Thursday, April 5, 2007
Diachasmimorpha juglandis is a parasitoid wasp in the family Braconidae. Like other braconids, it is relatively large; while most parasitic wasps are probably less than 5 mm long, D. juglandis can get nearly up to 1 cm.
This species is categorized as a “solitary larval-pupal endoparasitoid,” meaning that an egg is laid inside the host - in this case a larva (or close relative) of the walnut fly Rhagoletis juglandis, and a single adult wasp emerges from the pupal stage of the fly, having devoured it from within.
D. juglandis females, shown in these pictures, have to find their hosts without ever making visual or physical contact with them, because they are feeding within the husks of walnut fruits. Once a wasp has landed on the fruit, she walks around its surface, pausing every few seconds to feel for vibrations beneath her, caused by moving fly larvae. When she has located one, she inserts her long ovipositor through the husk (above right), and hopefully, into the body of the fly larva, where she lays a single egg. 
The tiny wasp soon hatches, and bides its time as a first stage larva, cruising through the bodily fluids of its host, sustaining itself on stored fat. Interestingly, the length of the first stage is variable, depending on what stage the fly was at when parasitized; the wasp needs to wait for the fly to begin its pupation (which it does after dropping out of the walnut and digging down into the soil). At this point, the wasp molts into the next stage and begins devouring the entire body of the fly, a task made especially easy because the fly tissues have begun breaking down on their own in preparation for reassembling as an adult fly.
Notice that in the picture above and at the left, of a recently hatched wasp larva, it has a rather large sclerotized head, somewhat like a helmet with jaws. This structure is present only in the first stage; after the first molt, the wasp larva looks like an amorphous white blob.
The reason for the sclerotized head and jaws has to do with the fact that these wasps are solitary - it is only possible for one wasp to emerge from a single host. It is possible for two wasps parasitize to the same host, so that there are two first-stage larvae present at the same time. Because the host only provides enough food for one, these wasp larvae must fight to the death within the host - which all the while continues on its merry way in the walnut, eating fruit until it is full grown and ready to pupate, oblivious to the battles occuring within its body. The jaws on the wasp allow it to easily attack and kill a rival wasp in the fly.
Which wasp wins? This is a fascinating question that has been studied in several solitary parasitic wasp species. In most cases, the one that got there first has the advantage, because it has had time to feed and grow a bit, making it stronger than any subsequent intruder. But back in the early part of the last century, some biologists studying these interactions in a related Diachasmimorpha species in Hawaii (imported there for biological control of medfly and oriental fruit fly) came to a fascinating conclusion following their dissection of hundreds of parasitized flies. They found that they could distinguish how old a first-stage wasp larva was by how fat it was - newly hatched larvae were skinny, but a wasp that had been in the host for a few days had begun to get tanked up on all the fat it was feeding on in the host. When they found two wasps in a death struggle, the skinny one had killed the fat one.
So apparently, when a wasp first hatches, it swims around the host before it tanks up, perhaps looking for any other wasp larvae. When it encounters a fat larva that has been there alone for a few days, it is an easy task for the skinny and much more maneuverable wasp to pop the older one with its jaws like a balloon (upper inset on photo above right).
By the time the fly begins to pupate, it is in the soil and will not be parasitized by another wasp; at this point there is only one wasp survivor inside of it. The sclerotized head of the first stage wasp larva is no longer necessary, and when it molts to the second instar upon fly pupation, the helmet is gone.
Both the host, and thus by necessity, the parasitoid, have just one generation a year. There are a few short weeks as adults for them when the flies emerge from the soil and oviposit in ripe walnut fruits, and then the emerging adult wasps try to find them and parasitize as many as possible. Flies and wasps then diapause (a type of insect hibernation) in the soil for nearly a year until ripe walnut fruits are available again.
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