Development of Vegetation Indices as Economic Thresholds for Control of Defoliating Insects of Soybean.
James Board1, Vijay Maka1, Randy Price2, Dina Knight3, and Matthew E. Baur4. (1) Department of Agronomy and Environmental Management, Rm. 104 Sturgis Hall, Louisiana State University, Baton Rouge, LA 70803, (2) Department of Biological and Agricultural Engineering, Rm. 149 Doran Hall Agricultural Engineering Building, Louisiana State University, Baton Rouge, LA 70803, (3) Center for Geoinformatics, Louisiana State University, Baton Rouge, LA 70803, (4) Louisiana State University Agriculture Center, 402 Life Sciences Bldg, Department of Entomology, Baton Rouge, LA 70803
Because of greater efficiency relative to conventional methods, interest has developed for using vegetation indices as economic thresholds for insecticide application to defoliating insects of soybean. These insects usually infest soybean fields in the southeastern USA during the seed filling period. Vegetation indices may be useful indicators of canopy leaf area index (LAI) and light interception levels required to avoid yield loss from these pests [3.5-4.0 LAI, 95% light interception]. Our objectives were to: 1) Determine the relative accuracy of three vegetation indices for predicting LAI and light interception; and 2) Outline a method for using vegetation indices as economic thresholds for insecticide application to defoliating insects during the seed filling period. Several commercial soybean cultivars were planted near Baton Rouge, Louisiana (USA) (30o N Lat) in May, 2004 and June, 2005. In 2004, differences in LAI and light interception were created by manual defoliation, whereas in 2005, LAI/light interception differences occurred because of cultivars and planting dates. Data were taken on LAI, light interception, and three vegetation indices: the normalized difference vegetation index (NDVI), green normalized difference vegetation index (GNDVI), and the simple ratio (SR). Results indicated that across canopies ranging from very low LAI to canopy closure (95% light interception), NDVI most accurately predicted LAI and light interception (r2=0.93-0.97). Light interception and LAI were linked to NDVI by strong linear regression models, and did not show the quadratic response reported by others. However, regression models were not homogenous across years, indicating that such models need to be year/site specific. A proposed method for adopting NDVI as an economic threshold was presented. In conclusion, NDVI was shown to be an accurate predictor for canopy parameters associated with defoliation-induced yield loss; and this vegetation index has potential use as an economic threshold for insecticide application to defoliating insects of soybean.