In my last post I wrote about how treehoppers can communicate with each other using complex songs that are transmitted through their host tree’s branches rather than through the air. That’s amazing enough, but there is more to the story, according to an old article in Natural History magazine by Rex Cocroft, a treehopper expert who appears in one of the TV clips I included in the previous thread.
Many treehoppers, including the thorn-shaped ones studied by Dr Cocroft in Costa Rica, are social, with the nymphs living near the mother. The nymphs are prey for wasps, who carry them off and mash them up. Here’s what happens when a treehopper baby is threatened by a wasp:
“As a wasp closes in, the nearest nymphs make a brief vibration that sounds, when converted for human ears, like “ch.” This starts a chain reaction as neighboring nymphs, sensing the vibration through their legs, respond immediately with “ch” signals of their own. The wave of signaling spreads in a flash, and the signals of the whole group pile up into one collective vibrational shout. Heard through headphones, these group efforts sound as if someone has tuned a radio to the static between stations and then flicked the volume from zero to maximum and back again every second or so…Alerted that her offspring are in danger, the mother moves to their defense.”
Dr Cocroft explains that the mother fights off the wasp, though often not successfully, since treehoppers are not terribly ferocious. After the danger as passed, the mother makes its own special noises that calm down the babies, a treehopper version of a “Soft kitty, warm kitty” lullaby.
Dr. Cocroft’s full article, in Natural History magazine, is worth a read.
Treehopper mothers aren’t very good fighters, but that’s alright. They and their offspring can afford to “hire” more ferocious babysitters. They are tapped into a nearly infinite supply of nutritious tree sap, far exceeding their needs. Many species secrete droplets of this sap, which attract nasty ants who “farm” and care for the treehoppers. Life is easy for ants that have herds of treehoppers under their control. These ants don’t want to lose their little gold-mines so they fight off predators that would eat the treehoppers.
These ants react appropriately to the same treehopper alarm calls that mother treehoppers respond to. The ants respond by becoming agitated, increasing the odds that they will encounter the predator. An article by Morales et al proved this experimentally: “In field trials, playback of a recorded treehopper alarm signal resulted in a significant increase in both ant activity and the probability of ladybeetle discovery by ants relative to both silence and treehopper courtship signal controls.”
Treehoppers are not the only insects that use ants for protection. Some butterfly caterpillars in the families Riodinidae (the “metalmarks”) and Lycaenidae (including the “hairstreaks” and “blues”) secrete nutritious liquids in order to manipulate ants. In the 1990’s my friend Phil DeVries discovered that these caterpillars also make special sounds, transmitted through plant leaves, to call their ant “babysitters”. The calls were roughly similar the ants’ own sounds. He called them “singing caterpillars”. Plants also pay ants with sugars and proteins to protect themselves against herbivores, but these “singing caterpillars” had evolved a way to subvert the ant defenders of plants by bribing them with higher-quality liquid. Once the caterpillars evolved the means to control ants, they could eat these plants, and could also have access to the aphid and treehopper larvae that would otherwise be protected by the ants. We normally don’t think of butterfly caterpillars as predators, but several species of Lycaenid caterpillars do eat these ant-tended bugs. Some Lycaenids are such masters at ant communication (chemical and/or sonic) that they can trick ants into feeding them or carrying them into the ant nest, where they eat the brood, right under the noses of the duped guardian ants. Some (all?) of these caterpillars protect themselves and manipulate the ants with fake ant pheromones and fake ant sounds. Some caterpillars specifically imitate the sounds of the queen ants, so that they too get “the royal treatment” inside the nest.
Remarkably, the plants themselves are not just passive instruments for all these insect noises. People have long claimed that plants can hear and react to music or speech, and while most of those claims were dubious, we now know that some of them are true: PLANTS CAN HEAR! Not only that, they can respond appropriately to what they hear.
Some of the most important noises for plants are the sounds of insects chewing leaves. Even a human can hear a caterpillar chewing, and a plant hears these same sounds as they are transmitted through their leaves, or through the leaves of nearby plants. Plants can distinguish caterpillar-chewing sounds from other less-threatening sounds. When they hear these sounds, they produce more defensive chemicals to fight off insects. Dr Heidi Appel and Dr Cocroft played recordings of chewing caterpillars to plants, and measured the plants’ production of defensive chemicals compared to control plants which only heard recordings of wind or leafhopper noises. They showed that the plants which heard chewing caterpillars produced significantly more defensive chemicals than the controls. Plant communication, even more than substrate-born insect communication, is a vast secret dimension of nature that we are only beginning to uncover.
DeVries, Phil (1991). Call production by myrmecophilous riodinid and lycaenid butterfly caterpillars (Lepidoptera): morphological, acoustical, functional, and evolutionary patterns.