Caterpillars sense electric field from wasp wingbeats

In the natural world, detecting a predator early, or getting the drop on your prey, is often the difference between life and death. This multimillion-year-old arms race has led to the evolution of giant noise detecting ears, and eyes that can see nearly a full 360 degrees. But now, scientists are uncovering another world of predator-prey interactions that make use of another field of senses: electroreception. A study from the University of Bristol has found that a wasp’s wingbeats generate enough of an electrical charge to be detected by the caterpillars they prey on, giving these caterpillars a heads up that danger is nearby. Author on the study Sam England explains…

Sam - Animals in nature pretty readily accumulate static charge when they rub up against leaves or, or the air or sand or other things in their environment. And just in the same way that electrostatically charged balloons can move your hair around, we were wondering whether or not the natural static charges of animals could move the hairs around on other animals and whether those animals could feel that. And the really obvious application of this is, well, could a prey animal like a caterpillar detect the static charge of a predator animal like a wasp?

Chris - What scale of static buildup accumulates on something the size of a wasp?

Sam - It's really small <laugh>. So, you know, when you talk about units as they get smaller and smaller, you kind of have milli and then micro. We're talking, in terms of the units of charge, it's Coulombs. And the thing that we put in front of that is pico, so it's picocoulombs. So these are really, really small amounts of charge. But actually the interesting thing is that we're also talking about quite small distances between these charges. So as a wasp approach is a caterpillar, they're at some point only going to be centimetres or millimetres away. And at this distance, even very, very tiny charges can actually have quite considerable forces if they're in a material that's an insulator like air.

Chris - So that's your hypothesis, is it, how the caterpillar might be sensing the wasp is that the wasp distorts the electric field around itself and that pushes on the hairs of the caterpillar? And the caterpillar can feel its hair standing on end.

Sam - Yeah. Yeah, exactly.

Chris - And does that actually happen? How have you tested that?

Sam - We first wanted to obviously double check that the wasps and the caterpillars are accumulating charge like we see in other animals because that hadn't actually been measured for these animals before. We measured that by getting the wasps to fly through an electrostatic charge sensor. And yeah, it turns out the wasps are nice and charged, so they're going to be a source of electric field that can be potentially detected. And we also measure the charge of caterpillars by dropping them through the sensor. And then once we had the two measures of how charged these animals were, it meant that we could do computational simulations of how strong that electric field would be between the two. We got the numbers from these simulations and then we presented electric fields of that strength to various different species of caterpillar. And then we watched them to see if their behaviour indicated that they were perceiving these electric fields as a threat, which is exactly what we ended up seeing. So depending on the species, we saw a few different defensive behaviours in response to these electric fields. So one thing that we saw was a defensive coiling behaviour. So caterpillars like to kind of coil up into a ball to protect themselves from predators. But we also saw other behaviours like flailing. And also we actually saw that the caterpillars try to bite the source of the electric field when we played like a wasp mimicking electric field to them. So it basically tells us that these sensory hairs on the caterpillars might actually be kind of electro mechanically tuned to the wing beat of their predators. So basically like they've evolved to be especially sensitive to the electric field of their predators which is, in my opinion, really cool.

Chris - Given that you've discovered this sensitivity among, initially these caterpillars, but probably many other species are gonna do a similar thing, aren't they? And given the ubiquity of the signals that we are polluting the environment with, we are producing electric fields all over the place with what we do. Is there a risk then that our behaviour is gonna have a knock on effect on these sorts of behaviours in the natural world?

Sam - Yeah, I think unfortunately this might very much be the case. When we did this survey of how the hairs respond to different frequencies of electricity, we also saw that in that kind of peak sensitivity window that they have, it also includes 50 to 60 Hz. And the issue with that is that those are the exact frequencies that most of our electricity comes from power lines or electrical equipment in our homes, that's the frequency of those ac electrical signals that we have in our electrical grid. So it's very, very likely that caterpillars and probably many other animals are actually sensitive to the electricity that's being emitted by, by power lines and other electrical equipment. And it might be hindering their ability to actually detect their predators, either because it basically completely drowns out that electrical sense, which is quite concerning. Because basically just means that we've essentially discovered another way in which we might be polluting the environment this time with electrical noise pollution. The good thing is that now that we've possibly unveiled this damage that we might be doing, we can now come up with ways of preventing it. For example, by trying to shield our lines better from emitting the electric fields that they do.