Effect of Velvetleaf Competition and Defoliation Simulating a Green Cloverworm (Lepidoptera: Noctuidae) Outbreak in Iowa on Indeterminate Soybean Yield, Yield Components, and Economic Decision Levels

Abstract

We evaluated the separate and combined effects of velvetleaf, Abutilon theophrasti (Medic), competition and simulated green cloverworm (GCW), Plathypena scabra (F), defoliation on soybean grain yield and components of yield in a 2-year study. Competition durations of velvetleaf were manipulated manually (roguing) and chemically (postemergence herbicide). A temperature-dependent developmental model was used to determine realistically the rate and intensity of simulated GCW defoliation (imposed by hole-punching). Velvetleaf stress was largely limited to soybeans near full-season weeds. As few as 4,386 velvetleaf plants per ha were capable of causing economic losses in soybeans. Increasing the intensity of defoliation resulted in a proportional linear reduction in grain yield. Under the conditions studied, 25 to 29 simulated GCW larvae equivalents per m of soybean row were required to cause economic loss in full-bloom soybeans. At the plot level, little definable evidence of velvetleaf and GCW interaction was detected. On a stratum basis, consistency in velvetleaf-induced yield reductions was observed in the upper two-thirds of the canopy, whereas 2-year consistency in simulated GCW-induced losses was restricted to the lower two-thirds of the canopy. Nonadditive treatment interactions were consistently evident only in the central canopy stratum of soybeans near weeds. This study indicates that knowledge of velvetleaf prevalence in a field probably is not a prerequisite for using conventional monospecific economic injury levels in managing the GCW on fullbloom soybeans, assuming that a runaway velvetleaf infestation is not present.