Overcoming Obstacles: Perspective on How Mediterranean Oaks Defend Their Acorns from Insect Seed Predators

Insect pre-dispersal seed predators attack a large proportion of the acorn crops in oak forests worldwide. Oaks (<i>Quercus</i> spp.) have evolved several strategies, including physical barriers, chemical defenses (e.g., tannins), and/or phenological predator avoidance, to reduce infestation rates. This study examines how four Mediterranean oak species cope with acorn-feeding insects. Nearly 4000 acorns were collected from five sites at two time points during the maturation period: in mid-September and mid-October. Infestation rates were higher in mid-September, when the pericarp is softer and easier to drill, but at that time, the cotyledon tannin content was higher. <i>Q. coccifera</i> acorns had the highest tannin concentration, which, we experimentally discovered, hampered weevil development (with longer development and a lower final larval mass). Infested acorn abscission was also more effective in <i>Q. coccifera</i>. Due to the smaller size and later maturation phenology of its acorns, insects depleted the cotyledons and suffered food shortages more frequently. In the end, <i>Q. coccifera</i> showed the lowest acorn infestation rates, although its strategy would have costs in further stages of the regeneration cycle. Tannins deter acorn dispersers, and their production is costly. Such trade-offs would favor the co-existence of different strategies evolved by <i>Quercus</i> spp. against pre-dispersal insect predators.