Chemotype-Dependent Bioactivity of Amazonian Piper Essential Oils: Linking Chemical Composition to Selective Phytotoxic and Antifeedant Effects

Plant essential oils (EOs) are promising sources of botanical biopesticides, and the genus Piper is particularly rich in bioactive constituents. This study investigated the chemical composition and biopesticidal potential of the EOs from seven Amazonian Piper species. The oils were characterized by GC–MS and evaluated for their phytotoxicity against Lolium perenne and Lactuca sativa, their antifeedant activity against Spodoptera littoralis, Myzus persicae and Rhopalosiphum padi, their antifungal activity against Aspergillus niger and Alternaria alternata, and their nematicidal activity against Meloidogyne javanica; composition–activity relationships were explored by multivariate analysis. The oils resolved into two chemotypes, one dominated by phenylpropanoids (mainly apiol) and the other by sesquiterpenes. Notably, all oils selectively inhibited the growth of the grass L. perenne while sparing or stimulating the dicot L. sativa. The apiol-rich oils of P. hispidum, P. stellipilum and P. demeraranum showed the strongest antifeedant effects on aphids, and all oils inhibited spore germination of both fungi. Principal component analysis associated apiol, but not dillapiol, with the phytotoxic and antifeedant activities, whereas antifungal activity was unrelated to the main compositional gradient. No nematicidal activity was observed. These results identify apiol-rich Amazonian Piper EOs as promising candidates for managing grass weeds, aphids and phytopathogenic fungi.