As in the case for sap-sucking insects, evidence of the presence and activity of leaf defoliators might be obvious on the photographs taken from a sUAV flying close to the crop canopy (Fig. 8). Important visual cues include the presence of caterpillars and/or excreta on the plants and chewing damage on the leaves (Fig. 9). UAV based survey methods outlined above for the sucking insects is applicable to survey and monitor leaf defoliators on the crops.
Reduced plant biomass because of leaf tissue removal by chewing herbivores could be identified using high resolution UAV data. Direct removal of foliar biomass through insect feeding, death of leaf tissues following feeding damage, and the ultimate cost to the plant in terms of growth rate can be mapped using vegetation indices associated to aboveground biomass. For example, lower NDVI was shown for cotton fields associated with higher beet armyworm activity (Sudbrink et al. 2003). Substantial decrease in NIR region was shown for the plants growing in high beet armyworm activity (Fig. 10). In forest ecosystems, spectral reflectance and various indices (eg., NDVI and enhanced vegetation index [EVI]) from satellite data were used to map insect outbreaks and their impact on various tree species (eg., Radeloff et al 1999, Townsend et al. 2004, Dennison et al. 2009). Incorporating LiDAR data that provide better estimation of plant biomass can substantially improve the accuracy of such efforts in crop areas.
Dennison, P. E., P. L. Nagler, K. R. Hultine, E. P. Glenn, and J. R. Ehleringer. 2009. Remote monitoring of tamarisk defoliation and evapotranspiration following saltcedar leaf beetle attack. Remote Sensing of Environment 113:1462-1472
Radeloff, V. C., D. J. Mladenoff, and M. S. Boyce. 1999. Detecting Jack Pine Budworm Defoliation Using Spectral Mixture Analysis: Separating Effects from Determinants. Remote Sensing of Environment 69:156-169
Sudbrink, D. L., F. A. Harris, J. T. Robbins, P. J. English, and J. L. Willers. 2003. Evaluation of remote sensing to identify variability in cotron plant growth and correlation with larval densities of beet armyworm and cabbage looper (Lepidoptera: Noctuidae). Florida Entomologist 86:290-294
Townsend, P. A., K. N. Eshleman, and C. Welcker. 2004. Remote sensing of gypsy moth defoliation to assess variations in stream nitrogen concentrations. Ecological Applications 14:504−516
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