Within-plant variation in chemical defence of Erysimum cheiranthoides does not explain Plutella xylostella feeding preference

Plants invest a substantial fraction of their resources into defence against herbivores, with the highest levels of defence expected to be allocated only to the most valuable tissues. In an evolutionary recent event, plants in the genus Erysimum (Brassicaceae) evolved the ability to produce novel cardenolides in addition to ancestrally conserved glucosinolates. While plants co-express both defences, tissue-specific expression for optimized defence might represent an effective cost saving strategy.

Larvae of the glucosinolate-resistant diamondback moth Plutella xylostella occasionally feed on Erysimum cheiranthoides in the field but tend to avoid younger leaves. Here, we quantified within-plant variation in chemical defences of vegetative or early reproductive plants and performed a set of feeding assays to identify traits involved in the preference of P. xylostella larvae.

In accordance with optimal defence theory (ODT), we found youngest leaves to contain the most nutrients, as well as the 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 on whole plants and with detached leaf discs, but surprisingly, chemical defence compounds in isolation were insufficient to affect caterpillar preference.

Physical and chemical defences, including cardenolides, co-vary within Erysimum to maximise 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 for plant functioning.