If you ever enjoyed collecting insects as a kid, you probably put some in a jar and watched them fight. It seems only in keeping with the rules of the natural kingdom that insects of differing species would try to eat one another. But if you think that the spirit of cooperation is a uniquely human trait, you might be surprised to learn about the relationships between certain treehoppers and ants.
Pictured above are a treehopper, Thelia bimaculata, and an ant from the collection. This hopper is common in the United States east of the Mississippi River, as well as in Canada. It makes its home on the Black Locust tree, which has been widely planted around the country. Treehoppers like these are a common sight around the TTD-TCN office, but the ant is unusual and intriguing.
The reason these two different insects have been pinned together is because of the mutualistic relationship that they share. In mutualism, two organisms cooperate so that they both benefit. This is opposed to parasitism, in which one-organism benefits and the other is harmed, or the rarer commensalism, in which one organism benefits and the other is neither benefited nor harmed.
Thelia bimaculata, like many other treehoppers, may use its unusual thorn-like appearance to blend in with their host plant and avoid predators. However, this may not always be enough to protect the treehopper colonies. In this case, they are protected by ants. These ants will surround the treehoppers, eliminating predators on the Black Locust trees that would otherwise eat the colony. With the pressure of predation diminished, the treehoppers are free to feed and reproduce as they please, causing the population to grow.
In exchange for the protection, the treehoppers allow the ants to collect a reward. This reward takes the form of a nutritious sugary substance called honeydew that the hoppers secrete. As anybody who’s been on a picnic can tell you, ants enjoy sweet substances, and they will vigilantly defend the hoppers so that they can harvest honeydew. This also has a secondary benefit for the hoppers, as the excess buildup of honeydew is harmful to them. In this manner, both the treehopper and the ant benefit and have a greater chance of survival than they would alone.
The insect world is fascinating and surprising, and the distinctive partnership between the treehoppers and ants is just one aspect of it. As we continue to digitize the plant bug collections, we will gain a better understanding of their place in the ecosystem.
Morales, M. A. Survivorship of an ant-tended membracid as a function of ant recruitment. 2000 Oikos 90: 469–476.