Bioactivity of essential oil from Lippia gracilis Schauer against two major coconut pest mites and toxicity to a non-target predator

The coconut mite, Aceria guerreronis Keifer (Acari: Eriophyidae), and the red palm mite, Raoiella indica Hirst (Acari: Tenuipalpidae), are serious pests of commercial coconut plantations worldwide. We assessed the toxicity and sublethal effects of the essential oil from Lippia gracilis Schauer on these two pests, and its toxicity against Amblyseius largoensis (Acari: Phytoseiidae), a predatory mite that is mainly associated with R. indica. Chemical analyses revealed that the essential oil of L. gracilis accession LGRA106 contains thymol as a major compound (52.41%). Based on concentration-mortality bioassays, the toxicity of the oil differs from that of thymol for A. guerreronis (LC50 = 4.28 mg/mL, LC50thymol = 5.34 mg/mL) and for R. indica (LC50=4.99 mg/mL, LC50thymol = 9.03 mg/mL). The LC50, as estimated for R. indica, proved to be toxic to the predatory mite A. largoensis, killing roughly half of its population. In addition, the essential oil of L. gracilis and thymol, at their LC50 for each mite species, repelled A. guerreronis and reduced the survival and oviposition of R. indica. The potential of L. gracilis as a natural pesticide on coconut plantations and their effects on beneficial mites are discussed herein.