Mites attack males of the sexually polymorphic tree Acer opalus more harmfully and more often.

1. Theory predicts that plants that achieve fitness more through male function may be less tolerant of herbivory than those that achieve fitness through the female function. We tested this hypothesis by measuring the degree of infection and damage inflicted by the gall-forming eriophyid mite Aceria macrorhynchus on three different sexual morphs (males, protandrous bisexuals and protogynous bisexuals) of the Mediterranean tree Acer opalus. 2. The dependence of each morph on male function, estimated by registering gender changes over 5 years and quantifying male function, decreased in the order males, protandrous, protogynous. 3. The same decreasing order was found in the degree of gall infection produced by the mite, and in the reduction of photosynthetic rates and shoot growth, but not in the carbon-isotope discrimination caused by the mites. 4. Mites attack the morphs of A. opalus that rely more on the male function more often and more harmfully than they do other morphs. Male-biased herbivory is a selective force that could have been driving the separation of sexes in the genus Acer. Functional Ecology (2004) 18, 592–597 [ABSTRACT FROM AUTHOR]