Ant protection of a host plant's defoliator: consequence of an ant-membracid mutualism [Vanduzea arquata, Formica subsericea, Maryland]

Black locust (Robinia pseudoacacia) is extensively defoliated by the locust leaf-mining beetle (Odontota dorsalis) in western Maryland. This plant is also the host of Vanduzea arquata (Membracidae), a gregarious, sap-feeding treehopper, which is tended by the ant Formica subsericea. Aggression by F. subsericea reduced adult density and density of egg masses of O. dorsalis on branches with Vanduzea tended by Formica (ant branches) compared to non-ant branches. Observations and staged encounters also showed that Formica ants excluded Nabicula subcoleoptrata, an important predator of O. dorsalis larvae, from branches with Formica and Vanduzea. Thus, ants indirectly protected the beetle larvae from this predator. In both 1980 and 1981 survivorship of beetle larvae was significantly higher in the presence of ants. In 1980 this resulted in nearly twice as many beetle larvae surviving on ant branches even when egg mass density was initially lower. In 1981 beetle survivorship was higher on branches with ants, although numbers of larvae remained higher on nonant branches compared to ant branches. There was no apparent net benefit or harm to black locust in having Formica ants and Vanduzea treehoppers, although these results suggest a mechanism by which ant attendance could harm the plant indirectly. These results are contrary to previous studies of ant-Homoptera-plant associations.