We have observed localized mating and feeding aggregations of Lytta sayi (Coleoptera: Meloidae) on wild lupine Lupinus perennis occupied by the Federally endangered Karner blue butterfly Lycaeides melissa samuelis (Lepidoptera: Lycaenidae) in Eau Claire County, Wisconsin. In 2004, we monitored butterflies and beetles within a 1,020m² stand of lupine where a beetle aggregation formed and dispersed within 11 days. Three beetles were observed on day 1 with a maximum of 951 beetles by day 7. By day 3, the beetles consumed all flowers from 83% of the flowering stems and by day 7, no flowers remained. Beetles did not eat lupine leaves, but ate seeds on 2% of all stems. Activity budgets of butterflies were conducted before and during the beetle aggregation. In the area of the highest beetle density, 77.1 % of all butterfly landings occurred before the beetle aggregation but declined to 57.9% during the aggregation. This area also contained the most Potentilla simplex. Before the beetles arrived, butterflies spent 34% of their perching time on P. simplex flowers, 1.1% on L. perennis flowers and 50.4% on L. perennis leaves (n=28). During the beetle aggregation, butterflies spent 12.2% of their perching time on P. simplex flowers, 0.98% on L. perennis flowers and 42.4% on L. perennis leaves (n=27). Our results suggest that adult Karners alter their choice of perches due to the presence of L. sayi aggregations. Although there is no direct effect on Karner larvae, as they eat lupine leaves, beetle removal of flowers reduces nectar, pollen and seed production.