Analysis of the toxicological and antifeedant activity of eugenol against Hyphantria cunea larvae

To investigate the toxicological and antifeedant activity of eugenol, a plant secondary metabolite, on Hyphantria cunea larvae, and to clarify the mechanism of action of eugenol on larval growth and development, feeding and digestion, and oxidative defense system.

The toxic effect of eugenol on 3rd instar larvae of H. cunea was analyzed by bioassay, in vivo enzyme activity assay and real-time fluorescence quantitative PCR (RT-qPCR) method to calculate the sublethal concentration (LC₃₀), and the larvae were treated with eugenol at LC₃₀, and their body weights, feed intake, food utilization, trypsin (TYP), α-amylase (α-AMS), lipase (LPS), superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activities as well as hydrogen peroxide (H₂O₂) content, malondialdehyde (MDA) content and total antioxidant capacity (T-AOC), and the expression levels of the above mentioned digestive and antioxidant enzyme genes were analyzed at mRNA level.

1) The corrective mortality rate of larvae reached 66.70% after 72 h of treatment with the highest concentration of 39.408 mg/mL eugenol. The lethal median concentration (LC₅₀) of eugenol-treated larvae after 72 h was 21.001 mg/mL and the sublethal concentration (LC₃₀) was 9.998 mg/mL; 2) After 72 h of eugenol treatment at LC₃₀ post-larval feeding was reduced by 15.10%, body weight gain was reduced by 32.28%, and food utilization (ECI) was reduced by 20.38% under eugenol treatment at LC₃₀; 3) TYP, α-AMS and LPS activities were significantly decreased in larvae after 72 h of eugenol treatment at LC₃₀, and the related genes, Hcα-AMS1, Hcα-AMS2, HcLPS1, HcLPS2, HcTYP1, and HcTYP2, were also significantly down-regulated; 4) Among the oxidative stress-related indicators in larvae, the H₂O₂ content increased to 2.93 times of the control group, the MDA content increased to 1.91 times of the control group, and the T-AOC decreased by 11.55%. The activities of antioxidant enzymes CAT, SOD and POD were significantly increased, and the expressions of related genes HcCAT1, HcCAT2, HcSOD1, HcSOD2, HcPOD1 and HcPOD2 were also significantly up-regulated.

Eugenol has good toxicity and food rejection effects on the 3rd instar larvae of H. cunea. Eugenol interferes with the digestive and absorptive systems and oxidative defence system of the larvae, resulting in oxidative damage, reduced feeding and impaired digestion, which are detrimental to the survival of the larvae and ultimately lead to their death.