I studied top-down effects on creosotebush (Larrea tridentata) insect herbivores by protecting them from bird and arthropod predation. The purpose of my study was to investigate experimentally the differential and/or interactive effects of two predator guilds on herbivore densities. I performed my experiments on 24 creosotebushes at each of three sites within the Jornada Long-Term Ecological Research Site in the Chihuahuan Desert of southwestern New Mexico, USA.... I conducted these experiments and censuses in 1993 and again in 1994. In both years the herbivore densities became significantly higher in experimental than in control creosotebushes. The effects of bird and arthropod predation on herbivore densities were additive in 1993, but they were compensatory in 1994.