Tolerance response of E. annuus against the attack of girdling beetle P. rufiventris
We characterized the life history of E. annuus with and without the attack of P. rufiventris (Fig. 1). Overwintered E. annuus began to bolt in April and produced flowers in May. The seeds germinated and overwintered at the rosette stage. P. rufiventrislaid an egg inside the stem of E. annuus after girdling the stem from April to May. Girdling behavior killed the upper part, stimulating axillary bud emergence. Seed production in girdled E. annuus was therefore delayed compared with that in non-girdled E. annuusplants.
To assess the fitness loss caused by girdling beetles, we measured the number of flowers in experimentally girdled and non-girdled E. annuus plants with and without larval attack. Three weeks after girdling and egg inoculation, girdled E. annuus produced significantly fewer flowers than the non-girdled E. annuus (Fig. 2a). Flower production was not affected by larval feeding inside the stems.
We then measured the long-term effects of girdling on E. annuusgrowth and reproduction of naturally girdled E. annuus in the field. Two weeks after girdling, girdled E. annuus plants had higher chlorophyll content than non-girdled plants (Fig. 2b). Subsequently, the axillary bud emergence was boosted in girdled E. annuus, and the number of branches exceeded that in non-girdledE. annuus 4 weeks after girdling (Fig. 2c). The extra branches of girdled E. annuus persisted throughout the flowering season ofE. annuus . The higher number of axillary branches in girdledE. annuus led to flower production restoration. Initially, girdled E. annuus plants produced significantly fewer flowers than non-girdled plants. However, the difference diminished over time and lost significance seven weeks after girdling (Fig. 2d). Consequently, the difference in the number of flowers in girdled and non-girdled E. annuus was not significant by 12 weeks after girdling when the flowering season of E. annuus was complete. Because seed number per flower is proportional to flower size inE. annuus , we further compared flower sizes. The flower size of girdled and non-girdled E. annuus did not differ significantly (Fig. 2e). Thus, we concluded that the loss of shoot apical meristems in girdled E. annuus restores reproductive production by boosting growth at the end of the life cycle.