We experimentally investigated the effects of plant quality and natural enemies on the abundance of different herbivore guilds on oak trees. Two oak species (Quercus laevis and Q. geminata) and four guilds of leaf herbivores (leaf miners, gall-formers, leaf-rollers, and chewers) were studied using a factorial design that manipulated predation/parasitism pressure and plant nutritional quality. Forty plants of each species were divided into four treatments: 1) control plants (nutrients and natural enemies unaltered); 2) nutrients added, natural enemies unaltered; 3) nutrients unaltered, natural enemies removed; and 4) nutrients added and natural enemies excluded. Fertilized plants exhibited significantly higher foliar nitrogen for both oak species, and tannins tended to increase over time and decrease with fertilization, but only for Q. geminata was this trend significant. Fertilized plants supported significantly higher densities of all herbivore guilds than did unfertilized plants, but exclusion of natural enemies did not significantly affect herbivore abundance for any guild studied. Our results demonstrate that all herbivores on oaks, regardless of guild type, respond more strongly to bottom-up effects such as host-plant quality than to top-down effects caused by natural enemies.
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Vol. 13 • No. 1