Electrostatic sense for enemies

When they feel threatened, the caterpillars of many butterfly species roll up. © Sam England

An alarm system of a special kind: Some caterpillars can sense the electrostatic charge of approaching wasps and thus quickly take protective measures, according to an experimental study. The perception is based on the reaction of the hairs of the butterfly larvae, which are apparently specifically tuned to the typical electrostatic signals of the predatory insects. This detection system, which may be widespread in insects, could be impaired by man-made electrosmog, say the researchers.

The effect of electrostatic charges is particularly well known from the balloon trick: if you rub the plastic object on textiles, a charge builds up that can attract hair when you hold the balloon to your head. This effect also occurs naturally in the environment: through various mechanical processes, many objects become electrostatically charged – including animals and plants. There is already evidence that the recognition of such charge patterns could have considerable biological significance. For example, it has already been shown that flying insects can use the electric fields of flowers to orient themselves when searching for food.

Predator-prey relationships in focus

These results now formed the basis of the study by Sam England and Daniel Robert from the University of Bristol. “We knew that many insects naturally accumulate static electricity on their bodies as they move around in their environment, and that these charges can influence objects. So we wondered whether a typical prey animal – like a caterpillar – could also detect its predators based on the electric field they emit,” says England.

To investigate this question, the two researchers conducted studies on caterpillars of three European butterfly species. They chose the common wasp as their model predator. First, England and Robert recorded the electrical charges that these predatory insects typically build up in flight using sensory measurements. They then entered these charge values ​​into computer models to predict how strong the electrical field would be when a wasp approaches a caterpillar on a plant. The researchers then artificially generated the corresponding voltage field – including the oscillations when the wings beat – in the laboratory and transmitted it to the test caterpillars using electrodes.

Hairs detect the threat signal

As the team reports, activating the wasp-typical field led the caterpillars of all three butterfly species to behave in the same way that they usually do when threatened: they curl up or throw their heads back and forth to defend themselves. The two researchers then investigated the mechanism of the caterpillars’ electrostatic perception ability. “We knew that the hairs of insects can be moved by the electric field emanating from statically charged objects, just as a charged balloon can move the hairs on your head,” says England. That’s why they used a laser method to closely monitor the reactions of the hairs of their test animals.

Not only did the movement of these tiny structures become apparent under the influence of the wasp-typical voltage field. The hairs also have an electromechanical resonance that exactly matches the wing beat frequency of the flying predatory insects, according to the results of the study. The researchers see this as an indication that the characteristics of these structures are specifically geared towards the perception of the electrostatic fields of the flying predatory insects.

“These results are the first evidence that land-dwelling animals can detect predators by the static electricity they emit,” England emphasizes. He believes that this concept is widespread among insects. “But it also indicates a previously unnoticed way in which humans could have a negative impact on wildlife,” says the scientist. It seems conceivable that electric fields emitted by power lines or electronic devices irritate or stress these creatures. “I therefore think it makes sense to investigate more closely to what extent we are actually affecting living creatures through this form of pollution,” says England.

Source: University of Bristol, Fasch article: PNAS, doi: 10.1073/pnas.2322674121

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