Singing in the soil

Most people know that birds can be identified by their songs, but not many people know that songs can also be heard underground. When I started my research on greenhouse gas emissions from scarab beetle larvae in soils, the main challenge was (and still is) monitoring these animals without disturbing the soil. The current standard monitoring method basically consists of taking a spade and digging holes, which is not suited for studying undisturbed larval activity at permanent measurement plots throughout time, e.g. throughout an entire vegetation period, as you can see in the picture below.

Therefore, I started searching for non-invasive monitoring methods, ultimately ending up in the research field of soil acoustics. In soil acoustic research, we insert small acoustic sensors (or microphones) into the soil and then simply record all underground sounds for as long as needed. As you can see in the second picture, the measurement site looks completely undisturbed although a soil audio recording is in progress. Scarab beetle larvae produce three different kind of sounds. First, you can hear them moving through the soil, the scraping of their bodies against soil particles. Second, you can hear them feeding on plant roots. When they bite into a root, a breaking plant fibre produces a very short click sound. Moving and feeding sounds are called incidental sounds since larvae do not produce them on purpose. However, the third sound type is actively produced for communication which is scientifically referred to as stridulation. Scarab beetle larvae stridulate by rubbing their mandibles together. Basically one could say, they grind their teeth to talk to each other underground. The beauty about stridulations is that they seem to be species-specific just like bird songs. Here you can listen to stridulation examples from cockchafer larvae (Melolontha spp.).

Melolontha melolontha – Common Cockchafer

Melolontha hippocastani – Forest Cockchafer

So potentially, we could identify scarab beetle species in soils non-invasively simply by listening to their chatter. Non-invasive soil monitoring techniques could improve our knowledge of belowground biodiversity as well as the ecology of single species, leading e.g. to the design of environmentally-friendly pest measurement controls. However, here are the caveats. We don’t really know why and when larvae communicate with each other, what the best measurement protocol in the field is for monitoring stridulations, and how to analyse vast amounts of soil audio data in an efficient way. Globally, there are more than 20,000 scarab beetle species, but soil audio recordings are available for only a few species. The less visible a species is, the fewer data is usually available.

In my research, I want to address these caveats. We just published a study, in which we monitored stridulations of scarab beetle larvae in the laboratory and designed the first data analysis routine for the automated detection of stridulations in soil audio recordings. However, laboratory conditions often resemble an ideal world. There is no disturbing background noise and we know exactly how many individuals of which species are in the test soil. Now it is time to test our new data analysis routine under real field conditions in my new research projects “Underground twitter – Developing acoustic monitoring tools to study the cryptic life of soil-dwelling beetle larvae” funded by the National Geographic Society.

Backpacking for field work

One of the blogs of the European Geosciences Union (EGU) has a nice feature about field rucksacks. Scientists reveal what they pack into their rucksack when they departure for field work. I have already introduced you to my field work at cockchafer infested sites in Germany, and now you will also get a chance to peek into my field rucksack.

Field transportation

But before I can unpack my field rucksack at a measurement site, I first have to get there. What I really like about Germany is the fact that one can reach almost any location by public transport. I usually take the train and for covering the distance between a train station and a field site, I have my bicycle. The bicycle is also very useful to carry the spade used for soil excavations.

Field equipment

And here is a list of all the items you can find when unpacking my field rucksack.

  1. A small suitcase with audio equipment (microphones and an external amplifier) for spying on cockchafer larvae in the soil.
  2. A folder with data sheets (one for each excavated soil plot).
  3. 35 glass test tubes (110 ml volume each) with rubber stoppers for a) incubation of larvae to quantify their methane emissions, and b) transport of bulk soil samples.
  4. A suitcase containing a sensor and data logger for measuring soil moisture and soil temperature at different soil depths. The suitcase can also be used as a small table.
  5. Stainless steel cylinders with caps to collect undisturbed soil samples. Undisturbed soil samples are necessary to quantify dry bulk density and gravimetric soil moisture content.
  6. A knife.
  7. A small scale to weigh excavated cockchafer larvae.
  8. A plastic box with useful small items (pens, batteries, hollow needles for syringes, ect.).
  9. A stop watch to time larval incubations in the glass test tubes.
  10. The other half of my audio equipment – headphones and an audio recorder.
  11. A water and dirt proof field book for taking notes in addition to what is recorded on the data sheets.
  12. A small shovel.
  13. A spoon (which I actually never used…)
  14. A box containing evacuated glass vials for gas sample collection and storage, and two plastic syringes (one spare) for gas sampling.
  15. A mobile, handheld weather station for measuring air temperature, air pressure, relative air humidity, and wind speed. A GPS device to determine the exact position of each dug up soil plot.
  16. From left to right: a tool to insert stainless steel cylinders into soil without disturbing it, a knife to remove protruding soil from the cylinders after sampling, a hammer to drive the steel cylinder tool into the soil, a folding yardstick, and wooden sticks to mark the plots for excavation.
  17. Sunscreen, insect repellent, disposable gloves (cockchafer larvae are not “house-trained”), plastic bags for bulk soil samples and trash, and tape.


In the basement of our research institute, I keep a few living common cockchafer larvae which we had dug out in a pasture in Blaubeuren-Weiler (Germany) in May. The original plan was to use these larvae in an acoustic experiment to learn more about the communication between them. Well as it turned out, the larvae didn’t care about my schedule for the experiment. They had already reached their third and final larval stage when we collected them in the field. This means that in August/September, the larvae should start to pupate to eventually transform into the adult beetles. When food is abundant and soil temperatures are sufficiently warm, larvae can develop much faster and start to pupate earlier. It seems that my common cockchafers feel very much at home in the basement of the institute because that is exactly what happened. The majority of the larvae has already turned into pupae.

A healthy well-fed cockchafer larva is always clean (despite of living in soil), shiny and has a dark butt. The dark colour stems from soil and organic material in the larval gut. You can see its content shimmer through the skin which is transparent at this part of the larval body.

Common cockchafer larva - third larval stage

Common cockchafer larva – third larval stage (= third instar)

When the larva starts to pupate, it becomes inactive, stiff and starts to look like a little mummy. The skin becomes very dry and people who are unfamiliar with these animals might just think that the larva is dead.

Common cockchafer larva starting to pupate

Common cockchafer larva starting to pupate

To finally change from the larva into a pupa, the animal has to completely shed its skin. In the last picture, you see the final result – a common cockchafer pupa. The animal doesn’t look like an adult beetle (= imago) yet, but already quite different from the original larva. At the bottom of the picture, you can see the shed skin. If you look close enough you can still recognize the shape of the larval head.

Common cockchafer pupa

Common cockchafer pupa

In our recent publication about the rose chafer – another scarab beetle – we could show that these insects stop emitting methane when they transform from the larva to the pupa. This makes sense because at this stage the insects completely stop feeding. And thus the methane-producing microorganisms living in the gut system of these insects eventually run out of food and their productivity goes down as well.

Ernsthaft? Ernsthaft!

Wie würden Sie reagieren, wenn Sie jemand fragt, ob Sie sich mal Stimmen von Maikäferlarven anhören möchten? Mit genau dieser Frage habe ich letzte Woche Besucher beim Hessentag in Rüsselsheim angesprochen. Ich hatte dort die Möglichkeit zusammen mit vier meiner Feldmaikäferengerlinge, mein Projekt CH4ScarabDetect am Stand von ProLoewe im Rahmen des Schwerpunktes FACE2FACE den Hessentagsbesuchern zu präsentieren.

FACE2Face-Stand bei ProLoewe

Meine kleine Projekt-Präsentation auf dem Hessentag, …

Die Reaktion der Leute auf meine Frage verlief im Großen und Ganzen immer nach dem gleichen Schema. Erst erkundigte man sich, ob man die Frage richtig verstanden hatte. Dann ließ man sich versichern, dass das ganze kein Scherz ist („Sie wollen mich jetzt nicht veräppeln, oder?) und wer dann noch Interesse hatte, der kam auch zu meinem Stand und setzte sich die Kopfhörer auf.


…wo es zwischenzeitlich auch mal eng werden konnte.

Die Lautäußerungen von Engerlingen sind in keiner Weise mit den Stimmen vergleichbar wie man sie von Säugetieren kennt. Aber Engerlinge sind in der Lage absichtlich Laute zur Kommunikation im Boden von sich zu geben. Der Fachausdruck dafür ist Stridulation. Bei der Stridulation werden zwei bewegliche Körperteile gegeneinander gerieben, wodurch Laute erzeugt werden. Bei den Maikäferengerlingen werden die Mundwerkzeuge, die Mandibeln, gegeneinander gerieben. Man kann also sagen, sie unterhalten sich untereinander durch Zähne- und Kieferknirschen. Davon überzeugte sich unter anderem auch der hessische Kultusminister Alexander Lorz.

Kultusminister Lorz Hessentag 2017

Besuch des Kultusministers am ProLoewe-Stand.

Innerhalb des letzten Jahres habe ich schon unzählige Audioaufnahmen von Engerlingen im Boden gesammelt (dafür steckt man einfach ein Mikrofon in den Boden), aber ich habe keine Ahnung was die Engerlinge mit ihren Lauten bezwecken wollen. Für mich dienen die Audio-Aufnahmen derzeit in erster Linie dazu eine Methode zu entwickeln, um die Engerlinge im Boden einfacher zu finden, damit ich anschließend ihre Methanemissionen bestimmen kann. Wenn ich an den Punkt angekommen war, an dem ich den Besuchern erzählte, dass Engerlinge Methan genauso produzieren und abgeben können wie Kühe, kam meistens der zweite ungläubige Blick („Ernsthaft? Wie kommt man auf die Idee, sowas zu erforschen?“) Tja, auf diese Frage hatte ich nicht wirklich eine Antwort, aber es war definitiv keine Schnapsidee. Davon überzeugte sich auch die hessische Milchkönigin.

Milchkönigin Hessentag

Besuch der hessischen Milchkönigin, Sarah I.

Engerlinglauschen am Hessentag

Wenn Sie mal Engerlinge beim Fressen beobachten und belauschen möchten, oder mehr über Methanemissionen und Bodengase im Allgemeinen erfahren möchten, besuchen Sie mich morgen oder am Samstag auf dem Hessentag in Rüsselsheim. Gerne stelle ich Ihnen mein Projekt näher vor. Unterstützt werde ich dabei von vier Feldmaikäfer-Engerlingen aus Blaubeuren und eine ganze Menge Messequipment werde ich auch dabei haben. Sie finden mich am Stand von Hessen schafft Wissen/ProLOEWE (Lageplan Punkt 8) auf dem Opelgelände (Geländeplan_Hessentag)


Snacking out of season

One objective of my research project „CH4ScarabDetect“ is to quantify directly in the field methane (CH4) emissions of cockchafer larvae at cockchafer infested sites. In May, I visited the Kaiserstuhl – a region in southwest Germany famous for its wine production. To the dismay of the wine-growers, cockchafer larvae like to feed on vine, but I didn’t plan to visit any vineyards that day.

When I started my journey in the morning, I thought I would spend my day amongst walnut trees in a tree nursery in a small town called Sasbach-Leiselheim. There is a family-owned tree nursery and the family Schott already has to deal with the common cockchafer (Melolontha melolontha) as a pest insect for over 30 years. The common cockchafer is one of the main pest insects at the Kaiserstuhl.

Leiselheim am Kaiserstuhl


To my surprise, Mr. Schott senior drove me to  a small Christmas tree plantation near Burkheim which belongs to another landowner. Christmas trees were never on my list of vegetation types I wanted to visit during my research project, but if the cockchafers like them, I like them as well.


Christmas trees which have been damaged by cockchafers.

For my soil excavations at cockchafer infested sites, I start with marking 50 cm x 50 cm large measurement plots with the help of a wooden frame. Then, I dig up the soil in these plots to a depth of about 25 – 30 cm. For each cockchafer larva that I find during digging I write down at which depth I found it and I measure its weight. At the Christmas tree plantation, I found up to 25 larvae per m² at different stages of larval development (weights ranging between 0.6 and 2.6 g). A fully-grown adult beetle was also among my findings.


Excavated measurement plot (50 cm x 50 cm, 25 cm deep).


A larva of the Common Cockchafer right below the soil surface.

To determine the CH4 emissions of the excavated larvae, each individual is placed inside a large glass test tube which is then sealed air-tight for an hour. We refer to this method as incubation and the time the test tube is closed as incubation time. During the incubation time, all CH4 which is emitted by the larva is collected in the test tube. After an hour, a small air sample (25 ml) is extracted from the test tube with a syringe. The air sample will be stored in a small glass vial and latter analysed with a gas chromatograph to determine its CH4 concentration. I don’t know yet how much CH4 the larvae emitted at the Christmas tree plantation on that day because the data of the gas chromatograph still has to be analyzed. However from other sites, I know that the CH4 concentration inside the glass test tubes can increase to more than 50 ppm within an hour which is far above the atmospheric concentration of CH4 in the air surrounding us, which is about 1.8 ppm.


Cockchafer larva in a glass test tube for a methane measurement.

Vom Rheingau an den Kaiserstuhl

Ein Teil meiner Arbeit im Forschungsprojekt „CH4ScarabDetect“ besteht darin, mit Maikäfer-Engerlingen befallene Flächen aufzusuchen und die Methanemissionen dieser Engerlinge direkt vor Ort zu bestimmen. Anfang Mai ging es von der im Rheingau gelegenen Hochschule Geisenheim an den Kaiserstuhl. Beide Regionen sind für ihren Weinanbau bekannt. Die Engerlinge des Feldmaikäfers haben zum Leidwesen der Winzer auch Weinreben zum Fressen gern, aber die Weinberge waren diesmal nicht mein Ziel.

Als ich am Morgen losfuhr, ging ich noch davon aus, dass ich den Tag in Sasbach-Leiselheim in einer Baumschule zwischen Nussbäumen verbringen würde. Leiselheim ist der Sitz der Baumschule Schott, die sich auf Nussbäume spezialisiert hat. Die Familie Schott muss sich leider schon seit über 30 Jahren mit dem Thema Fraßschäden durch Maikäferengerlinge befassen, da der Feldmaikäfer einer der Hauptschädlinge am Kaiserstuhl ist.

Leiselheim am Kaiserstuhl

Leiselheim am Kaiserstuhl

Informationstafel der Baumschule Schott

Informationstafel der Baumschule Schott

An der Baumschule wurde ich von Herrn Schott senior mit dem Auto abgeholt. Ohne die Zusammenarbeit mit der lokalen Bevölkerung wäre mein Projekt, so wie ich es mir vorstelle, nicht durchführbar. Die vom Maikäferfraß betroffenen Landnutzer wissen am besten, wo es sich am meisten lohnt, nach Engerlingen zu graben, und natürlich brauche ich die Zustimmung der Eigentümer bevor ich auf einer Fläche graben darf. Außerdem lohnt es sich, den Leuten vor Ort gut zuzuhören, denn einen großen Teil ihrer Beobachtungen der Landschaft direkt vor ihrer Haustür findet man in keinem Lehrbuch oder Fachartikel. Von Herrn Schott habe ich zum Beispiel gelernt, dass Feldmaikäfer-Engerlinge sich in einer Stunde durchaus 40 cm horizontal durch den Boden graben können, und dass auf Äckern Wühlspuren von Wildschweinen auf Engerlinge im Boden hinweisen können, denn Wildschweine fressen gerne Engerlinge.

WIldschweinspuren bei Burkheim am Kaiserstuhl

Wildschweinspuren in der Nähe von Burkheim am Kaiserstuhl

Zu meinem Erstaunen brachte Herr Schott mich zu einer kleinen Anpflanzung von Weihnachtsbäumen in der Nähe von Burkheim, die einem Bekannten von ihm gehört. Weihnachtsbäume standen nicht auf meinem Plan, aber ich grabe da, wo sich die Engerlinge wohlfühlen. Für meine Grabungen markiere ich mit einem Holzrahmen 50 cm x 50 cm große Messflächen, die ich dann bis zu einer Tiefe von 25 – 30 cm aufgrabe. Von jedem ausgegrabenen Engerling werden die Fundtiefe und das Gewicht notiert. In der Weihnachtsbaumanpflanzung gab es bis zu 25 Feldmaikäfer-Engerlinge pro Quadratmeter in verschiedenen Stadien der Larvenentwicklung (Körpergewicht zwischen 0.6 und 2.6 g). Ein ausgewachsener Feldmaikäfer war auch dabei.


Weihnachtsbaum mit Fraßschäden. Im Hintergrund ein markierter Grabungsplot.


Bodentemperaturmessung in einer aufgegrabenen Messfläche.

Zur Bestimmung der Methanemissionen der Engerlinge wird jedes ausgegrabene Individuum nach dem Wiegen für eine Stunde in ein großes Reagenzglas gelegt, das luftdicht verschlossen wird. Diese Methode bezeichnen wir als Inkubation und die Verschlusszeit als Inkubationszeit. Während dieser Inkubationszeit sammelt sich das vom Engerling ausgestoßene Methan im Reagenzglas an und am Ende der Stunde wird mit Hilfe einer Spritze eine Luftprobe (25 ml) aus dem Reagenzglas genommen. Später wird im Labor mit Hilfe eines Gaschromatographen die Methankonzentration in dieser Luftprobe bestimmt. Wieviel Methan die Engerlinge in der Weihnachtsbaumplantage an diesem Tag ausgestoßen haben, kann ich Ihnen noch nicht sagen, da mir die Ergebnisse des Gaschromatographen noch nicht vorliegen. Aber von anderen Grabungen weiß ich, dass die Konzentration in diesen Reagenzgläsern durch die Engerlinge innerhalb von einer Stunde auf über 50 ppm ansteigen kann, was weit über der Konzentration von ~1,8 ppm Methan in der uns umgebenden Luft liegt.


Feldmaikäferengerling direkt unter der Grasnarbe.


Methanmessung an einem Feldmaikäferengerling.

A first time for everything

This is the 6th time that I am attending the EGU and yesterday I managed for the first time to be busy the entire day with meetings and actually not listening to any presentation or looking at any poster. That’s what scientists normally do most of the time at conferences. Listening to other scientists presenting their studies. The EGU is thematically subdivided into 22  divisions. I am usually jumping around between sessions of Atmospheric Sciences, Biogeosciences and Soil System Sciences. This is enough to bring your calendar to explode because normally one ends up with at least three different sessions running at the same time, and of course, at opposite ends of a four-storey building. However at the EGU, it does not stop there. For me it is THE PLACE for networking and there are loads of additional sessions dealing exactly just with that.

First of all, there are so-called non-public splinter meetings. You want to sit down with scientists from other universities and work on something? You can book a meeting room at the EGU. Yesterday morning we were seven people sitting together for one and a half hour working on drafts for scientific papers from a measurement campaign. These meetings can also be organized as public meetings for networking in the scientific community (e.g. “Ideas and perspectives for future research on forests and the CH4 and N2O cycles” organized by Mari Pihlatie from Helsinki) and it is also possible to contribute actively to shaping the programme of the EGU General Assembly in 2018 by joining the subdivision meetings (e.g. Subdivision SSS4: Soil Biology, Microbiology and Biodiversity). If you are a young scientist, go to these meetings! It is a great way to increase your visibility in the research community. In addition to the scientific and administrative sessions and meetings, there is also a huge exhibition with companies presenting their scientific instruments. These companies are keen on getting to know our research interests and needs to design new product lines. In the best case, it is a win-win situation for both sides. I had a meeting with a LI-COR Biosciences representative to discuss what I like about the currently availabe suit of gas analyzers and what improvements and new developments I am looking for to use in future measurement campaigns. And of course at the end of the day, I had to present my poster.


Poster presentations at the EGU can be a bit daunting and overwhelming for both the presenters and the audience. There are huge halls where you have nothing but posters. “That poster abstract sounds interesting, but do I want to walk another kilometer today to find it? Brain, are you still capable of taking in new information?”.



And thus it can happen that you have invested a lot of time in preparing a nice poster and you are at a conference with over 10.000 scientists, but maybe only two or three actually stop at your poster and maybe one asks you a question. Yesterday, I had the best poster session at the EGU ever! I love my research topic and think it is absolutely awesome and cool. What a feeling when other people come to you and have the same opinion. As it turned out, my boss did a great job in advertising my poster during her talk in the morning. A lot of people came and said “I heard this talk in the morning, can you explain me more about this and that”. One and a half hours of author’s attendance time (the time you actually should stand next to your poster to answer questions) were just flying by and I stopped counting to how many people I spoke during that time. Yes, fresh motivational boost for the upcoming work!

To finish the day, I went to listening to a talk by Pete Smith. For all of you soil scientists out there, yes, THE Pete Smith. Yesterday, he received the Philippe Duchaufour Medal for his outstanding contribution to the field of soil science and this guy is just an awesome presenter. The title of his lecture might give a hint (“Soil science is way more fun than a proper job”). You should really have a look at his abstract. And don’t get back to the first sentence of this blog post. This didn’t count as listening to a presentation. That was relaxing after work.



Goodbye solitude

Spring marks the beginning of a new field season. When I spent my days in the field measuring CH₄ fluxes between soils and the atmosphere, I am mostly all by myself. Last Thursday at my mesocosm experiment that little insect kept me company which didn’t mind being carried around with the measurement equipment.

However right now, I am in Vienna to visit one of the biggest conferences I can possibly attend in my reasearch area. The European Geosciences Union General Assembly 2017 has begun.


A full week of listening, watching, discussions, networking … over 900 sessions and more than 10000 attendees. Topics ranging literally from the Earth’s core all the way into outer space. You can even watch part of the action from home. For me, it will be the first time to present my project CH4ScarabDetect to a really large and diverse audience. If you are actually at the conference this week and interested to learn more about my project or insect CH₄ emissions in general, you can find me in poster hall X1 at board 17:30-19:00. My project supervisor, Prof. Claudia Kammann, is going to talk about rose chafers (Cetonia aurata) early in the morning in session SSS4.7.

EGU2017-12499 | Posters | SSS4.7

The influence of cockchafer larvae on net soil methane fluxes under different vegetation types – a mesocosm study
Carolyn-Monika Görres, Claudia Kammann, David Chesmore, and Christoph Müller
Tue, 25 Apr, 17:30–19:00, Hall X1, X1.221

EGU2017-12198 | Orals | SSS4.7

Stimulation of methane oxidation by CH4-emitting rose chafer larvae in well-aerated grassland soil
Claudia Kammann, Carolyn-Monika Görres, Stefan Ratering, and Christoph Mueller
Tue, 25 Apr, 08:45–09:00, Room -2.32