Nematotoxic activity of essential oils from Monarda species

Bioactivity of essential oils (EOs) from Monarda species has never been investigated on phytoparasitic nematodes. In this study, the EOs from two Italian ecotypes of Monarda didyma and M. fistulosa and their main compounds, carvacrol, γ-terpinene, o-cymene, and thymol, were evaluated for their in vitro activity on the infective stages of phytoparasitic nematodes Meloidogyne incognita and Pratylenchus vulnus, as well as on M. incognita egg hatch. Soil treatments with the two EOs were also investigated for their suppressiveness on M. incognita on tomato. Both EOs were strongly active on M. incognita juveniles, as a only 1.0 μL mL−1 LC50 value was evaluated after a 24-h exposure to both EOs, whereas a lower activity was recorded on P. vulnus (15.7 and 12.5 μL mL−1 LC50 values for M. didyma and M. fistulosa EOs, respectively). Among the EOs’ main compounds, carvacrol was highly active also at a short exposure in low concentrations, whereas γ-terpinene and thymol were much less active on both nematode species and o-cymene showed a discrete activity on P. vulnus only at the highest concentration. Hatch percent of M. incognita eggs treated with M. didyma and M. fistulosa EOs was always significantly lower than in water or in Tween 20 and Oxamyl solutions. In the experiment in soil, the multiplication of M. incognita and gall formation on tomato roots was significantly reduced by soil treatments with both EOs. The strong nematicidal activity of both Monarda EOs may suggest them as potential sources of new sustainable nematicidal products.