Uncovering different parameters influencing florivory in a specialist herbivore

1. It has become increasingly recognised that several herbivores switch from folivory (leaf‐feeding) to florivory (flower‐feeding) during larval development. Yet, it remains poorly understood which cues influence this behaviour, whether a switch to florivory is consistently shown on different hosts, and to what extent florivory could be hindered by plant traits. 2. Using the sawfly Athalia rosae and two Brassicaceae differing in architecture and surface structure, the cues that influence larval movement to the flowers were investigated. A broad set of behavioural assays was employed and physical and chemical plant traits potentially affecting the larvae were analysed. Furthermore, the consequences of folivory versus florivory on insect performance were studied. 3. The larvae preferred flowers over leaves. Consumption of particular flower parts correlated partly with measured plant traits such as glucosinolate distribution. Visual cues were of higher importance than volatile cues. The initial position of newly hatched larvae on plants influenced the probability of the larvae reaching the flowers during development. Trichomes and surface waxes hindered the larvae from moving upwards to the flowers. Larvae developed slower and gained less body mass when feeding on inflorescences of Brassica nigra than when feeding on leaves, in contrast to the patterns observed before on Sinapis alba, where florivory led to an improved performance. 4. This study demonstrates that florivory depends on various host plant traits. It reveals new insights into different parameters influencing this multifaceted phenomenon and into the expected impact on the ecology and fitness of both the attacking herbivores and the plants.