Flies from a Goldenrod Leaf

As part of my survey of gall-making and leaf-mining species of Nantucket last summer, I collected a number of Elliott’s goldenrod (Solidago latissimifolia) leaves with mines like this one: Beginning three days later, agromyzid flies emerged from these mines over the course of a week.  Owen Lonsdale identified them as Calycomyza solidaginis. On August 15, the day the last agromyzid emerged, I was surprised to see this little midge in the rearing vial: I dumped out the leaves and scoured them for the source.  Finally, I spotted it, on the same leaf as the mine shown above: The midge’s pupal skin was projecting from an inconspicuous, flat, circular gall on the other side of the midrib.  The midge is Asteromyia modesta, a close relative of A. carbonifera, which forms much more conspicuous galls with a symbiotic fungus inside.  Or rather, a more conspicuous symbiotic fungus inside.  When I showed John Stireman these photos, he said of A. modesta:

They just seem to reside in inconspicuous “leaf pockets” rather than using fungi to make galls. However, they still seem to have a fungal association as the inside of their pockets tend to have a thin film of mycelia. Sometimes it is not obvious.

Dr. Stireman has studied Asteromyia carbonifera intensively, and his website states:

Although taxonomically described as a single species, genetic data indicates that populations of these gall midges are undergoing an extensive adaptive radiation onto different Solidago species. This diversification appears to be strongly influenced by a symbiotic fungus necessary for gall formation as well as a diverse suite of parasitoid wasp enemies that select for divergence in gall shape and perhaps host associations.

In his note to me, he said:

Like A. carbonifera, modesta is found on a wide range of goldenrods.  It also appears to consist of many morphologically cryptic but genetically distinct populations.

Speaking of parasitoid wasps, I got some of those from the leaves in that vial too.  On August 21 a braconid emerged (identified by Michael Sharkey as a member of the subfamily Opiinae): The next day, along with a second braconid, this pteromalid emerged:

The wasps were parasitoids of the leaf-mining flies, as evidenced by the exit holes they chewed in the undersides of the leaf mines:

A mine from which a fly has emerged (like the one shown at the top of this post), instead of a chewed hole, has a curved slit at the end, on the upper side of the leaf.

I’m not entirely sure how this happens.  The fly uses its ptilinum to pop the lid off its puparium, but I can’t quite picture how it then escapes from the leaf through a slit several millimeters away.  Most flies I’ve reared that pupated within their mines created an opening in the leaf in the process of opening their puparia.