Exogenous Melatonin as a Potential Biopesticide: Effects on the Growth, Development, and Reproduction of Helicoverpa armigera

The cotton bollworm, Helicoverpa armigera, is a major polyphagous agricultural pest that is notoriously difficult to control due to its broad host range, scattered oviposition, larval boring behavior, and strong resistance to both chemical insecticides and Bt proteins. Melatonin, an endogenous hormone ubiquitously present in living organisms, plays critical regulatory roles in various physiological processes, including metabolism, immunity, stress responses, and reproduction. Moreover, numerous studies have demonstrated that exogenous melatonin plays an important role in helping plants defend against diverse environmental stresses. In the present study, an artificial diet supplementation method was employed to investigate the effects of different concentrations of exogenous melatonin on the growth, development, and reproduction of H. armigera, with the aim of evaluating its potential application in integrated pest management (IPM). Larvae were treated with melatonin at concentrations of 0, 0.02, 0.2, 2, and 20 μg/g. The results showed that with increasing melatonin concentration, larval developmental duration was significantly prolonged, whereas pupal duration was significantly shortened (p < 0.05). At 20 μg/g, the longest larval duration (16.90 d) was recorded, representing an extension of 2.69 d compared with the control, while pupal duration (8.04 d) was reduced by 2.16 d relative to the control. Furthermore, higher melatonin concentrations significantly decreased larval head capsule width, larval body and pupal weight, and also significantly shortened adult longevity and oviposition period, while markedly reducing both total fecundity and daily fecundity per female. At 20 μg/g, the mean total fecundity per female reached its minimum (224.13 eggs), representing a reduction of 62.81% compared with the control. Collectively, these findings demonstrate that exogenous melatonin can significantly inhibit the growth, development, and reproduction of H. armigera, highlighting its potential application as a biorational tool in crop protection and integrated pest management.