No abstract

In this study, the performance and sequestration specificity of the sawfly Athalia rosae L. (Hymenoptera: Tenthredinidae) reared on two wild Brassica oleracea L. (Brassicaceae) populations (KIM and WIN), which express indole glucosinolates (GSs) in very high concentrations, were investigated. The local and systemic plant responses to larval feeding also were analyzed. Indole GSs predominated in KIM plants, whereas aliphatic GSs prevailed in the WIN population, which had several times higher total GS concentrations than KIM plants. Plants of the KIM population had lower specific leaf areas, and higher carbon/nitrogen ratios than WIN plants. The insects reared on WIN plants performed better for most traits than insects reared on the KIM population. The larvae preferentially sequestered aliphatic GSs but when feeding on KIM plants, indole GSs were also concentrated in the hemolymph. In response to feeding by A. rosae larvae, indole GSs were induced locally and systemically, whereas aliphatic GSs were reduced only in systemic leaves of the WIN population. Soluble myrosinase activities were constitutively higher in WIN than in KIM plants, and feeding damage led to a significant reduction of this glucosinolate-degrading enzyme in WIN plants only. The data suggest that the sawfly is well adapted to high concentrations of aliphatic GSs, which are sequestered by the larvae. In contrast, the larvae may be less adapted to plants dominated by indole GSs. Selective induction of indole GSs by the plants in response to larval feeding may be adaptive as accumulation of indole GSs may lower the performance of the larvae.

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.