Variable chemical defence in the checkerspot butterfly Euphydryas gillettii (Lepidoptera: NymphaIidae)

1. Like other checkerspots, Euphydryas gillettii butterflies may contain the defensive chemicals, iridoid glycosides, which are sequestered from their hostplants during larval feeding. 2. We analysed the iridoid glycoside content of E.gillettii adults from two different populations, Warm Lake, Idaho, and Granite Creek, Wyoming, that have different patterns of hostplant use. 3. Gas chromatographic analysis of thirty butterflies from the Wyoming population showed that they contained a mean of 1.27 (k0.19 SE) % dry weight iridoid glycosides. Notably, 20% of these butterflies contained no detectable iridoid glycosides. 4. In contrast, nineteen butterflies from the Idaho population contained ii mean of 3.89 (k0.38 SE) % dry weight iridoid glycosides, and all butterflies contained iridoid gl ycosides. 5. These results illustrate how the chemical defence of herbivorous insects varies according to differential use of potential hostplants.