The aim of the invention is to provide a biocide composition which consists of a commercial natural olive leaf extract (olea europea) and a carrier which allows administering the plant extract in the beehive, preferably syrup (50% w/v sucrose dissolution) to inhibit the growth of the Paenibacillus larvae bacteria in all the phases of its biological cycle, both in its vegetative state and as a spore, along with Melissococcus plutomius and/or Nosema spp. achieving a curative treatment or the prophylaxis of the disease in the Apis mellifera larvae and adult bees. The active substance is preferably oleuropein, a secoiridoid glucoside, naturally present in plants of the Oleaceae family, although in lower concentrations.