Infestation Dynamics and Quantitative Impact of the Seed Borer, Trachylepidia fructicassiella (Lepidoptera: Pyralidae), on Cassia fistula Pods

The lepidopteron seed borer, Trachylepidia fructicassiella, is a pest that causes severe damage to the pods of the golden shower (Cassia fistula L.) and several other forest trees. The caterpillars burrow into the pods, devouring the seeds and pulp, which later impairs seed viability. Molecular characterization and phylogenetic studies of the caterpillars have confirmed that the specimens collected from infested C. fistula pods in this study were T. fructicassiella. This study investigated the infestation dynamics and quantitative impact of T. fructicassiella on C. fistula pods over two consecutive years (2022–2023). Field surveys revealed a high and persistent infestation rate, with 56.40% and 60.66% of pods affected in 2022 and 2023, respectively. Infestation led to severe physiological damage, significantly reducing pod length by 28.43% (p < .0001) due to early-stage larval boring. Most critically, the pest caused a catastrophic 78.47% reduction in seed viability, reducing the mean seed count from 63.49 in healthy pods to 13.67 in infested pods. The study recorded an average load of 9.97 insects per infested pod, comprising overlapping generations of larvae, pupae, and adults. Correlation analysis indicated no significant relationship between insect density and residual seed count (r = −.04), suggesting that even low-density infestations result in maximal economic damage. The findings of this study contribute to a better understanding of pest ecology and may assist researchers in developing management strategies for the cassia seed moth in C. fistula, thus shedding light on future studies of the cassia seed borer.