ECOLOGICAL AND BEHAVIORAL BASIS FOR REPRODUCTIVE ISOLATION IN THE SYMPATRIC ENCHENOPA BINOTATA COMPLEX (HOMOPTERA. MEMBRACIDAE).

Abstract

A pervasive question in evolutionary biology is whether sympatric speciation can be promoted by the exploitation of new plants or changes in life history. Recent studies (Maynard Smith, 1966; Alexander, 1968; Huettel and Bush, 1972; Knerer and Atwood, 1973; Bush, 1969, 1975; Tauber and Tauber, 1977a, 1977b; Edmunds and Alstad, 1978) have made sympatric speciation more readily accepted than in the past (Mayr, 1970) but it is still subject to debate (Futuyma and Mayer, 1980). Our past work (Wood, 1980; Guttman et al., 1981) with the treehopper Enchenopa binotata Say suggests that this insect diverged along plant lines into a complex of sympatric reproductively isolated species. Enchenopa is common in eastern North America on deciduous trees and shrubs as far south as Panama. Enchenopa from each of seven North American plants are all listed taxonomically as the same species (Metcalf and Wade, 1965), although they differ in nymphal coloration, oviposition site, egg froth composition, time of day they deposit eggs, and nymphal feeding sites. Females from a given species select that for oviposition when given a choice, and when confined to a single small cage select mates assortatively by plant species (Wood, 1980). Insects from each species are electrophoretically distinguishable although differences within host races can be found among samples collected from different conspecific trees. Enchenopa on Juglans nigra, Ptelea trifoliata, Robinia pseudoacacia, and Cercis canadensis are electrophoretically different from each other and from those on two species of Viburnum and Celastrus scandens, which are similar to each other (Guttman et al., 1981). Thus E. binotata is a complex of reproductively isolated species. The North American plants of Enchenopa represent evolutionarily diverse plants which are broadly and locally sympatric. When several species are found together Enchenopa may occur on each host. Females insert eggs into the branch of the plant and cover them with a secretion known as egg froth, which protects overwintering eggs (Wood and Patton, 1971), and contains an ovipositional attractant which is responsible for the clumping of egg masses on single branches. The large aggregations of nymphs that result attract mutualistic ants, which apparently increase nymphal survival (Wood and Seilkop, unpubl.). Wood (1980) suggested the following hypothesis based on the work of Bush (1975) to explain the divergence of the Enchenopa binotata complex. Based on present geographical distributions (Metcalf and Wade, 1965), plant and seasonal records in Costa Rica (Ballou, 1936) and observations by one of us (T.K.W.), the original stock was postulated to be tropical, polyphagous and multivoltine. As Enchenopa progenitors colonized north temperate climates they encountered deciduous hosts which promoted selection for coordination of treehopper life history