We investigated the interaction between the honeydew-producing whitefly Aleurothrixus aepim and tending ants on shrubs of Croton floribundus (Euphorbiaceae) in a semideciduous forest in southeast Brazil. Whitefly eggs underside leaves were tagged during early March 1998 and randomly divided into two experimental groups: control (ants present, n = 75) and treatment (ants excluded, n = 52). During the 75-d experiment, honeydew-collecting ants visited 79% of the control groups of A. aepim. Adults emerged in significantly greater numbers from control than from treatment groups, the latter being heavily attacked by fungi due to accumulation of honeydew. Complete contamination by fungi was three times more frequent at ant-excluded (39%) than at ant-tended (13%) groups. Control groups with low levels of ant-tending produced significantly fewer adults than those more frequently tended by ants. Encarsia parasitoid wasps were more frequently seen on ant-excluded than on control A. aepim groups. Predatory arthropods, however, were equally frequent in either experimental group. This is the first experimental study to demonstrate ant-derived protection in honeydew-producing whiteflies. Given that aleyrodid honeydew can cause considerable damage to the host plant, our results suggest that the honeydew-gathering activity by tending ants is an important factor mediating such multitrophic interaction.302295297Bellotti, A.C., Smith, L., Lapointe, S.L., Recent advances in cassava pest management (1999) Annu. Rev. Entomol., 44, pp. 343-370Bronstein, J.L., Conditional outcomes in mutualistic interactions (1994) Trends Ecol. Evol., 9, pp. 214-217Buckley, R., Interactions involving plants, Homoptera, and ants (1987) Annu. Rev. Ecol. Syst., 18, pp. 111-138Buckley, R., Ant-plant-homopteran interactions (1987) Adv. Ecol. Res., 16, pp. 53-85Byrne, D.N., Bellows T.S., Jr., Whitefly biology (1991) Annu. Rev. Entomol., 36, pp. 431-457Cushman, J.H., Addicott, J.F., Conditional interactions in ant-plant-herbivore mutualisms (1991) Ant-Plant interactions, pp. 92-103. , C. R. Huxley and D. F. Cutler [eds.]. Oxford University Press, OxfordCushman, J.H., Whitham, T.G., Conditional mutualism in a membracid-ant association: Temporal, age-specific, and density-dependent effects (1989) Ecology, 70, pp. 1040-1047Cushmam, J.H., Whitham, T.G., Competition mediating the outcome of a mutualism: Protective services of ants as a limiting resource for membracids (1991) Am. Nat., 138, pp. 851-865Dansa, C.V.A., Rocha, C.F.D., An ant-membracid-plant interaction in cerrado area of Brazil (1992) J. Trop. Ecol., 8, pp. 339-348Del-Claro, K., Oliveira, P.S., Honeydew flicking by treehoppers provide cues to potential tending ants (1996) Anim. Behav., 51, pp. 1071-1075Del-Claro, K., Oliveira, P.S., Ant-homoptera interactions in a Neotropical Savanna: The honeydew-producing treehopper, Guayaquila xiphias (Membracidae), and its associated ant fauna on Didymopanax vinosum (Araliaceae) (1999) Biotropica, 31, pp. 135-144Del-Claro, K., Oliveira, P.S., Conditional outcomes in a neotropical treehopper-ant association: Temporal and species-specific variation in ant protection and homopteran fecundity (2000) Oecologia, 124, pp. 156-165Dowell, R.V., Integrating biological control of whiteflies into crop management systems (1990) Whiteflies: their bionomics, pest status and management, pp. 315-335. , D. Gerling [ed.]. Intercept, AndoverFransen, J.J., Natural enemies of whiteflies: Fungi (1990) Whiteflies: their bionomics, pest status and management, pp. 187-210. , D. Gerling [ed.]. Intercept, AndoverGerling, D., (1990) Whiteflies: their bionomics, pest status and management, , Intercept, AndoverMessina, F.J., Plant protection as a consequence of ant-membracid mutualism: Interactions on goldenrod (Solidago sp.) (1981) Ecology, 62, pp. 1433-1440Vinson, S.B., Scarborough, T.A., Interactions between Solenopsis invicta (Hymenoptera: Formicidae), Rhopalosiphum maidis (Homoptera: Aphididae), and the parasitoid Lysiphlebus testaceipes Cresson (Hymenoptera: Aphidiidae) (1991) Ann. Entomol. Soc. of Am., 84, pp. 158-164Way, M.J., Mutualism between ants and honeydew-producing Homoptera (1963) Annu. Rev. Entomol., 8, pp. 307-344Wood, T.K., Life history patterns of tropical membracids (Homoptera: Membracidae) (1984) Sociobiology, 8, pp. 299-344