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.