The Noise of Locusts – The sound of locusts was so loud that it was captured by fiber optic cables.

Considered one of the strangest testing laboratories in the world, it stretches over Princeton in New Jersey. It is a fiber optic cable suspended between three utility poles and then extends underground before feeding into a “probe device.” This device emits a laser through the cable and analyzes the returning light. It can capture small disturbances in this light caused by seismic activity or even loud noises, such as the passing of an ambulance. It’s a modern technology known as distributed acoustic sensing or DAS.

Discovering a New Use for the Technology

As DAS can track earthquakes, an increasing number of scientists are using it to monitor seismic and volcanic activity. (The buried system is highly sensitive; it can even detect pedestrians and cars above.) However, scientists in Princeton stumbled upon a noisier use for the technology. In the spring of 2021, Sarbar Ozharar – a physicist at NEC Laboratories, which operates the Princeton Lab – noticed a strange signal in DAS data. “We realized that there were strange things happening,” says Ozharar. “Something that shouldn’t be there. There was a distinct frequency everywhere.”

Using DAS to Monitor Insects

Add insects to the rapidly increasing list of things that DAS can spy on. Thanks to some specialized anatomy, locusts are considered the loudest insects on the planet, but all kinds of six-legged insects make a lot of noise, like crickets and grasshoppers. Using fiber optic cables, insect specialists may have found a new and powerful way to listen cheaply and continuously to species – from a distance. “Part of the challenge we’re facing at a time of declining insects is that we still need to collect data on population sizes and species and their locations,” says Weir. “Once we figure out what can be achieved with this kind of remote sensing, I think we could really get creative.”

How DAS Works in Monitoring Insects

DAS technology relies on vibrations, whether it’s the sound of a singing locust or the movement of a geological fault. Fiber optic cables transmit information, like high-speed internet, by emitting light pulses. Scientists can use the probe device to shine a laser through the cable and then analyze small amounts of light that return to the source. Since the speed of light is a known constant, they can determine where a specific disturbance is occurring along the cable: if something shakes the cable 100 feet away, it takes a tiny bit longer for the light to return to the device than if something happens at 50 feet. “We can turn every meter of fiber, almost, into some kind of microphone,” says Ozharar.

Using DAS to Monitor Lobsters

Ozharar’s team focused on a loop of cable at the top of one of the utility poles, which you can see in the picture above. (The loop is highlighted in red.) “If the fiber is in a linear shape, the sound interacts with the fiber once and then continues to travel,” says Ozharar. “But if you have a coil, the same signal travels multiple times through the fiber.” This makes the system much more sensitive, like recording a concert with multiple microphones, instead of having one fan in the crowd capturing it with their smartphone.

Using DAS to Monitor Cicadas

When cicadas emerged in the spring of 2021, Ozharar’s DAS system happened to be listening. This type of “periodical cicada” develops underground and emerges every 13 or 17 years to mate, depending on the type. “Due to climate change perhaps – although we’re not exactly sure why – there were some latecomers, so the populations that came out early were visited by the populations that came out later than they were temporally programmed to do,” says Weir. “Having a way to monitor this timeframe could be really useful.”

Using DAS in Monitoring…

DAS in Insect Monitoring

The male locust possesses an organ known as the tymbal, which vibrates like a drum to produce the eye-catching song. Each species has its own tune, allowing suitable males and females to find each other. There is additional information embedded in this sound as well: males tend to call during the hottest part of the day, which requires energy. This allows females to assess the quality of their partners – they prefer to choose the fittest males so they can pass on the best genes to their offspring.

Using DAS in Locust Monitoring

DAS can listen from the absolute beginning of the emergence through the peak and down to the decline as the collective celebration of mating recedes. The volume of the noise is a strong indicator of the number of locusts, allowing entomologists to determine the size of the locust population. They can even see the effect of temperature: when the weather is warmer, it is harder for male locusts to sing. “You can see that when you go through the five days that we have monitoring data, when the temperatures are a bit cooler, they have slightly different frequencies in hertz of calling,” says Weir.

Using DAS in Insect Monitoring

Fiber optic cables are already everywhere, waiting for scientists to utilize them. They are plentiful in urban areas, of course, but they also run between them, which is beneficial for entomologists wanting to monitor insects in rural areas. “We’re just using them to transmit data – zeros and ones – but we can do more than that,” says Ozharar. “That’s why fiber sensing will become more important and widely used in the near future.”

Using DAS in Insect Monitoring

But no one is suggesting that DAS will replace other methods of insect monitoring – while fiber optics are widespread, they are not everywhere. Instead, DAS can complement other techniques. The field of bioacoustics is already using microphones to listen to species in remote areas, sometimes aided by artificial intelligence to analyze the data. This method can help confirm data coming from fiber optics. Scientists are also experimenting with “environmental DNA” or eDNA, for example, using air quality stations to collect biological material shed in a specific area. And like Weir, entomologists still need to collect field samples to check the health of individual animals.

Using DAS in Insect Monitoring