Behavior of the weevilNeochetina eichhorniaeWarner, a specialist herbivore, varied with leaf age of waterhyacinth (Eichhornia crassipes(Mart.) Solms-Laubach [Pontederiaceae]). In the field, adult feeding decreased as the leaves aged. Accrued feeding damage on 8-d-old leaves was only about 60% of the sum of two respective sets of 4-d-old leaves. Likewise, on 40-d-old leaves the accrued feeding damage was only 21% of that from 10 successive 4-d-old sets. Laboratory studies confirmed a preference for the youngest tissue available and discounted the influence of leaf arrangement. The weevils preferred unfurling bud leaves by a 5-fold margin over open immature leaves. Olfactometer studies showed a 2.4-fold greater attraction to young leaves than to mature leaves. Further, adult feeding exhibited under-dispersed patterns on young leaves but random distributions on old leaves. In contrast, a generalist, the yellow woollybear,Spilosoma virginica(F.), preferred mature waterhyacinth leaves and 14-d-old larvae weighed 85% less when provided young leaves instead of mature leaves. The youngest leaves were higher in N, P, K, and Mg, but low in Ca and Mn compared with older leaves. Total phenolics (compounds reduced by Folin reagent) were also highest in young tissue, but analyses of leaf extracts by thin-layer chromatography and high-performance liquid chromatography showed lowest concentrations of phenolic compounds in the youngest leaves. These data suggest that natural plant products, not necessarily phenolics, attract the weevils to young tissue and stimulate them to feed, especially at sites of previous injury. These constituents also might deter generalists likeS. virginica, forcing them to consume only mature foliage.