Manure was applied in corn plots in 1967 at the rate of 50 tons per acre. The northern and western corn rootworms, Diabrotica longicornis (Say), and D. virgifera LeConte, populations in these plots were reduced to about half the level of those in the check plots. Quantitative observations of the predaceous arthropods in the manured and the check plots showed that the total populations of ground beetles and spiders were not changed because of the manure application. However, the populations of mites, both predaceous and non predaceous, were 3 or more times as high in the manured as in the check plots. On the basis of the vertical distribution of these mites and of corn rootworms, it was concluded that the predaceous mites could be feeding on corn rootworms. This trophic relationship was confirmed by laboratory observations on the feeding activity of some predaceous mites. The effect of manure on mite populations dissipated in the following year, 1968, and the difference in rootworm population also diminished. All the above facts suggested that the predaceous mites were responsible for the Teduction in rootworm populations. Data also show that the presence of the corn rootworms may have served as a stabilizing factor in the predaccous mite populations. Manure was again applied in 1969. Adult rootworm population was lower in this area than in the 1967 manured and check areas. This fact confirmed earlier findings. The data of 1967 suggested that predation by mites accounted for a 19.7% control of corn rootworms under natural field conditions and a 63.0% control when manure was applied in the field.