Biology in the News Explained

Cool Bugs #2 – Rhagoletis juglandis

What do you think this sound is? Continue reading for the answer…

Today’s subject is a fruit fly, Rhagoletis juglandis. This is not related to the fruit fly of genetics fame, Drosophila melanogaster, which is in a different family. Nearly all drosophilids only eat fruit once it is rotting; flies in the family Tephritidae, including the genus Rhagoletis, feed on ripe fruit and thus are known to entomologists as the “true” fruit flies.

Walnut fly larvae

I will admit up front that these flies are mainly of interest to me as larvae (at the left), because they serve as hosts for one of my favorite parasitic wasps, Diachasmimorpha juglandis, below. R. juglandis larvae feed on and live in the fruit of the Arizona walnut (Julglans major) (i.e., the husk surrounding the actual nut), and D. juglandis females parasitize them through the walnut fruit skin.

The fly larvae live in groups in the walnut husk, sometimes by the dozens. All the larvae in a fruit may or may not have the same parents, if there have been multiple ovipositions in the fruit.
The parasitic wasp Diachasmimorpha juglandis
In the picture below is of a mating pair of R. juglandis adults on a plastic walnut model. Males and females mate multiply, with several individuals if given the opportunity.
Walnut flies mating
There are territorial contests by the males on the ripe walnuts while they are still hanging in the tree. This behavior is known as “boxing.” The males stand on their hind legs and bat their forelegs and wings together. (In the picture to the left, the wings are only a blur.) The idea is that the winners of these contests have access to more females, who will come to the walnut to mate and lay eggs. Some poor females are forced to mate as they extrude their ovipositors to dig a hole in the husk in which to lay eggs; the males will grab them from behind and mate with them before they have a chance to oviposit. Sometimes, though, males are so intent on fighting with each other that they don’t seem to notice a third male that is mating with the female on the fruit while they are going after each other.

Walnut fly males 'boxing'
While males are duking it out, mated females also get the opportunity to finally oviposit without harassment (left). A female drills a hole in the husk with the tip of her ovipositor (which eventually shows signs of wear) and deposits several eggs in a cavity just beneath the surface of the husk. These grow and feed inside the husk until they are ready to pupate, when they exit the fruit and burrow into the soil. Sometimes there are so many larvae within the husk of a walnut that their feeding is audible. Click here to listen to the sounds of feeding fly larvae in a walnut.
Walnut fly female attempting to oviposit

Unfortunately for the larvae, the racket they make chowing down on the walnut is their undoing… as will be revealed in the next Cool Bug of the Fortnight!

Here are references for more information on Rhagoletis juglandis:

Papaj, D.R., 1994. Oviposition site guarding by male walnut flies and its possible consequences for mating success. BEHAVIORAL ECOLOGY AND SOCIOBIOLOGY 34 (3): 187-195.

Henneman, M.L. and Papaj, D.R., 1999. Role of host fruit color in the behavior of Rhagoletis juglandis (Diptera: Tephritidae). Entomologia Experimentalis et Applicata 93:247-256.

Nufio CR, Papaj DR, Alonso-Pimentel H, 2000. Host utilization by the walnut fly, Rhagoletis juglandis (Diptera : Tephritidae). ENVIRONMENTAL ENTOMOLOGY 29 (5): 994-1001.

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