These file contain data supporting all results reported by Carlson et al. in "Divergent dietary and defensive adaptations of closely related coexisting seed bugs". In Carlson et al. we retrospectively investigated adaptive divergence in diet and defense among seed bugs that co-occur at small spatial scales. The small milkweed bug (Lygaeus kalmii) primarily feeds on milkweeds (Apocynaceae) and sequesters toxic cardenolides as an anti-predator defense, both traits representing the ancestral condition of Lygaeinae. Little was previously known about the false milkweed bug (L. turcicus); we report the primary host of L. turcicus is false sunflower, Heliopsis helianthoides (Asteraceae) and that its seeds produce cardenolides, the second report of this toxin in Asteraceae. The dominant cardenolide was a novel structure which we resolved and named heliopside. We found divergent trends in performance and defense of the two bugs. Lygaeus kalmii had >40% higher survival and growth on milkweeds compared to false sunflower, an effect that was primarily explained by false sunflower’s thick seed shell. Nonetheless, L. kalmii sequestered toxins equivalently from the two seed species. Conversely, L. turcicus had equally high survival and growth on the two seeds, but sequestered toxins more effectively from false sunflower than milkweed, likely due to specialized detoxification of false sunflower’s cardenolides. Thus, adaptation to host-plant traits in these two seed bugs is asymmetric for diet and defense, and trade-offs are likely to constrain the bugs’ host ranges and contribute to coexistence. Shared seed chemistry apparently facilitated host-shifts in the Lygaeinae from Apocynaceae to Asteraceae, with divergent adaptation to their food niches contributing to diversification.