<i>Piper hainanense</i> Hemsl. and <i>P. thomsonii</i> (C.DC.) Hook.f.: Essential Oil Compositions, Antimicrobial and Mosquito Larvicidal Activities

Background and Objectives The genus Piper (family Piperaceae) includes aromatic plants widely used as spices and in traditional medicine. Essential oils from Piper species are known for their antimicrobial and pesticidal properties. This study aims to characterize the chemical profiles of the stem bark and leaf essential oils from two Vietnamese Piper species, Piper hainanense and P. thomsonii, and evaluate their biological activities. Methods Essential oil components were identified using gas chromatography-mass spectrometry (GC-MS). Antimicrobial activity was assessed using the broth microdilution method, while mosquito larvicidal activity was evaluated against fourth instar larvae of Aedes aegypti. Results The major constituents of P. hainanense essential oils were sabinene (14.4-15.9%), δ-selinene (6.7-14.6%), β-pinene (13.5-13.9%), β-selinene (5.9-12.4%), α-pinene (7.0-9.0%), and β-elemene (6.1-6.6%). In P. thomsonii stem bark essential oil, elemicin (23.8%), spathulenol (14.5%), and caryophyllene oxide (7.4%) predominated, while its leaf essential oil contained elemicin (23.0%), β-pinene (15.6%), and γ-elemene (13.6%). Antimicrobial assays revealed that P. thomsonii essential oils exhibited strong antifungal activity against Aspergillus niger (MIC = 32 µg/mL). P. hainanense essential oils demonstrated strong mosquito larvicidal activity, with 24-h LC 50 values of 26.72–32.57 µg/mL and LC 90 values of 34.15–43.48 µg/mL. Conclusion P. hainanense and P. thomsonii essential oils exhibit potential as natural agents for creating antimicrobial medications and mosquito-control strategies. These results serve as a foundation for additional investigation into the medicinal and insecticidal uses of essential oils from Piper.