Within‐plant variation in chemical defence of <i>Erysimum cheiranthoides</i> does not explain <i>Plutella xylostella</i> feeding preference

Plants invest a substantial fraction of their resources into defence against herbivores, with the highest levels of defence often allocated only to the most valuable tissues. Plants in the genus Erysimum (Brassicaceae) have evolved the ability to produce novel cardenolides, in addition to ancestrally conserved glucosinolates. While these plants co-express both defences, differences in tissue-specific expression might represent an effective cost-saving strategy. Larvae of the glucosinolate-resistant diamondback moth Plutella xylostella occasionally feed on Erysimum cheiranthoides but tend to avoid younger leaves. Here, we predict that caterpillar feeding preference is shaped by variations in cardenolide levels. Thus, we quantified within-plant variations in defence and nutritional traits of vegetative or early reproductive plants and performed feeding assays to evaluate the relative importance of cardenolides. In accordance with optimal defence theory (ODT), the youngest leaves contained the most nutrients and had highest levels of cardenolides, glucosinolates and trichomes, with more extreme within-plant differences found in reproductive plants. Caterpillars consistently avoided the well-defended youngest leaves, both on whole plants and detached leaf discs. Surprisingly, neither experimental addition (external application) nor removal (CRISPR-Cas9 knockout) of cardenolides significantly affected caterpillar feeding preference. Physical and chemical defences, including cardenolides, co-vary within E. cheiranthoides to maximize defence of youngest leaves. While P. xylostella clearly responds to some of these traits, the prominent cardenolide defence appears to lack potency against this specialist herbivore. Nonetheless, the careful regulation and re-mobilization of cardenolides to younger leaves during plant development suggests an important role in plant functioning.