These young bugs were congregating mostly around the leaf axils (the acute angles where the leaflets meet the stems). Treehoppers tend to be very host specific, and it’s likely that these hoppers feed only on walnuts. Although Membracids feed on trees and some other plants, only a few species are considered to be economically important pests (Triplehorn and Johnson 2005).
Treehopper adults are known for their enlarged and sometimes ornate pronotums. Some of them look humpbacked, some resemble thorns, some have keels, and others are even more ornate. The nymphs shown here will develop some such structure with a future molt.
Triplehorn, C.A. and N.F. Johnson. 2005. Borror and DeLong’s Introduction to the Study of Insects. Seventh Edition. Thomson Brooks/Cole, Belmont, CA.