During yesterday’s 83rd Ohio Diagnostic Workshop held in Secrest Arboretum, OSU CFAES, Wooster, OH., participants found 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 common name refers to the two distinct marks that are highly visible on top of the treehopper. The females use their sharp, saw-like ovipositors to cut slits in the bark of their host trees and insert eggs into the stems.
The two-marked treehopper females cover the bark wounds with a white, sticky, sculptured waxy coating presumably to protect the eggs from dehydration, parasitism, and predation. The plugs also contain a chemical attractant that draws other females to lay their eggs near one another.
The egg plug exudate is extruded through the female’s ovipositor and remains spongy; it never hardens. It’s not unusual to find insects such as flies trapped in the sticky egg plugs as shown in some of the images in this Alert. This implies that the origin of the frothy plug material is connected to sugary sap extracted by the females as they feed.
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 treehopper variants all look the same and practice the same egg-laying behavior. However, the host-specific 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. One of my favorite titles is a paper published in 2018, "You stay, but I Hop: Host shifting near and far co-dominated the evolution of Enchenopa treehoppers." You can read this and other scintillating papers by referring to the section below titled, “Selected References (For Late-Night Insomnia Treatment).”
I'm Pickin' Up Good Vibrations (and Other Beach Boy Favorites)
Male two-marked treehoppers entice females by vibrating on plant stems and leaves to send a "cuddle up" message to females. The females receive the “wouldn't it be nice” message through specialized structures on their legs.
Using sophisticated voyeur equipment, researchers have listened in and discovered males produce different “fun, fun, fun” vibration patterns depending on their tree host. Males on red buds sound different than males on wafer-ash.
Of course, females are likewise tied to specific hosts meaning wafer-ash females only hear a “let’s go away for awhile” message from wafer-ash males. Redbud treehopper males may get around, but if they land on wafer-ash, it’s tears in the morning and sail on sailor.
Researchers speculate that out-of-sync life cycles may have contributed to the host-specific divergence of two-marked treehoppers. The treehopper's life cycle varies based on the host. Researchers have found that egg hatch in the spring is tied to sap flow. Consequently, 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.
No Management 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 References (For Late-Night Insomnia Treatment)
Fowler-Finn, K.D. and Rodríguez, R.L., 2013. Repeatability of mate preference functions in Enchenopa treehoppers (Hemiptera: Membracidae). Animal Behaviour, 85(2), pp.493-499.
Fowler‐Finn, K.D., Kilmer, J.T., Hallett, A.C. and Rodríguez, R.L., 2015. Variation in signal–preference genetic correlations in Enchenopa treehoppers (Hemiptera: Membracidae). Ecology and Evolution, 5(14), pp.2774-2786.
Fowler‐Finn, K.D., Cruz, D.C. and Rodríguez, R.L., 2017. Local population density and group composition influence the signal‐preference relationship in Enchenopa treehoppers (Hemiptera: Membracidae). Journal of Evolutionary Biology, 30(1), pp.13-25.
Hsu, Y.H., Cocroft, R.B., Snyder, R.L. and Lin, C.P., 2018. You stay, but I Hop: Host shifting near and far co‐dominated the evolution of Enchenopa treehoppers. Ecology and Evolution, 8(4), pp.1954-1965.
Fowler‐Finn, K.D., Kilmer, J.T., Cruz, D.C. and Rodríguez, R.L., 2018. Female mate choice of male signals is unlikely to promote ecological adaptation in Enchenopa treehoppers (Hemiptera: Membracidae). Ecology and Evolution, 8(4), pp.2146-2159.