Emulsion oil in water formulation of methyl-eugenol increases its insecticidal activity against Cydia pomonella L. (Lepidoptera: Tortricidae)

The codling moth Cydia pomonella is the major pest of apple orchards worldwide. Chemical insecticides are commonly used against pests; however, increased number of resistance cases highlights the urgent need for alternative control methods. Plant-derived essential oils are promising alternatives to chemical insecticides. Although they show great efficacies under laboratory conditions, their efficacy are lower in the field; requiring their formulation. Methyl-eugenol (ME) is a plant essential oil with insecticidal activity; however, knowledge on the efficacy of formulated forms and mode of action of ME is limited. Here, we developed two formulations of ME, an emulsion oil in water (EW) and an emulsifiable concentrate (EC). Bioassays on neonate larvae revealed a limited efficacy of non-formulated ME with the highest mortality of 34.1% at 15% concentration at 120 h after the application. The mortality caused by EW formulation without ME led to a maximum of 10% mortality, while the mortality caused by EW formulation with ME varied between 26.7 and 100% for the lowest and highest concentrations, respectively. As the EC formulation without ME led to 90% mortality, no further bioassay on the EC formulation of ME was conducted. EW formulation of ME showed strong contact toxicity against C. pomonella larvae after 120 h of application (LC₅₀ = 40.5 µL/cm²). Since ME is known to inhibit acetylcholinesterase, the relative expression levels of AChE-1 and AChE-2 genes were examined in ME-exposed surviving larvae upon its plain application. Only AChE-1 was upregulated with a five-fold increase 24 h after treatment, which might be the reason for the survival of the larvae. Overall, the EW formulation of ME has great potential against codling moth. Further studies under field conditions will be helpful to use this essential oil in integrated pest management programs.

Graphical abstract