Evaluation of two invasive plant invaders in Europe (Solidago canadensis and Solidago gigantea) as possible sources of botanical insecticides

Solidago gigantea and Solidago canadensis (Asteraceae) are two invasive weeds native to North America and introduced in Europe and Asia, where they are spreading quickly threatening the stability of local secondary ecosystems. These two plant invaders may represent an ideal bioresource to be exploited for production of green pesticides. Therefore, herein we evaluated the efficacy of the essential oils (EOs) obtained from their different parts, i.e. leaves, inflorescences and roots, against Culex quinquefasciatus, Spodoptera littoralis and Musca domestica. The essential oil composition was investigated by gas chromatographic–mass spectrometry (GC–MS) analysis. S. canadensis leaf EO was the most toxic to C. quinquefasciatus, with a LC50 of 89.3 μl L−1. The two most effective oils against M. domestica adults were S. canadensis leaf and flower EOs, with LD50 values of 206.9 and 207.1 μg adult−1, respectively. Three EOs highly toxic to S. littoralis were also identified, namely S. gigantea leaf EO, S. canadensis leaf EO and S. gigantea flower EO, with LD50 values of 84.5, 98.9 and 107.4 μg larva−1, respectively. Since the S. canadensis leaf EO was the only green product effective against all the tested insect pests, we selected it for non-target toxicity assays on Eisenia fetida earthworms, along with the leaf EO from S. gigantea. Both the S. canadensis and S. gigantea leaf EOs did not led to mortality of E. fetida adult earthworms, at variance with the positive control α-cypermethrin, allowing us to propose them for pest control purposes in IPM and organic farming.