Last August, on one of my visits to Nantucket for my survey of gall-making and leaf-mining insects on the island, I spotted a conspicuous gall that was new to me, in a beggarticks (Asteraceae: Bidens) stem.
I broke off the stem below the gall so I could put it in a bag, and right at the point where the stem broke, there was a moth pupa inside it. Apparently I never got a picture of the pupa, but I did get some of the moth (about 1 cm long) that emerged five days later:
Cutting the stem lengthwise, it was clear that the moth had nothing to do with the gall that had caught my attention. The moth’s “gall,” if you could call it that, was a slight swelling surrounding the frass-filled area where the caterpillar had fed. In the cross-section below, you can see the branch that diverges from the feeding gallery, leading to the exit hole.
The moth, fittingly, is known as the “Bidens borer” (Tortricidae: Epiblema otiosana). Related species bore in stems of other members of the aster family, such as E. scudderiana in goldenrod (Solidago) and E. strenuana in ragweed (Ambrosia).
I hung onto the stem to see what else might emerge. Within a few days, several of these ~2-mm flies appeared in the bag:
I sent them along with a batch of agromyzids to Owen Lonsdale, who identified them as frit flies (Chloropidae) in the genus Elachiptera. He noted that this genus needs revision, and so the specimens will sit in the Canadian National Collection without a species name until someone decides to do something about that. Larvae of many frit fly species feed in grass stems, while others are scavengers, parasites, or predators. I’m not sure exactly what these Elachipteras were up to, but looking closely I was able to find some larvae and puparia.
By the end of the month, several individuals each of two different beetle species had emerged in the bag.
The 2-mm beetle above is a “shining flower beetle” (Phalacridae), which Vassili Belov has tentatively identified as Stilbus apicalis. The 1.5-mm one below was identified by Brad Barnd as a “minute brown scavenger beetle” (Latridiidae) probably in the genus Melanophthalma.
Minute brown scavenger beetles are associated with rotting vegetation. Shining flower beetles mainly feed on fungi. With this in mind, I’m fairly sure the gall on the Bidens stem was caused by a fungus, which really is pretty clear from its external appearance. Insect-caused stem galls tend to be much more regular in shape and size. However, the galls of the midge Asteromyia tumifica (Cecidomyiidae) on goldenrod stems are quite irregular and lumpy, so it wasn’t totally unreasonable to imagine that some kind of insect was responsible for this.