Acorn Weevil

Curculio glandium — the European acorn weevil — is one of the most morphologically extraordinary insects in temperate woodland and a flagship example of insect-plant coevolution. The species is small (5-8 mm body length excluding the snout) but has a dramatically elongated rostrum (snout): the female's downward-curving rostrum is approximately equal to her own body length, and the male's is somewhat shorter. The rostrum carries the mouthparts at the tip and is used as a drill to bore through the tough developing acorn shell of oak (Quercus species). The drilling process is precise and time-consuming: a female lands on a developing acorn (still on the tree, in late summer), positions her snout at the chosen drilling site, and rotates her body in a slow rocking motion that grinds the rostrum through the acorn shell. A single hole takes 30-60 minutes to complete, and the female may need to start over multiple times if the acorn shifts. Once the hole is bored, the female turns around and uses her ovipositor to deposit one egg through the hole into the developing acorn cotyledon. The egg hatches in 1-2 weeks; the larva develops over the next month inside the acorn, eating the cotyledon flesh while the acorn matures and falls from the tree. After acorn fall, the mature larva drills a small round exit hole (the familiar 'wormhole' in collected acorns), drops to the ground, and burrows into the soil to pupate. Adults emerge the following autumn or up to 3-4 years later (variable depending on conditions). Acorn weevils are a major check on oak seed survival in temperate forests — in heavily-infested years, 30-90% of an oak's acorn crop can be destroyed by acorn weevils, which has driven the evolutionary 'masting' behavior of oaks (synchronized mass acorn production every 3-5 years to satiate weevil populations).