During yesterday’s Greater Cincinnati BYGLive! Virtual Diagnostic Walk-About, Dave Shetlar (Professor Emeritus, OSU Entomology) showed pictures he’d taken late last week in central Ohio of snowy-white masses on the stems of redbuds (Cercis canadensis). The agglomerations could easily be mistaken for mealybugs, felt scales, or soft scales, particularly cottony scales.
However, the white masses are the "egg plugs" of the Two-Marked Treehopper (Enchenopa binotata). The 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 near one another
Treehoppers (family Membracidae) are sometimes called “thorn bugs” for their unusual appearance. The first segment of an insect’s thorax located just behind the head is called the prothorax and the plate covering the top of the prothorax is called the pronotum. Treehoppers commonly have enlarged pronotums configured into a range of fantastical shapes from pointy knobs to hatchet-like protrusions. If you look closely at the two-marked treehopper, you’ll note that the pronotum forms a knob at one end and is elongated to fit between the wings and cover the abdomen.
A Host of Variations
The two-marked 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.)
The females and males found on the different hosts all look the same. However, it’s been discovered that the exact “version” of the two-marked treehopper depends on the host. In other words, each of the different host plants 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 two-marked 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 '.
The driving forces behind the development of multiple variants of the two-marked treehopper have been the subject of several scientific papers. This is 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 paper and a few others that describe fascinating discoveries made about this treehopper (see "Selected Reading to Cure Insomnia" below).
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 host plant produce entirely different vibration patterns compared to males on other hosts. They don’t catch the same waves.
Out-of-sync life cycles may also have contributed to the host-specific divergence of two-marked treehoppers. The treehopper variants all 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 laid on another host species depending on when the sap begins to flow for the two species. Adults that accidentally land on their non-host may think “I just wasn’t made for these times.”
No Controls 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 required.
Selected Reading to Cure Insomnia
2013 Repeatability of mate preference functions in Enchenopa treehoppers
2015 Variation in signal–preference genetic correlations in Enchenopa treehoppers
2017 You stay, but I Hop: Host shifting near and far co-dominated the evolution of Enchenopa treehoppers
2017 Local population density and group composition influence the signal‐preference relationship in Enchenopa treehoppers
2018 Female mate choice of male signals is unlikely to promote ecological adaptation in Enchenopa treehoppers