Overcoming obstacles: perspective on how mediterranean oaks defend their acorns from insect seed predators

Abstract

Simple Summary Oak seeds (acorns) are an important food source for insect larvae, such as acorn weevils, moth caterpillars, and gall wasps, but they have evolved several strategies to defend themselves from these seed predators. This study looks at how four types of oak trees, Q. coccifera, Q. ilex, Q. faginea, and Q. pyrenaica, use different defenses to protect their acorns. These defenses include chemical compounds like tannins, physical traits, and the timing of acorn development. We collected nearly 4000 acorns from five different locations at two different stages of growth and analyzed the rate of infestation, the tannin content, and the size of the acorns. We also examined how larvae of a specific acorn weevil, Curculio elephas, developed in the acorns of each oak species. The results showed that oak species with higher tannin levels, like Q. coccifera, had fewer pests and slower larval development. These findings help us understand how plants and insects interact and how oak trees balance protecting their acorns with their need to spread seeds, which can be important for maintaining healthy ecosystems.

Abstract Insect pre-dispersal seed predators attack a large proportion of the acorn crops in oak forests worldwide. Oaks (Quercus 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. Q. coccifera 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 Q. coccifera. 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, Q. coccifera 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 Quercus spp. against pre-dispersal insect predators.