Florivory is an alternative but suboptimal diet for an invasive leaf‐feeding beetle

Flowers can be a valuable food source for primarily leaf‐feeding insects. They may also be an alternative diet when preferred foods are unavailable. Here we explored the value of florivory for the invasive viburnum leaf beetle (VLB). In the field, VLB larvae have been observed to feed on V. tinus inflorescences in the absence of young leaves, their preferred food. Under controlled conditions, we measured VLB larval performance when larvae were fed young V. tinus leaves, old (from the previous year's growth) V. tinus leaves, and V. tinus inflorescences. VLB larvae could complete their development by feeding solely on V. tinus inflorescences, with a negative impact on performance: larval survivorship to adult stage and adult mass decreased and developmental time increased when larvae fed on inflorescences. Survivorship was null when larvae were fed old leaves. Florivory appears to be a suboptimal diet for VLB, which may be used as a last resort when better food sources are absent. These results illustrate the importance of plasticity for the nutritional ecology of herbivorous insects. Dietary flexibility may have facilitated VLB's adoption of novel host plants in its invasive range. Larval performance of viburnum leaf beetle was compared when feeding on inflorescences, young (tender) leaves, and old (tough) leaves of Viburnum tinus. Larval survivorship, adult mass, and development duration worsened when larvae fed on inflorescences compared to young leaves, and old leaves were unsuitable. Florivory is therefore a suboptimal diet for VLB larvae but may be lifesaving when young leaves are absent or have been depleted.