Treehopper

Abstract: We investigate the ant fauna associated with the Neotropical treehopper, Guayaquila xiphias, on shrubs of Didymopanax vinosum in the cerrado (savanna) of SE Brazil. Treehoppers infested plants at the border of the cerrado almost exclusively and preferably fed near the apical meristem. During the reproductive phase of the host plant, however, the vast majority of the treehoppers aggregated on the inflorescences. We found 21 ant species harvesting honeydew at G. xiphias aggregations, the most frequent being Camponotus rufipes, Ectatomma edentatum, C. crassus, and C. renggeri. Such a taxonomic diversity of ants tending G. xiphias aggregations in the cerrado is far greater than that reported for any other ant-homopteran system. Daily turnover of ant species at a given treehopper aggregation was observed on 29 percent (64 out of 222) of the G. xiphias aggregations recorded on D. vinosum shrubs. Species replacements probably reflect distinct humidity and temperature ranges tolerated by the species, and may ultimately reduce interspecific competition at homopteran aggregations. Since predation and parasitism on G. xiphias can be severe, and tending ants protect the homopterans against predators and parasitoids, the round-the-clock activity by ants at G. xiphias aggregations is regarded as crucial for the survival of these treehoppers in the cerrado.

Abstract: I examined mechanisms and patterns of benefit for the membracid (treehopper) Publilia concava tended by the ant Formica obscuriventris to test two hypotheses: that treehoppers benefit from ant attendance only by protection from predators, and that density-dependent benefit depends on the presence of predators. I used a factorial design, manipulating ants and predators in 1996, and ants and removal of uncollected honeydew in 1997. Results showed that treehoppers benefit from ant attendance by protection from predators. Additionally, results suggested that treehoppers benefit from ant attendance in ways other than protection from predators; tended treehoppers outperform untended treehoppers even with predators excluded. There was no support for the hypothesis that a proximate benefit of ant-tending includes removal of uncollected honeydew. A possible benefit (untested) of ant-tending is increased feeding rates. Treehoppers in small aggregations benefited more than treehoppers in large aggregations, indicating a density-dependent benefit in this mutualism, independent of predator level. Correspondingly, the number of ants per treehopper was highest for small aggregations. This study suggests that individuals can benefit from mutualisms in complex ways. Additionally, it adds to a growing number of studies that support the hypotheses that mutualisms may be stabilized by density-dependent benefit and that density-dependent benefit may be driven by the recruitment patterns of mutualists.

Abstract: A pervasive question in evolutionary biology is whether sympatric speciation can be promoted by the exploitation of new host 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 host 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 host 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 host species select that host 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 host 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 host plants of Enchenopa represent evolutionarily diverse plants which are broadly and locally sympatric. When several host species are found together Enchenopa may occur on each host. Females insert eggs into the branch of the host 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), host 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