Gall Watching Is the New Bird Watching

This is the authors' abstract. We don't add key points for this paper.

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! 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.