Effects of Insect Herbivory on Bilberry Production and Removal of Berries by Frugivores
​Berries of bilberry (Vaccinium myrtillus L.) in different phases of ripening

​Ripening of berries, foraging by frugivores and biochemical composition of berries

The experiment was conducted in 20 forest patches in Turku, Finland in 2013 and 2014. Each patch had six plots: three herbivore-treatment plots and three control plots. In each herbivore-treatment plots, two bilberry ramets were bagged with mesh bags containing autumnal moth (Epirrita autumnata Borkhausen) larvae. In control plots, two ramets were bagged with empty mesh bags. Two non-bagged ramets in herbivore-treatment plots and control plots were also selected, and due to phalanx growth strategy of bilberry plants, these ramets were expected to have root- or rhizome connection to the bagged ramets growing in respective plots. Before the experiment, the number of flowers and raw berries were counted, and the ripening as well as disappearance of ripe berries (interpret eaten by frugivorous birds such as thrushes, as these were the most common seed dispersers in the study areas) were surveyed during fruiting season. In addition, berry samples were collected to determine the profiles and concentrations of anthocyanins, sugars and organic acids.
In accordance to previous studies, we found that birds preferred plants with high fruit yield. Interestingly, herbivory treatment indirectly affected the probability for berries to be foraged, as the berries in non-bagged, intact ramets in neighbouring herbivore-treated ramets had the lowest probability to be foraged by frugivores. These ramest likely had root-, or rhizome-connections to the neighbouring herbivore-treated ramest, which may have enabled the priming effect in these non-defoliated ramets. It is also possible that the ramets neighbouring larval-defoliated ramets may have responded to a release of herbivore-induced volatile organic compounds (HI-VOCs) from defoliated ramets. Further studies on the mechanism(s) on how herbivory affects neighbouring ramets in bilberry and how this affects foraging preferences of frugivores, are needed to confirm our ramet-ramet signalling and defence-induction hypotheses.  Herbivore-treatment did not affect the probability of ripening or the biochemical composition of berries. The likeliest reason for this is that storages in rhizomes, or e.g. increase in photosynthetic biomass may have compensated the resource loss caused by larval-defoliation.
Our results indicate that attraction of frugivores to bilberry is partly dependent on the size of fruit yield, and that herbivory may cause ecological costs to clonal plants by lowering removal and dispersion of berries by frugivores from the undamaged neighbouring ramets.

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Tuuli-Marjaana Koski