Spectral Responses to Larval and Artificial Defoliation in Eucalyptus dunnii: Implications for UAV-Based Detection of Gonipterus Damage

Remote sensing advancements have enhanced defoliation monitoring in forests, but distinguishing insect-specific damage from general canopy stress remains challenging due to overlapping spectral signatures. This study addresses this gap by analyzing multispectral reflectance changes in Eucalyptus dunnii caused by Gonipterus sp. n. 2 larval feeding and artificial defoliation (AD). A randomized complete block design with five replicates tested four treatments: No Damage, Medium (100 larvae/tree) and High (200 larvae/tree) larval inoculation, and AD (80% leaf removal). Twenty potted E. dunnii trees were monitored over 16 days using UAV-based multispectral 10-band imagery. Five multispectral flights were conducted during the experiment. The reduction in visible and near-infrared (NIR) reflectance likely reflects structural changes in canopy composition, namely an increased proportion of mature foliage. Both larval feeding and AD treatments decreased reflectance in these spectral regions, probably due to the removal of young leaves and exposure of older, darker leaves. This explanation is inferred from morphological observations; further biochemical measurements would be required to confirm the underlying mechanisms. Larval feeding and AD reduced chlorophyll-related vegetation indices (CVI, NDRE), decreased anthocyanin-related vegetation indices (mARI, ARI), and also caused a drop in relative carotene content (MTVI, CTRI/RE). The effects were strongest in the AD and peaked soon after the treatment, indicating that these pigment effects can mostly and also be attributed to the older leaves becoming more exposed. Statistically significant interactions between date and treatment were found for the pigment-sensitive indices, the Anthocyanin Reflectance Index (ARI) and the Chlorophyll Vegetation Index (CVI). They displayed opposite reflectance trends—CVI increased while ARI decreased—but followed a consistent pattern aligned with insect feeding. EVI values also exhibited a distinguishable pattern that matched this trend. Due to the inherent difficulty of studying insect feeding in natural settings, AD trials may serve as a practical proxy for assessing the impact of pest-induced damage on vegetation reflectance and physiological indices.