangered Mottled Duskywing (<i>Erynnis martialis</i>) butterflies select their host plants based on patch size, we created a Resource Selection Function from 115 observations at a successful reintroduction site. We assessed selection of host plants distributed as individual plants (&lt; 1 m²), small patches (&lt; 10 m²), medium patches (&lt; 30 m²), and large patches (&gt; 30 m²) in females, males, and all observations combined (including unknown sex). All three classes of observations showed strong selection for large host plant patches, whereas medium host plant patches were moderately selected when all observations were considered, but not selected when males or females were considered separately. Individual plants and small patches were used in proportion to their availability. At two additional nearby sites where reintroduction of Mottled Duskywing was less successful, the density of large host plant patches was 25 and 44 times lower than the successful reintroduction site.</p> Implications for insect conservation <p>Our results suggest that further reintroductions of Mottled Duskywing should consider availability and size of host plant patches at potential release sites. Our work highlights the value of post-release monitoring for assessing habitat preferences of reintroduced individuals and provides one example of how adaptive management could be used in reintroductions of endangered species.</p>