Stenogastrine wasps are the most primitive social wasps. Some of them are not completely social but simply quasi-social or at most, semi-social, meaning they nest in groups but often raise their larvae individually and do not cooperate. Some, however, have interesting dominance structures. These wasps are small to medium-sized and very slender. They are quite closely related to the Eumenines, commonly known as potter wasps , and share similar characteristics. For instance, larvae of Stenogastrines, instead of filling out the cell vertically, curl up in a semicircle in the bottom of the cell. They then turn over and pupate in a weird arched posture.
The entire subfamily Stenogastrinae is a rather poorly studied group, and taxonomically very complicated; although most species were simply described as Stenogaster in the past, they have since been split into several different genera, such as Eustenogaster, Liostenogaster, Parischnogaster and others.
I have so far observed around 10 different species in Singapore. Shown below are photos of three species I have managed to photograph so far. I shall cover these wasps on a single page for now, till I learn more about them and get the species I found identified; I may then write individual species descriptions at a later date.
The nesting habits of Stenogastrine wasps are fairly varied. Unusual for social wasps (and possibly demonstrating their link with the potter wasps), their nests are usually made of a coarse mixture of mud, sometimes with just a little coarsely ground plant fibres blended in. The nests are very brittle, and presumably would not last in event of heavy rain, which is common all the time in the tropics. The nest is always attached to a hanging projection such as a plant rootlet; the attachment is very weak and the nest can be dislodged by a mere nudge This is probably the reason why all nests I have found so far have been in well-sheltered locations where the rain would not penetrate. They like damp, dark places close to a water source. Some species build open nests, while others have nests with a very interesting outer envelope. A couple of nests I found in Singapore are shown below.
This nest was found under a dark, damp tree root, just three minutes away from high-end apartment district and touristy shopping areas!
This nest was found in the Botanic Gardens, under a wooden structure designed to protect plants.
I am still not certain of the identity of most of the Stenogastrine wasps 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.
I do not know the full range of prey used by these wasps. I have seen the second species in the first two photos on this page actually catch Aedes mosquitoes, which are serious pests and carriers of dengue in Singapore! Other species frequently steal food from spider webs, and appear quite specialised to do so! They hover in front of the web, plucking at it with measured precision in an apparent attempt to distract the spider. Once the spider runs towards them to investigate the source of disturbance, the wasp quietly hovers back to prey trapped in the nest and carefully removes it, taking off in mere seconds! See here for more info on hunting strategies of wasps (the Stenogastrine habit of stealing from spider webs is covered at the bottom of the page).
Stenogastrine wasps are not aggressive. They can give a sting, but it is generally fairly mild. They usually flee when the nest is disturbed, returning only much later, when the source of irritation is gone. Furthermore, they do not usually nest in human dwellings, except for houses which have long been abandoned. Thus they pose absolutely no threat to us and should not be killed.