The sun has just risen. You have your binoculars, your guidebook, and that special notepad where you keep your life list and take notes on each foray into the field (Fig. 1). What will you see today? Will there be more of the same things you’ve seen before, observations that could be made hanging around the trees in your neighborhood? Or will there be something new—a new species, a new county record, a new host record? This is why you got into gall watching!
A fine day for gall watching.
If you are new to this hobby, the oak tree down the street could be a fine place to start (Herrmann et al. 2012). Maybe you are on crutches. It won’t matter much; you probably don’t need to go far. The gall wasps (Cynipidae) work their trickery and hormonal magic throughout the plant kingdom, but nowhere are they as exuberant as on oaks (Stone et al. 2002). Galls are made of plant material: tissues whose development has been co-opted, at times with remarkable precision, by other organisms. Galls can be produced by fungi, bacteria, viruses, nematodes, or, most commonly, insects or other arthropods (Egan et al. 2018). Arthropods spanning several orders, including large groups such as eriophyid mites, cecidomyiid flies, and cynipid wasps, induce plant galls. Arthropod-induced plant galls have been observed on fossilized plant material dating back 384 million years—the time when seed plants and terrestrial arthropods both got their starts—and it is likely that microbes have been galling plants over a similar time frame (Labandeira 2021). Somewhere around 10,000 arthropod species produce plant galls, outpacing other oddities of natural history that have emerged in the tree of life, like animals whose juveniles consume milk produced in their mother’s ventral glands. Perhaps examples of both can be found hiding in that oak tree.
The first gall you spot is lumpy and colorful, like a miniature bunch of grapes glued to the underside of the oak leaf (Fig. 2a). Until recently, determining what the thing was would have been an exercise in navigating academic libraries. You might have pored over the smudgy black-and-white images of Lewis Weld’s 1950s-era treatise on gall wasp taxonomy, a self-published book you’d be hard-pressed to find outside of specialized collections. You might have trawled through a century’s worth of back issues from all sorts of entomological journals. But now, you pull out your phone to consult gallformers.org, and the matter is simple. Gall wasps, like many galling insects, are extremely choosy about the plants that they gall (Ronquist and Liljeblad 2001), so your first step is telling what oak you’re looking at. Know the plant, and you’ll have a much better sense of the identity of the gall. It’s a bur oak, Quercus macrocarpa, with strongly lobed leaves and chunky bark. There are around 40 gall wasp species that commonly make their home in this tree species, often also parasitizing a handful of other closely related oaks. Some of these galls are specific to particular plant tissues: stems, leaves, buds, acorns, and even roots. Your gall is on a leaf, which narrows down the possibilities even further. It matches closely with a photograph and description of Andricus dimorphus.
Galls formed by cynipid wasps on oaks come in many shapes and sizes. Galls of (a) grape-like Andricus dimorphus, (b) pink and fluffy Druon quercusflocci, (c) ball-like Philonix fulvicollis, (d) saucer-like Phylloteras poculum, (e) spindly Andricus chinquapin, and (f) hairy Druon ignotum were found on Midwestern bur, white, and chinquapin oaks in the fall of 2023.
The list of gall photographs that you flipped through includes a dizzying array of shapes and colors. There are woolly fluffballs colored gaudy pink (Fig. 2b), woody spheres ranging in size from a poppy-seed to a softball (Fig. 2c), shapes that look like saucepans or floppy hats (Fig. 2d), and hairlike spires that resemble a miniature Seattle Space Needle with a gall chamber at the end (Fig. 2e). These general forms have evolved multiple times in the cynipid family history, in which insects from distinct genera have converged on remarkably similar gall shapes and placements on the tree (Stone and Schönrogge 2003). Gall wasps still confound some of the cleverest molecular biologists. No one quite knows how the wasps trick a plant into creating these distinct shapes, though it is clear that they have profound control over the development of the plant tissue that they inhabit. Plant hormones seem to be involved, but the process is probably complex (Martinson et al. 2022). These are not some sort of unstructured tumor; the gall maker (a tiny, grub-like wasp larva, either alone or with its kin) sits somewhere inside of this structure, having pointed the plant’s vascular tissue toward itself: a buffet or, more accurately, a feeding tube for its entire juvenile life.
After documenting six species of cynipid wasps on your neighborhood oak, a decision point comes. You could simply upload your observations to the active cynipid research community on iNaturalist and leave the flow of arboreal life unimpeded, but meddling is the hallmark of a curious mind, and you want to know more. You pick off a few examples of your favorite gall species, and you put them in deli cups to try to rear the adult insects from the galls (Fig. 3). A garage or garden shed is a good place for this, because it retains the natural cycles of temperature and light that the gall wasp is accustomed to. You may need to replace the lid of the cup with some mesh if it gets moist or moldy (though some mold is okay).
The collections from a fruitful gall-watching journey in September. What insects will emerge from these galls to be found in the deli cups over the next year in our garage?
The life of a gall wasp is complex and also insular. Even their main antagonists have lives that revolve largely around the gall and the tree it is in. At the end of the season, many galls drop from the tree and overwinter in the leaf litter below. A couple of species jump like jumping beans, perhaps to get into hiding places down in the litter; another species has fatty blobs on the outside of the gall, attracting ants that carry the gall away (Warren et al. 2022). The main hypothesis for the benefit of these and many other gall structures is protection against parasitioids—other insects that may lay their eggs in the gallmaker and kill it. From the ten galls you collected and identified as Druon ignotum (Fig. 2f), three wasps eventually emerge, but none are the Druon species. They are parasitoids that were lurking inside of—and feeding on—the developing gall wasp while the gall was up in the tree. Some gall wasps even engage in theft of galls. An entire tribe of cynipid wasps, the Synergini, do not induce galls themselves, but instead co-opt them from other cynipid species. Such uninvited guests are referred to as inquilines.
The cynipid wasps that did successfully emerge in your deli cups chewed out of the galls in early May, coinciding almost exactly with the bud break and leaf expansion of bur oak. In case you’re interested in the sex of those tiny wasps, they’re all females. Not an aedeagus in sight, no matter how many you look at, even if you have the high-powered dissecting microscope and the patience needed for the search. Gall wasps make a mockery of sexual norms, even among the insects who are already pretty adventurous in that regard, by undergoing cyclical parthenogenesis. Most gall wasps have two generations per year; the one in the fall produces most of the vibrant and whimsically shaped galls that we see, and it consists entirely of morphological females. However, hiding out in those overtly maternal ovipositors are the genetic signatures of sex determination: without any sort of intercourse, some will lay male eggs in the spring, and others female. From these eggs, later in the spring or summer, males or females will emerge and then mate. Those mated females lay the eggs that become the asexual fall generation. Many of the spring and fall galls are so outrageously distinct that they have been described as different species, only to be recognized through genetic analysis as two halves of a whole (Hood et al. 2018). In other common gall wasp species, sexual generations have never been observed, even if we can find the genetic signatures of sex occurring in the shadows, where we have yet to see it (Stone et al. 2008). Maybe you will be the lucky gall watcher who unravels that mystery…
We wake up from our daydream. Clearly, gall watching is not (yet) the multibillion-dollar industry that bird watching is. Most gall wasps lack common names, and we rarely see them embroidered on tea towels or baseball caps. To some degree, this is the fate of an insect in our anthropocentric world. Silent Spring (Carson 1962) made the moral case against the unregulated use of chemicals that kill insects—largely because those chemicals harm birds. What would the world be like if we cared about at least some insect species in the way we care about birds? We may find out: gall wasps’ fellow Hymenoptera, such as bumble bees and other native pollinators, have recently garnered substantial acclaim among humans. So, perhaps there is still hope for gall watching—for the thrill of the chase (or in this case, the search in the tree) and the excitement of not knowing what will emerge from the next gall. Perhaps we will see you with binoculars or pole pruners near that oak somewhere in your neighborhood. Until then, we’ll be in the garage, looking at something mysterious in a deli cup.
*Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. government.
Ian Pearse is a research ecologist at the U.S. Geological Survey, Fort Collins Science Center where he studies plant-insect interactions. Leah Samuels was an intern at the Morton Arboretum, a student contractor with the U.S. Geological Survey and is most recently beginning graduate school in molecular ecology.
Gabe Ribicoff is an undergraduate student at the University of Chicago.
Jorge Jaime Rivera worked on this and other projects at the Morton Arboretum’s herbarium that is led by Andrew “Oak Origins” Hipp. All authors attest to a fondness for oak galls and admit to occasional stiff necks from staring up into oak trees.
References Cited
