Ropalidia are very small paper wasps, averaging less than 10mm in many species. They are more slender in shape than similar to Polistes but stouter than Parapolybia.
Ropalidia is a very large genus, with many species distributed all across Asia and Africa. The nesting habits of different species of Ropalidia are equally varied. Some start small nests singly; these nests stay small and open, without an outer covering. Nests of other species look like miniature hornet nests, round and with an outer covering, and sometimes containing thousands of wasps; alternatively such nests can be built inside crevices such as tree hollows. Yet others form "supercolonies", where there can be many nests in close proximity to each other!
I have personally observed several Ropalidia species in Singapore and Hong Kong. I found new nests of one such species in several locations in Singapore, with just one female, such as the one below.
However, many people have reported seeing nests of the same species locally which reached unusual sizes and shapes! Long nests shaped like a small dangling rope may be more common than thought, even in urban areas, as shown below. Photos copyright and courtesy of Jacqueline Lau.
I have also found a very large colony of another species in a hollow in a palm tree. It appears to be a swarm founding species. I could not see the nest, since the hollow extends quite high up the trunk inside, but I could see rows and rows of wasps lining the inside; the wasps visible here are a mere fraction of the total number inside!
Ropalidia nests are not much of a threat when small. The wasps usually flee when disturbed. However, when the colonies get large enough, the wasps can mount a vicious attack, and the stings can be quite painful, considering their lack of size. But this only happens is the nest is brushed against or otherwise contacted; the wasps will not generally rush out and attack people moving around the vicinity. I do not know the full range of prey used by these wasps. I have personally seen them catch caterpillars and even small flies! They should be fairly beneficial to say the least.
I am still not certain of the identity of most of the Ropalidia I have observed. I shall update this page in future and give each species individual coverage once I learn more about them and get them identified. Here are two common species found in Singapore.