The effect of wheat trichome length was distinguished from the effect of density of trichomes by determining the oviposition and larval survival ofOulema melanopus(L.) on 9 wheat lines. The 3 densities and 3 lengths of the trichomes quantified 6 profiles that were defined as few-very-short, few-short, very-many-short, few-long, many-long, and very-many-long. Eggs per plant and percentage larval survival decreased in the same order. Trichome length is therefore an important resistance factor.