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?
November 4, 2018 at 9:12 am
November 4, 2018 at 1:47 pm
Fascinating. A micro-ecological mystery. I wonder if mycologists and entomologists could get together to solve this one? As for dispersion, I like the bobbing cork theory since Cypress and H2O seem to go together,
November 4, 2018 at 8:39 pm
Yes, and thanks…somebody will come along and look into it all properly. Of the three, I like the cork as well. I’ve had one of the galls afloat for several days…just as high and dry and happy as can be.
November 10, 2018 at 8:45 pm
Added on Nov. 16—the gall is still floating high and dry.
November 9, 2018 at 9:49 am
You always offer us a lot of tasty morsels to ponder.
Might the fungal pathogens already be active on the twigs chosen by mama midge, and like other fungal pathogens (say, like those on some plant roots that purportedly offer root knot nematodes a soft or vulnerable spot in the tissue to infiltrate), make the affected twig easier for the midge mama to penetrate to deposit her egg(s)? (Or, conversely, does the midge ovipositor initiate the wound that allows subsequent fungal infections?)
Are the galls in any way nutritious (offering sugars or protein to potential herbivores / opportunivores, or are the galls, conversely nasty-tasting or smelling, cautioning potential herbivores to avoid?
Are there any studies that suggest these galls affect longevity of the affected/infected trees? (I wonder if galls are like tree cancers in some way, gradually wearing down the tree’s immune system, making it a weaker individual more prone to other opportunistic illness). On the other hand…maybe some day some curious researcher in an oncology lab might discover some extract or component of the gall or its host that illuminates some aspect of host / pathogen survival or tolerance that may be useful to other species as well.
Always fun letting our minds toss around the tidbits you toss us, George :).
November 10, 2018 at 8:45 pm
Well, let’s see. On the fungi…sure, it is possible that the fungus softens up the target, but to make a total guess, I’d suspect the opposite…that gall-forming insects are evolved to avoid unhealthy egg deposition sites. With no data, I’ll guess that oviposition introduces fungi, or that the correlation of fungal diversity with egg success develops for some reason, perhaps not very exciting, after oviposition. On longevity, the low-data general consensus it seems to me is that the galls don’t harm the tree much, especially since they fall off with the twigs. But as you note, the signalling between gall and host is certainly complex and intricate, and goes to the DNA of the host. We’ll have to tune in again in another decade..