Participants in the S.W. Ohio BYGLive! Diagnostic Walk-About held this past Monday in the Boone County Arboretum (Union, KY) observed wafer-ash (Ptelea trifoliata) stems festooned with small, sticky, snowy-white masses. The masses could easily be mistaken for a soft scale, mealybug, or perhaps an insect egg mass. In fact, they are the "egg plugs" of the Two-Marked Treehopper, Enchenopa binotata 'Ptelea' (order Hemiptera; family Membracidae).
Regardless of the host, all members of the two-marked treehopper Enchenopa species complex have one generation per season in Ohio. All of the species look the same and practice the same egg laying behavior. Eggs are laid in late summer with females using their saw-like ovipositors to cut slits in the bark of their host trees. After they insert their eggs, they cover the bark wound with the white, sticky egg plugs that serve 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.
The oviposition behavior appears to cause no appreciable harm to the tree hosts; stem dieback has not been observed with this insect. 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.
On the Origins of the Species
Based on phylogenetic data, it is now believed the single host (monophagous) E. binotata complex that we find in North America evolved from two closely related Enchenopa species found in Central and South America that are polyphagous; they feed on many hosts. Indeed, the two-marked treehopper has served as a model for investigating sympatric speciation; species that share a common ancestor.
The studies have shown that life-cycles of the two-marked treehopper Enchenopa species complex vary based on host phenology. Eggs hatch in the spring; however, the exact timing is tied to sap flow. 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. It is believed this host-driven out-of-sync egg-hatch played a major role in driving the species divergence.
A Song and Dance Man
Male two-marked treehoppers entice females by vibrating on plant stems and leaves to produce a "come hither" vibration that is transmitted through their plant host and detected by the females using specialized structures on their legs. Using sophisticated voyeur equipment, researchers have listened-in and discovered that males belonging to the different species produce different vibration patterns. Not surprisingly, only a female belonging to same species as the male will respond; they are "tone deaf" to messages sent by males belonging to another species.
Exactly how this evolved is not known. Some researchers speculate that it may have been connected to the way sound vibrations travel through different substrates; the different hosts. It's well known that the structural characteristics of wood can significantly influence both the amplitude and frequency of sound vibrations traveling through the wood. It's easy to imagine that successful treehopper males use just the right rhythm and beat to send their desires through their tree host to maximum effect.