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.