Morphological and olfactory tree traits influence the susceptibility and suitability of the apple species Malus domestica and M. sylvestris to Anthonomus pomorum

The florivorous apple blossom weevil, Anthonomus pomorum (Coleoptera: Curculionidae), is the most economically relevant insect pest of European apple orchards in early spring. Neither efficient monitoring nor ecologically sustainable management of this insect pest has yet been implemented. To identify heritable traits of apple trees that might influence host selection of A. pomorum, we compared susceptibility of apple tree species using infestation rates of the domesticated apple, Malus domestica (Rosaceae: Pyreae), and the European crab apple, M. sylvestris. We evaluated the suitability of the two apple species for A. pomorum by quantifying the mass of weevil offspring. Because volatile organic compounds (VOCs) emitted from flower buds of domesticated apple have previously been suggested to mediate female weevil preference via olfactory cues, we conducted bioassay experiments with blossom buds of both apple species to explore the olfactory preference of adult weevils and, furthermore, identified the headspace VOCs of blossom buds of both apple species through GC-MS analysis. The infestation analysis showed that A. pomorum infested the native European crab apple stronger than the domesticated apple, which originated from Central Asia. The European crab apple also appeared to be better suited for weevil larval development than the domesticated apple, as weevils emerging from M. sylvestris had a higher body mass than those emerging from M. domestica. These field observations were supported by olfactory bioassays, which showed that A. pomorum significantly preferred the odor of M. sylvestris buds compared to the odor of M. domestica buds. The analysis of headspace VOCs indicated differences in the blossom bud volatiles separating several M. domestica individuals from M. sylvestris individuals. This knowledge might be employed in further studies to repel A. pomorum from M. domestica blossom buds.