What family has the most species in the animal world? Here is a contender, observers estimate up to a million species in the Gall Midge Family, with over 1000 named in North America alone. They are micro-flies able to induce galls on plants as larval homes. Many arthropods make galls, and today’s galls are the big waxy-blue eye-grabbers of the Cypress Twig Gall Midge.
John and I were working yesterday in the aptly named Cypress Creek Natural Area, walking along the edge of a compelling Bald Cypress population. This species has the most intriguing quirks, for instance some of the most “ornamental” galls I’ve ever seen. The galls can be numerous, on the tips of its twigs, looking from the distance like some ripening fruit. They are the work of the Cypress Twig Gall Midge (and maybe sometimes a second related species). It decorates Bald Cypress, Pond Cypress, and the Montezuma Cypress native to Mexico.
Members of the Gall Midge Family in a general sense can be pests and parasites on plant pests, that is, they can seem to protect their host tree, a benefit employed in horticulture for natural biocontrol. I don’t know if the Cypress Twig Gall Midge (CTGM) bugs other pests, probably not, but it does suffer its own parasitoids…parasites on the parasite. The structure of the gall therefore no doubt serves to protect the CTGM larvae cowering within from parasitoids, and from larger predators.
What is the gall’s structure? It is soft, spongy, surprisingly large, to over an inch long, and coated with a blue-white powdery material suggestive of ripening fruit. Larvae embedded in it may be nestled safely away from most parasitoids and predators. But there could be more to the gall structure.
And with that, we enter the speculation zone. Beyond protecting the larvae, are there additional reasons why the galls are big, lightweight and spongy, and colorful? How about helping to disperse the midges? Not just storage…but moving and storage.
The galls are the color of juniper “berries” and suggest bird-dispersed fruits. I don’t know if birds peck them, but there a hint of plausibility hidden in a small literature on insect larvae dispersing via a bird’s gut. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1617192/
Dispersal could occur even if a bird merely pecks at the soft gall or rips part of it free and drops an uneaten fragment elsewhere. The midges reportedly mate upon emerging from the gall, so a gall chunk with even two of the average reported 16 larvae per gall could relocate potential mates together.
The galls occupy the twig tips. The twigs are deciduous, so the galls land on the ground. Rodents and ground-dwelling birds, even large insects, could move them or fragments hither and thither.
The galls bob like corks, remaining dry and waterproof. The twigs and galls drop more or less during the relatively dry season, but then again, it does rain during their “on ground” time, some places such as creek banks have water year-round, and we don’t know the entire temporal-spatial history of the galls anyhow. Maybe that waxy coating has to do with flotation, water-proofing, and decay delay.
As a closing note, biologists George Washburn and Sunshine Bael last year found a connection between midge success and fungal diversity within the gall. The galls are little fungus gardens. Who knows why? Do the fungi help sustain or protect the midges? Or do midge larvae in the gall promote fungi? Or both? Neither? Are larger galls merely better habitats for larvae and fungi? Does the mother midge inject fungi during oviposition, and if so, why?