Diet Switching from Leaves to Flowers – is this Beneficial for the Mustard Leaf Beetle?

Many herbivorous species are considered specialists that feed not only on specific plant taxa, but also on certain organs. Numerous species feed on leaves (folivory), while supplementary feeding on flowers (florivory) or switching in diets is less commonly considered. We used the mustard leaf beetle, Phaedon cochleariae, known to feed on leaves of Brassicaceae, such as Nasturtium officinale, to test whether it also accepts flowers, whether different stages show different preferences for certain plant organs, how the glucosinolate contents of the plant parts differ and how individuals develop on either single or mixed diet. In preference tests, larvae and adult males did not differentiate between young and old leaves or leaves versus flowers, but rarely accepted fruits. Adult females preferred young over old leaves and leaves over flowers, while they did also accept fruits. Total glucosinolate concentrations were higher in young leaves and flowers than in old leaves and fruits. In development assays with four different groups of diets, larvae that were exclusively fed on leaves or switched to flowers over the larval development developed faster than those only fed on flowers. However, exclusive feeding on leaves led overall to the highest fertility, while individuals fed exclusively on flowers showed by trend the lowest survival. Since P. cochleariae can apparently also feed and develop on flowers, a switch to this organ may be beneficial if leaves turn senescent or to exploit a competition- or enemy-free niche. Partial florivory amongst species considered to be folivores may be more widespread than assumed.