Spittlebugs are a group of insects lovingly named for the sticky foam that they leave on plants. These insects attach to a host plant and feed on the sap that moves through the plant tissue. The sap passes through the spittlebug's digestive system and is excreted from the anus as a clear, sticky fluid that the spittlebug then breathes into, creating foam. The spittlebugs can produce 150–280 times their body mass in this fluid every day. The foam plays many important roles in the spittlebugs’ lives, including keeping them moist, repelling predators through the unpleasant smell or touch, protecting them from the sun's heat and light, and acting as an antimicrobial to keep germs at bay. While some of these amazing properties of this foam are known, there is still much to discover. To better understand how the foam interacts with structures such as the host plant or predators, Hannelore Hoch from Museum für Naturkunde, Germany, and colleagues from Technische Universität Dresden, Entofilm, Germany, and Christian-Albrechts-Universität zu Kiel, Germany, decided to document how the foam interacts with surfaces and how some spittlebug predators may behave when dealing with the foam.
To uncover more about the foam, the researchers collected spittlebugs and predators, such as ants and spiders, from various grasslands and marshes in Berlin, Germany. They recorded how the spittlebugs produce the foam, whether the predators avoided the foam and how predators behaved when forced to interact with the foam. The researchers also measured the contact angle, evaporation and pull-off forces of the foam to study its physical properties.
The team discovered that although the spittlebugs were surrounded by the foam, they were still able to move about as normal; the foam did not hinder them. When they checked how predatory ants interacted with the foam, they discovered that the insects visited a droplet of foam placed on a glass dish as often as they did a droplet of water, which may suggest that the ants try to use the foam to satisfy their thirst. However, when the ants walked into the foam, they struggled to escape and had to groom vigorously to clean themselves after breaking free. The foam was also capable of spreading over various surfaces, including the water-repellent, hydrophobic, surfaces found on many plants. It also turns out that the foam evaporates faster than water, which likely explains why the spittlebugs have to produce so much. And, as the foam dries, it gets stickier, which probably makes it even more difficult for predators when they make physical contact with it.
Understanding more about the sticky substance that the spittlebugs produce can teach us a lot about the ecological interactions between the spittlebugs, the plants upon which they reside and the predators that feast on them. The physiological process of creating copious amounts of foam can protect the insect from predators and the harsh environment around them, while the unpleasant flavour is also likely to deter animals from eating the plants that they live on. Interestingly, the sticky wet foam could also inspire the next generation of medical adhesives. While the thought of this foamy excrement many not be particularly appetizing, spittlebug foam is remarkably versatile, providing shelter and safeguarding young spittlebugs as they grow and transition to the next phase of life.