Interaction Among a Nematode (Heterodera glycines), an Insect, and Three Weeds in Soybean.

A 2 x 3 x 4 factorial field experiment was established to determine the interaction among a nematode, an insect, and three weed species on soybean in 1983-86. Low (nematicide treated) or high (untreated) population densities of the soybean cyst nematode (SCN), Heterodera glycines, and 0, 30, or 70% main stem girdling by the threecornered alfalfa hopper (TCAH), Spissistilus festinus, were combined with no weeds, one common cocklebur (CC), Xanthium strumarium, one sicklepod (SP), Cassia obtusifolia, or one pitted morningglory (PMG), Ipomoea lacunosa, per meter of row in all possible combinations. Most of the losses from the pests were significant (P </= 0.05) and additive. The high population density of SCN suppressed soybean seed yield by 14%. Girdling of 30 and 70% by TCAH suppressed yields by 10 and 25%, respectively. One CC, SP, or PMG per meter of row suppressed yield by 22, 14, and 12%, respectively. The addition of loss predictions for each pest was approximately the actual treatment losses recorded. The pests did not have an evident interactive effect on yield losses; however, the losses attributed to each pest were additive.