Simple and gland-tipped trichomes occur in varying proportions in the cosmopolitan weed black medick (Medicago lupulina L.). Densities of both simple and glandtipped trichomes were separately negatively correlated with the amount of damage to leaves in herbarium collections from throughout the world. This indicates that both types of trichomes are protective against herbivores, but suggests that the two types of hair differ in effectiveness against different species. Since an average of less than 1% herbivore consumption of the foliage of black medick was observed, it is remarkable that adaptive value of the trichomes was demonstrable. In a greenhouse experiment, both gland-tipped and simple hairs provided considerable resistance against whitefly oviposition. For both herbarium collections and the greenhouse experiment, conspicuously fewer gland-tipped hairs than simple hairs were required to achieve a given level of resistance to insects. Low trichome density considerably reduced damage in comparison to the absence of trichomes, while increasing density was progressively less efficient, suggesting that moderate pubescence is the most desirable strategy.