Attraction of a leaf beetle (Oreina cacaliae) to damaged host plants

Early in spring, just after the snow melts, the leaf beetle Oreina cacaliae feeds on flowers of Petasites paradoxus. Later in spring they switch to their principle host plant Adenostyles alliariae. The attractiveness of short- and long-term damaged host plants was studied in a wind tunnel. The spring host P. paradoxus was more attractive to the beetles after it had been damaged overnight by conspecifics or artificially, but not when the plants were damaged half an hour before the wind-tunnel experiments. Contrary to P. paradoxus, the principle host plant, A. alliariae was more attractive shortly after an attack by conspecifics (half an hour before the experiment) compared to a undamaged plant, but lost its increased attractiveness when damaged overnight. The enhanced attraction of damaged plants was longer lasting in the spring host P. paradoxus than in the main host A. alliariae. Volatiles emitted by host plants were collected and gas chromatographic analyses of the odors collected showed qualitative and quantitative differences between damaged and undamaged plants. Among the volatiles recorded, green leaf volatiles and mono- and sesquiterpenes dominated. In overnight damaged P. paradoxus plants with an enhanced attractiveness, limonene was emitted in higher amounts. In freshly damaged A. alliariae leaves, more alpha-humulene and germacrene D were emitted compared to (E,E)-alpha-farnesene, whereas in the less attractive A. alliariae plants, more (E,E)-alpha-farnesene was emitted compared to alpha-humulene and germacrene D. In the field, the long lasting attraction of flowering P. paradoxus early in the season may facilitate mating in O. cacaliae after a successful overwintering.