Stinging nettle (Urtica dioica) as a potential control agent for Varroa mite (Varroa destructor) in honeybee colonies (Apis mellifera).

The parasitic mite Varroa destructor poses a significant threat to honeybee (Apis mellifera L.) colonies, leading to substantial losses in the beekeeping industry worldwide. This study evaluated the efficacy of stinging nettle (Urtica dioica L.) as a potential control agent for Varroa mites. Two application methods were tested: a 20% w/v ethanolic extract spray and 10 g dried plant smoke, and compared them with formic acid, a commercially available treatment. Experimental colonies, naturally infested with Varroa mites, were divided into four groups: those treated with nettle extract, dried plant smoke, formic acid, or left untreated as a control. Several bioassays were conducted to assess these treatments' efficacy, including infestation reductions and mortality assessments. Additionally, oxidative stress levels, indicated by malondialdehyde (MDA) concentrations, and glutathione S-transferase (GST) activity in honeybee workers and pupae were analysed. Gas Chromatography-Mass Spectrometry (GC-MS) was used to identify key bioactive compounds in the stinging nettle extract. Results showed significant reductions in mite infestations (88.93%, 76.28%, and 100% on adult bees; 86.73%, 94.82%, and 100% within sealed brood for nettle extract, nettle smoke, and formic acid, respectively). The stinging nettle treatments exhibited lower bee mortality compared to formic acid, which concurrently elevated oxidative stress in honeybees. Nettle extract increased GST activity in workers while reducing it in pupae, and both treatments decreased MDA levels. Gas Chromatography-Mass Spectrometry (GC-MS) analysis of the ethanolic extract identified bioactive compounds, including isopropyl palmitate, (9Z,12Z,15Z)-octadeca-9,12,15-trienoic acid, trans-totarol, 4',6-dimethoxyisoflavone-7-O-β-D-glucopyranoside, and 1,2,4-butanetriol. These findings suggest that stinging nettle is an effective, eco-friendly alternative for managing V. destructor, enhancing honeybee health and promoting colony sustainability.