FAQ About Undescribed Galls

- Adam Kranz


At the time of writing, of the 2806 galls listed on the Gallformers database, 963 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 have a physical specimen of the inducing organism in front of them. 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 in front of 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 section galls. e.g., a small sharp pocket knife
  • Containers for collection. The easiest and most readily available are zip-top plastic bags
  • A way to capture the details of the collection. Could be a marker to write on the collection bag, a field notebook, voice memos on your phone, etc

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 and Time of Collection (make sure to specify the timezone!)
  • 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.

So, You Have an Unknown 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.

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. Try to make a shallow cut to one side of the midline and then to 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 there are no holes, it may be too early to collect.

There are a few ways to find out, 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 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 can either collect the gall 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, 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 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 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. The specifics of this container don't matter all that much; we have had success with anything from mesh bags to sealed jars. Leave the container as close to the normal ambient temperature and humidity of the collection area as you can reasonably get, but it can be eg air conditioned without causing problems.

For most cynipid wasps, if the collection timing is correct, the inducer should take it from here. The only way can fail at this point if the container is allowed to get too dry (sitting in front of a heating vent is not great) or too wet and thus moldy (don't put too much plant matter other than the gall inside a sealed jar, and if you see condensation, dry it out). Note that the actual 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.

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

For other arthropods, adults should ideally be stored in ethanol and put in a freezer until they can be mailed to a taxonomist. 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 can be preserved with other methods but likely they are no longer worth preserving by the time the adult emerges.

In the short term, putting the insect in the freezer dry is fine. For a medium term, ethanol at room temperature works. Isopropanol and even high-proof vodka can be used as stop-gap preservatives, but the ideal medium is 95% ethanol (or higher), which (unlike 70% ethanol) effectively preserves DNA.

Note that when using ethanol or isopropanol, these solvents will dissolve pen ink and some pencil graphite, so take care to keep it away from labels. 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.

Small centrifuge tubes (0.5 mL should fit any inducer we know of) are a good and cheap storage container that will seal completely.

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.