Corn yields were measured after application of nematicides in 16 experiments, mostly in medium-to-heavily textured soil, at 12 locations in Iowa during 1973-1976. The average maximum yield increase in plots treated with nematicides was 21% over yields in untreated plots. Yields were correlated negatively with nematode numbers or nematode biomass in nearly all comparisons. Correlations of nematode numbers in the soil with yield averaged -0.56 for Helicotylenchus pseudorobustus, -0.45 for Hoplolaimus galeatus, -0.51 for Pratylenchus spp., and -0.64 for Xiphinema americanum. Correlation coefficients for numbers of nematodes in the roots and yield averaged -0.63 for Pratylenchus spp. and -0.56 H. galeatus. Correlation coefficients for yield and total number of nematodes averaged -0.65 in roots and -0.55 in soils. Negative correlations also were greater for comparisons of yield with total parasitic-nematode biomass than with numbers of individual nematodes of a species or total numbers of parasitic nematodes.
Keywords: Helicotylenehus pseudorobustus; Hoplolaimus galeatus; Pratylenchus hexincisus; Xiphinema americanum; rootworms.