Sustainable enhancement of sweet marjoram production: effects of biostimulants and biological pest control

This study explored sustainable approaches for enhancing productivity and quality of Majorana hortensis (sweet marjoram) using biostimulants and biological pest control. Effects of two biostimulants: humic acid (HA) and effective microorganisms (EM), applied at concentrations of 5 and 10 mL L−1, on sweet marjoram were evaluated under net-house conditions in 2022 and 2023, using a randomised complete block design. In addition, the predatory insect Orius albidipennis was assessed for its efficacy in controlling whitefly (Bemisia tabaci) populations. In both years, plant growth parameters, leaf pigment content, essential oil yield and composition and insect control were monitored across three harvests (May, July, and September). Both HA and EM significantly enhanced plant height, biomass, and essential oil yield. The highest essential oil yields of 206.5 and 177 L ha−1 were recorded for the HA 10 mL L−1 treatment, in the first and second season respectively, approximately a fourfold increase compared to the control (53.5 and 43.5 L ha−1). HA treatments notably enhanced accumulation of key bioactive compounds (terpinen-4-ol and β-phellandrene) and elevated levels of endogenous hormones gibberellic acid (GA3) and indole acetic acid (IAA) (second harvest). The predatory insect Orius albidipennis effectively controlled whitefly populations, providing a viable non-chemical pest control option. Economic analysis confirmed that HA application at 10 mL L−1 achieved the greatest net revenue of 239,862 Egyptian pounds ha−1. This integrated approach of using biostimulants with biological pest control enhanced marjoram productivity while also improving economic and environmental sustainability.