Small, sticky, snowy-white masses are appearing on the stems of redbuds (Cercis canadensis) in southern Ohio. They could easily be mistaken for soft scales, mealybugs, or insect egg masses. However, they are the "egg plugs" of a treehopper originally named the Two-Marked Treehopper (Enchenopa binotata, family Membracidae).
The treehopper females use their sharp, saw-like ovipositors to cut slits in the bark of their host trees and insert eggs into the stems. They cover the bark wounds with the white, sticky egg plugs presumably to protect the eggs. The plugs also contain a chemical attractant that draws other females to lay their eggs in close proximity to one another. Eggs are laid in late summer and there is one generation per season.
The treehopper lays eggs on a wide range of hosts beyond redbuds. In fact, their egg plugs may be found on the stems of 15 plant species across 8 plant orders. This includes American bittersweet (Celastrus scandens), black locust (Robinia pseudoacacia), hickory (Carya spp.), tuliptree (Liriodendron tulipifera), viburnums (Viburnum spp.), wafer-ash (Ptelea trifoliata), and walnuts (Juglans spp.)
A Complex Story
Oviposition on a wide range of hosts is not unusual. However, it was eventually discovered that the exact "version" of the two-marked treehopper depended on the host. In other words, each of the hosts boasts its own two-marked treehopper. For example, the two-marked treehopper on redbud is specific to redbud; it does not occur on any of the other hosts. The treehopper on wafer-ash is confined to wafer-ash; the one found on black walnut is only found on black walnut, and so on.
The treehoppers are now collectively referred to as the "two-marked treehopper species complex," or the "Enchenopa binotata complex," or simply the "Enchenopa complex." When referring to hoppers found on a specific host, authors will sometimes assign the plant genus to the scientific name. For example, the two-marked treehopper found on redbud is sometimes written as Enchenopa binotata 'Cercis '.
A Song and Dance Man
Male two-marked treehoppers entice females by vibrating on plant stems and leaves to produce a "come hither" vibration detected by the females using specialized structures on their legs. Using sophisticated voyeur equipment, researchers have listened-in and discovered the males on one plant host produce entirely different vibration patterns compared to males on other hosts; they sing a different tune.
Regardless of the host, all of the treehopper variants look the same and practice the same egg-laying behavior. However, their life-cycles vary based on the host. Researchers have found that egg hatch in the spring is tied to sap flow. The eggs laid on one host species may hatch at a different time compared to those on laid on another host species depending on when the sap begins to flow for the two species. This out-of-sync development may have helped drive the divergence, but other factors may also have played an important role.
In fact, the driving forces behind the development of multiple variants of the two-marked treehopper have been the subject of a number of scientific papers. One of my favorite titles, "You stay, but I Hop: Host shifting near and far co-dominated the evolution of Enchenopa treehoppers." I've included links to this and a few other papers under "More Information" below.
No Control Needed
Oviposition by the two-marked treehoppers appears to cause no appreciable harm to the tree hosts; stem dieback has not been observed with this insect. While the strikingly white egg plugs are often very evident, particularly on wet stems, the foamy exudate doesn't last long.
Although both the adults and nymphs suck juices from leaf veins and petioles, their feeding damage is also considered inconsequential even when high populations occur. So, control of these treehoppers is not generally required.