Oak seed predatory weevils occurring in Poland are prone to increased interspecific competition due to the limited number of Quercus species, compared to southern Europe, in which they can develop. Therefore, analyses on the preferences of three weevil species for acorn sizes chosen for reproduction, as well as on reproductive period duration, were performed. Cafeteria-type experiments were set for females of three species associated with one oak species. Females were allowed to choose and oviposit in acorns of different sizes and growth stages. Research revealed statistically significant differences between the masses of acorns chosen for oviposition by females of Curculio glandium (the biggest), C. pellitus (medium), and C. venosus (the smallest). Studied weevils also differed in terms of the beginning of the reproductive period, which corresponded with the increasing mass of growing acorns. Moreover, C. glandium was observed to be the only species to perform radial egg galleries and lay a considerably higher and varied number of eggs. The results support the hypothesis of a strategy aimed at reducing interspecific competition between Curculio spp. in terms of limited host plant species number.

Curculio glandium females associated with the pedunculate oak were investigated in order to fill the knowledge gap on acorn structure preferences and shell-hardening influence on ovipositional behavior and fecundity. Shell solidification progression of weekly harvested acorns was measured using penetration and force–deformation tests along with fruit mass within a time frame covering weevils’ reproduction period. Captured females were offered regular acorns (uncracked, soft enough to drill into) and older seeds (with natural cracks) for behavior recordings and preferences tests. Young acorns and fruits ripe enough to be too hard for females to drill (artificially pierced and untouched) were used for egg output assessment throughout the shell-hardening progression. Experiments revealed that naturally cracked acorns were chosen significantly more often, which resulted in reduced drilling-phase duration. Egg number did not differ significantly before and after the threshold of acorn hardening; however, having passed it, females significantly more frequently deposited eggs in artificially pierced seeds. Results indicate the opportunistic nature of female preferences. Oviposition in cracked or pierced acorns facilitates the process and lowers competition for relatively shortly available soft and healthy seeds, the first of which reach impenetrability in the third week of August.

Curculio glandium is one of the pre-dispersal seed predators occurring in Central Europe. It is associated with Quercus robur, the acorns of which are shelter and food sources for developing larvae. Females of the species, to our knowledge, are lacking in marking pheromones or do not use them; therefore, in nature, multiple infestations (over 10 eggs or larvae) of the same host fruit can be found. Such density can provoke very strong competition, which was verified in this study. The survival rate and body mass of 695 second-instar larvae, competing in various test groups (one, three, five, eight and ten larvae) offered one acorn, were measured and video recordings made in order to describe their behavior and determine differences between groups. Experimental observations indicated that when the density of larvae in an acorn increased, the survival rate and body mass significantly decreased—being the lowest in test groups consisting of eight and ten individuals. In the latter groups, also the acorn embryo was completely consumed. Video footage, along with the presence of dead, nibbled larvae and living ones covered with scars resembling mouthparts, is evidence for aggression and cannibalism in the second and the third larval instars—behavior scarce in weevils and in phytophagous insects in general. Results confirm the assumption that in heavily infested oak fruits, competition between individuals is so strong that it involves cannibalism, which at the same time provides the strongest larvae with additional nutrients.

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.