Bees and wasps play a fundamental role in the health of ecosystems through processes such as pollination (ensures the reproduction of plants of nutritional and economic importance) and natural biological control (regulates populations of spiders, arthropods and other crop pests, diseases and weeds). Both bees and wasps are experiencing population declines and the main causes are urbanization and climate change. Urbanization can exert selective pressure on certain traits in animals, leading to ecological and evolutionary changes. There are characteristics in bees and wasps that can make them more vulnerable to certain ecosystems. Body size is a determining trait for individuals in a population. Larger individuals are known to be better pollinators than smaller individuals of the same species because they carry more pollen and have greater flight ranges. Females with larger body sizes have the ability to generate a greater number of offspring and the body size of an adult is directly proportional to the amount of food that the larva ingests during its development. A larger wasp will have an easier time hunting. Solitary females with good feeding conditions, the sex ratio of the offspring is biased towards females, who have higher energy requirements than males. Urbanization could be affecting body size and/or sex ratio. In any case, little is known about the situation of bees and wasps that nest in pre-existing cavities within cities with different degrees of urbanization.