When the Macoun Club's study-tree project began, in 1991, all three children in the Oda family were on hand to choose their trees. Michael chose a Sugar Maple so tall that we never knew what was going on up there. But his sisters Terri and Susan chose something accessible -- they chose Leatherwood shrubs. And what they discovered on their study trees were colonies of a curious-looking insect, the Leatherwood variety of which had never been reported in the scientific literature.
Leatherwood gets its name from its tough bark and pliable twigs, which can be tied in knots without breaking them. The shrub is shaped kind of like a bonsai tree, but with large deciduous leaves and an open crown of branches. It's latin name is Dirca palustris. It has only two close relatives in North America, out west and in Mexico. Our species grows in hardwood forests across the eastern part of the continent, where its showers of tiny yellow blossoms are among the very first spring flowers.
Treehoppers are such distinctive-looking creatures that they too are readily identified as such. Since Leatherwood doesn't have thorns, these slow-moving creatures are easy to spot -- if you know to look underneath the twigs and leaves. Their marshmallow-white egg masses are even more obvious, but again, are always on the lower side of the twigs. They are in the family Membracidae.
Over a period of 13 years Terri and Susan reported each year on their study trees in the Macoun Club's annual publication, making a permanent record of their observations that we can draw on now. Terri's shrub gradually succumbed to a mysterious ailment that caused it to shed its branches one by one, until nothing was left. Susan's Leatherwood also lost all its branches, but in this case the cause is all too well known to us. Each winter, hungry deer came through the woods and browsed on the twigs, and every year they bit a little deeper. The ate until nothing was left.
The deer won't touch Leatherwood in summer -- not a nibble. There is something in its chemistry that is so distasteful it must be toxic to them. But in winter, the bare twigs are palatable, and in our Study Area, it has seemed that they preferred Leatherwood to all other deciduous browse. Because of a population explosion among White-tailed Deer, 99.9% of this once common shrub has disappeared. The Leatherwood Treehopper has disappeared with them. Though we check the few surviving shrubs, we haven't been able to find a Treehopper here in 10 years.
Both the Oda girls have now graduated from high school and from the Macoun Club, but inspired by their long-term work, Macoun Club leader Rob Lee has picked up the thread of the story. Rob lives in a place where Leatherwood is the dominant shrub, and where the damage to the plants is still minor.
The eggs are laid in late summer, in a cluster of niches cut into the bark. The Odas observed that what we call an egg mass is actually a sticky white coating that covers the eggs and lasts all winter. In spring, the dried coating splits open and after the eggs hatch, tiny nymphs emerge. For the next two months, the nymphs suck at the Leatherwood's sap, slowly growing and moulting. They pierce the bark with a sharp beak, and though they can walk, they stay in one place for an extended period.
In midsummer, the nymphs shed their skins one last time, and emerge as shiny new adults. The females are brown, the males blackish. Both have two white spots on the back. They continue to feed on their home branches for a few more weeks, and then begin to explore. Treehoppers can walk, jump, and fly, and by late summer they have dispersed. As they find each other in new places, males call and females respond (see video below left). In mid-August, the first egg masses are being laid. Egg-laying will continue into September.
As nymphs, the different treehopper species are distinguishable by colour and pattern, and there are also differences in the adults' behaviour and the appearance of the egg masses, but the adults themselves cannot really be told apart, except by the type of plant they live upon. Although other members of the species complex collectively go under the name Enchenopa binotata, our Leatherwood Treehopper turned out to be an unrecognized species (which, however, could be associated with an existing name Enchenopa brevis).
The Leatherwood shrub is found from southern Ontario and Quebec to Florida, and westward to Oklahoma, but how widespread are Leatherwood Treehoppers? Until recent weeks, we knew of them only in and west of Ottawa, Ontario. Now it has been reported from east of Ottawa, and from southwestern Indiana.Observations from other localities would help establish the range of this species. Contact Robert E. Lee using his initials (rel) at his ISP (magma.ca).
All images provided by Rob Lee; sound recorded in Ontario courtesy of Rex Cocroft, of the University of Missouri. Created August 20, 2004, and last modified Aug. 6, 2015.