Wandering around the more overgrown corners of Montjuïc and other “wild” fringes of Barcelona, like the sunny slopes of Collserola, you may well come across Kermes oak, Quercus coccifera, an evergreen low growing oak found across the Mediterranean.

During the Roman Empire, Hispania had to pay a tribute to Rome in the form of an insect also called kermes insects (Coccus ilicis) that live on the Kermes oak. The unfortunate insect was squeezed in the thousands to produce a crimson dye – the most highly prized dye for the robes of senate members. The dye was not replaced until the arrival of cochineal, an scale insect that lives on cacti of genus Opuntia , after America was “discovered”. Although it is not easy to see one of the actual kermes insects on the oak, it is easy to spot the little red galls formed by a tiny wasp called Plagiotrochus quercusilicis. A plant gall is an abnormal growth of a part of the plant usually triggered by chemical signals produced by immature insects or other organisms. In a way they are like plant “tumors”.