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.

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.