FAQ About Undescribed Galls

- Adam Kranz


At the time of writing, of the 3112 galls listed on the Gallformers database, 1117 are undescribed. That ratio has not changed significantly since the early days of the site and isn't likely to change much in the future. You might reasonably wonder, if a gall can be distinctly identified by its morphology and host plant, such that we can represent it with its own Gallformers entry and consistently apply a Gallformers Code to observations, in what sense is it "undescribed", and what would it take to change that?

A gall is part of the extended phenotype of the inducing organism, and in many cases the traits of the gall are sufficient to identify the species that induced it. The traits of the gall can often even be used to make an educated guess about what other species the inducer is related to.

Regardless, the rules of taxonomy dictate that in order to formally describe a new species, taxonomists need to examine a physical specimen of the inducing organism. The species isn't considered "described" until their description is published in a peer-reviewed academic journal (so species described in grad school theses but never published in a journal are still considered undescribed).

One of our main goals at Gallformers is to facilitate you, an amateur or academic naturalist, in the process of collecting and rearing such a specimen and getting it to an appropriate taxonomist.

Raising an inducer is not hard, but each individual attempt has low odds of success. The biggest reason experts are more likely to succeed is not any particular technique but because they search harder and collect galls in larger numbers than any individual amateur. As a community, we can distribute that effort across many observers. So while you might not be successful yourself, you are still contributing to the process that builds the knowledge we need for someone to eventually succeed.

Being Prepared

When in the field looking for undescribed galls you need to be prepared if you want to maximize your chances of success in collecting and rearing. Bringing a couple of basic tools with you will greatly improve your odds.

  • Something to cut or cross-section galls. e.g., a small sharp pocket knife. I like pruning shears
  • Containers for collection. Ziploc bags, organza bags, and small plastic vials are each useful for different purposes
  • A way to capture the details of the collection. Ideally a geotagged photograph of the gall, plus a written tag to associate the physical collection with that image and track it going forward

When you collect a gall it is critically important that you capture several pieces of information. Without this information the specimen can be useless:

  • Date of collection
  • Location of collection: ideally Lat/Long (smart phone cameras often append this information to photographs automatically, but check first to make sure if you plan to rely on this method); if not, at least write down locale info and a rough description of the location so a future observer could find the site
  • Host plant species. If there is any uncertainty, and even if not, it's best to take photos of diagnostic features of the plant that will allow others to confirm your ID. This is especially important if you're in a location you can't conveniently return to

So, You Have an Interesting Gall, Now What?

So if you find a gall you determine to be undescribed (or perhaps a described gall that is of interest for other reasons), what should you do? The answer varies significantly depending on the taxon of the inducer.

1. Broadly place the taxon of the inducer.

Most galls can be placed taxonomically by comparison to other galls using the ID tool. For a truly new, unknown gall, or a gall listed as Unknown on Gallformers, the first step is to figure out what the likely taxon of the inducer is. This can sometimes be done with obvious external features, like rust fruiting bodies or mite erineum, but in general it requires dissection.

Carefully cut apart the gall. For most galls, a scalpel is a good tool for this (disposable ones are cheap online); thick-walled or woody galls you'll be better off with pruning shears or a sharper wood-carving knife. Try to make a shallow cut and then pull or pry the gall apart rather than passing the knife through the center of the gall, which destroys the larva.

Once you've made the section, photograph both the structure of the gall and the larvae as well as you can. This information should allow us to approximately place the inducer relative to known species.

2. Determine the gall's development timeline

Generally speaking, the most difficult part of collecting an inducer specimen is making your collection at the right time. To do that, you need to have a reasonable idea of when the inducer is likely to reach different points of its life cycle. If you find a gall that already has emergence holes, you're either too late or just in time (if the galls are abundant, section one to determine if others may still have inducers within).

To help determine when a gall should be collected, I've created a phenology tool that presents records from the literature and from iNaturalist and extrapolates to latitudes with no data. The records in the tool are incomplete both because existing information hasn't been imported and because it simply doesn't exist yet. Use phenology of apparently-related galls where possible. Otherwise, any information you obtain in collecting and rearing will help improve the tool for future users.

There are a few ways to investigate the phenology of a gall, and all of them are valuable even if you don't end up successfully rearing an inducer. If you can conveniently revisit the site, the most informative and least invasive is to simply check the gall at frequent intervals (2x a week is ideal) until you see evidence of emergence, which will give us at least one estimate of its emergence timing.

If you aren't likely to see it again, you should collect it immediately and try to rear it (see below). If you succeed, great; if not, then we know to wait a bit longer next time. If you don't want to take it home, or you have a lot of galls in front of you, it's once again informative to cut one open to see what developmental stage the inducer is in. This also lets us better calibrate future collections.

3. Collect at the right time

Once your gall is in the right stage for collection (pupae or adults; sometimes large larvae), depending on the taxon, you can either collect the sample directly or take the gall off the plant and bring it home to complete maturation. When removing the gall from the plant, it's often wise to collect not just the gall but the general area of the plant the gall is on, like stem sections above and below a stem gall or the full leaf or even twig for a leaf gall.

In every case, collecting more specimens is better for science and generally (but not necessarily or universally--use your discretion) not a threat to populations.

In a Pucciniales rust or an eriophyid mite gall, #2 is where the tricky part ends: if you collect the gall at the right time (when it is sporulating for a rust, when it is fresh for a mite gall), you just need to dry it and store it in an envelope.

For other taxa, like aphids, midges, or wasps, collections can't be made until after the point when the inducer no longer relies on the plant to complete its maturation. This happens at different life stages for different inducing taxa, but there are some general patterns.

Hemipteran inducers like aphids, phylloxera, and psyllids exist as nymphs for much of the gall's growth, and eventually produce winged adults at maturation. These winged adults are the ones necessary for description, and they typically hang out in the gall for some time before leaving through an opening called an ostiole. These can be collected directly from the gall as adults and preserved (see below).

Cynipid wasps exist as larvae for most of the gall's growth, and if the gall is collected in the larval stage they will likely die rather than emerge. Once they begin to pupate, however, they no longer need to feed on the gall and will likely survive to emerge from pupation and chew their way out of the gall.

Cecidomyiid midges exist as larvae in the gall and either pupate in the gall or emerge as a larva and pupate in the soil. The appropriate time to collect may vary by species for this group.

4. Bring the gall home to rear

Now that you've collected the gall, you need to store it in a sealed container so that whatever emerges won't escape. For spring galls on fresh, succulent, tissue, these need to be watertight so that the gall doesn't dry out. These will likely emerge within a relatively short timespan, so mold is often not a fatal issue. Ziplocs are a good choice but jars also work. Don't worry about air holes; inducers are small and don't use much oxygen before they emerge. Agamic cynipini or other detachable overwintering galls need to be kept humid but not too much so. Try to replicate the conditions they might experience overwintering outdoors in the leaf litter to the extent possible. We have had success for some galls with simple mesh bags indoors, however. Note that the emergence may be within a day or less of collection, but it may also take more than two years. If nothing has emerged yet, that may mean it's just waiting for the right moment.

For cecidomyiid midges, the process can be more involved. See this post by Charley Eiseman for more information, but you may need to transfer the larva/pupae from the gall container into soil and potentially refrigerate it over the winter before an adult can emerge.

5. Preserve what you reared

Once you have a specimen in hand, you need to preserve it to make sure your hard work doesn't go to waste through rot or degradation. The primary concern here is water: DNA-destroying enzymes are only functional when water is present. Water can be removed either by storing the specimen in a low-humidity environment like a freezer, or using a high-proof ethanol. Low proof ethanol (70%) is not ideal because it contains substantial proportion of water; 95% is great.

For eriophyid mite and rust galls, preservation means drying the tissue and storing it in a paper envelope.

For other arthropods, adults should be killed in a freezer and stored there dry until they can be mailed to a taxonomist. To ship the specimens, pack with cotton or tissue to prevent them from being damaged by rattling around the container. The galls from which these arthropods emerged should be preserved as well if possible--in many cases they can also simply be dried; if they are especially succulent or fleshy, they are likely no longer worth preserving by the time the adult emerges.

Some small specimens are prone to collapse or shrivel if dried. These can be better preserved in ethanol, but it must be high proof (95%). If you do choose to use ethanol, note that it dissolves both pen ink and graphite, and care must be taken to avoid smearing labels. A key concern is ensuring that the specimen can always be associated with its collection information, so making sure the label remains legible is crucial. Ideally, labels should be printed on a printer rather than written with pen or pencil, but if you do use pen or pencil, make sure the alcohol is sealed properly and the writing will not be in contact with it.

5.5 Inquilines

Unfortunately, it's as likely as not that you've gone through this whole process and ended up with something that isn't the inducer specimen needed to describe the gall. Depending on the gall, the adult arthropod emerging from your gall may be vastly more likely to be another species that displaced the inducer. Luckily these are also of scientific interest and can be preserved the same way. This is another reason rearing an inducer often takes many attempts.

6. Mail your specimens to a taxonomist

Contact the Gallformers team and we will do our best to help you network with someone who can describe your specimen. We have contacts working on most groups, with the conspicuous exception of eriophyid mites, which seem to be underserved taxonomically right now.