<scp> <i>Eurosta solidaginis</i> </scp> ‐induced stem galls affect strength and colour of botanical dyes extracted from giant goldenrod ( <scp> <i>Solidago gigantea</i> </scp> )

Societal Impact Statement With increasing demand for sustainable goods, botanical dyes are of increasing interest and value to consumers, industry and small‐scale artisanal producers. Goldenrod is a widely used dye plant with a long history of traditional use. The impact of the well‐studied goldenrod stem gall produced by Eurosta solidaginis on goldenrod botanical dyes—and the broader impact of insect herbivory on plant dyes—has not been explored. Our research provides an initial investigation into how dye strength and colour are affected by the complex gall‐maker‐host–plant relationship—an essential step in understanding the value insect herbivory may bring to botanical dye production, as well as potential limitations of this dye variation in larger scale production. Summary Goldenrod ( Solidago spp.) plants produce botanical dyes with a long history of traditional use. The concentration and profile of phenolic compounds in goldenrod, influenced by biotic and abiotic factors, impact the properties of the resulting dyes. We investigated whether the presence of the stem‐galling insect E. solidaginis (Diptera: Tephritidae) affected dye produced from goldenrod. Leaves collected from upper and lower stem sections of galled and non‐galled goldenrod plants were processed to extract dye compounds into a soluble form. The dye was analysed through colourimetric comparison of dyed fabric swatches and spectrophotometry of aqueous extracts to evaluate phenolic content. Through colourimetric analysis using a paired t ‐test, we found that the colour difference between upper and lower leaves was significantly affected by whether the plant was galled (t[6] = −2.80, p = 0.02). Comparing dyes made from lower versus the upper leaves of the same plants showed a larger colourimetric difference in non‐galled than galled plants. This indicates that the galling insect affected the availability of dye compounds. Spectrophotometric analysis showed that increases in total phenolics between lower and upper leaves of galled plants were not significantly greater than in non‐galled plants ( p = 0.103), indicating a possible response to insect herbivory. The differing effects of the gall on dye content and total phenolics illustrate the complexity of plant–herbivore interactions and provide insight into the influence of biotic factors on botanical dyes.