A couple of years ago, my husband and I set out to see how many species of wasps and bees we could document with photos in our small urban yard. We’re up to nearly 90, which covers all the conspicuous and easy-to-ID stuff. One of the challenges has been to differentiate among all the black and yellow wasps. The yellowjackets and other hornets aren’t too bad, but there are many species of potter and mason wasps in the family Eumeninae that are very similar except for the number or arrangement of yellow spots or stripes on a black body.
A series of good photos from multiple angles often cinches the ID. Occasionally we have to resort to netting an individual, chilling it, and looking at some features under a microscope while consulting detailed keys.
That is how I discovered a remarkable structure found on some bees and wasps, especially the Eumeninae: the acarinarium.
Acarinaria are special structures on the body of bees and wasps that function exclusively to harbor mites. They may be a hollow chamber, a hairless area that is easy for mites to cling to, or a series of pits along the edge of an abdominal segment. In my photo of the potter wasp above, the mites are carried on the thorax.
The mites on the wasp are benign – they are in a non-feeding phase while on board. If the mites are on a male wasp, they transfer to the female when the wasps mate. Potter and mason wasps are not social wasps; each female constructs a separate nest made of mud, in the ground, in wood, or some other cavity. As the female provisions her nest, the mites disembark. There they continue to develop, and once the immature wasp pupates, the adult mites feed on the young wasp. Amazingly, this apparently does not harm the wasp. A new generation of mites hitches a ride out of the nest on the wasp when it emerges.
Evolving modified body parts to accommodate mites makes sense so long as the mites are beneficial. It’s presumed that the mites perform a service in the nest prior to them feeding on the wasp, such as combating fungi, predators, or parasites that might damage wasp eggs or young. Each genus of wasp is associated with a particular genus of mite, so this relationship is very specialized. Yet the precise mechanism of this mutualism is still poorly understood.
Now, do we start a mite list for the yard?