oa African Entomology - Determination of economic thresholds for semiloopers (Lepidoptera: Noctuidae) on four soyabean cultivars in Zimbabwe



The yield reduction in four soyabean, Glycine max (L.) (Merrill) (Leguminosae), cultivars, Duiker, Gazelle, Roan and SCSI, was measured in relation to different levels of artificial defoliation. Defoliation was induced manually, either as a single defoliation or sequentially over three different growth stages. At maturity, the yields of each cultivar at each defoliation treatment were obtained, and the reduction in yield compared to the undefoliated controls were converted to a percentage. Linear regressions of the percentage yield and percentage defoliation were fitted. The reduction in yield was influenced by cultivars, levels of defoliation, and the number of defoliations. The most critical growth stage for defoliation was the seed development stage, but differences in yield due to defoliation at the different growth stages were small and not significant. SCSI was the most resistant to defoliation, while Duiker was the most susceptible to single defoliations. Economic threshold levels were determined for the different soyabean cultivars based on the data that had a statistically significant linear relationship between percentage yield and defoliation. Based on three insecticides and two application methods, the economic thresholds were determined at the flowering stage for Duiker, at the pod development stage for Duiker and Roan and at the seed development stage for the Duiker, Gazelle, Roan and SCSI cultivars. In general, the estimated economic thresholds for the semiloopers ranged from 45 to 145 per metre of row, depending on the cultivar, insecticide and method of insecticide application. These thresholds equate to between two and seven semiloopers per plant.


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