The common perception of wasps as an insect living within a colony composed of hundreds to thousands of individuals, which eventually could sting you if you happen to share your picnic with them, is mainly incorrect, as the vast majority of the 100,000+ species of wasps are in fact solitary and do not sting humans (even if they would like, they physically cannot).
The potter-wasps (Eumeninae), are mostly solitary wasps, i.e., one female individual builds a nest containing several successively built cells (from bottom to top). After capturing preys (caterpillars) and placing them within cells, the female lay a single egg into each cell, and then seal those with various materials, often “mud” in Hong Kong. This cell construction is continued until the nesting cavity is fully occupied, at which point the female leaves and seeks another location to construct a new nest. This lack of maternal care allows a great many “predators” to penetrate the nest; to rob the food and or attack directly the brood and ultimately kill the original host for its own development, phenomena generally known as parasitism. However, despite leaving the nest at the mercy of parasites and predators, some potter-wasps may enlist the help of tiny creatures, mites (Acari), to defend and protect the brood and food source.
These mites are often found within the nest, walking around the cell, where they were originally introduced by the female wasp that carried them on her body, thanks to special morphological structures called acarinaria. When the wasp builds and furnishes the cells, the mites migrate from the acarinia to the cell and are imprisoned inside. There, they will feed and reproduce on the wasp’s food provision, reaching sometimes impressive numbers and both immature mites and gravid females can be found. Although this sounds like parasitism, their development does not destroy the food provisions nor the brood and, the wasp larvae develop normally. However, if another parasite species attempts to lay eggs, these will be rapidly located and destroyed by the mites, henceforth permitting normal development of the wasp brood; although this does not always succeed.
Once the entirety of the food provision is exhausted and the wasp larvae start metamorphosis, the mites will move to the pupae and remain there; probably feeding on it, waiting for its complete development into an adult with fully formed acarinaria on which the mites will fix themselves. The newly emerged wasp will transport them to a new nesting site and there, the mites will be able to reproduce and develop again. These mites can be found on female wasps but also on males, transferred to a female during copulation (and possibly vice-versa).
This is one of the many examples where different species have co-evolved to cooperate (mutualism) in order to sustain their mutual development and increase their reciprocal chances of producing successive generations, in this case, food, shelter and dispersal for the mites being exchanged against defense and security for the wasps.
In Hong Kong, there are no less than 38 species of potter-wasps that have been recorded, including two new species described in 2019, and more of these elegant and colorful wasps may be hiding around you. So, pay attention to these delicate wasps and to their fascinating biology and ecology.