Acute toxicity of terpenoids on the green lacewing Chrysoperla externa

Terpenoids are secondary metabolites and are considered one of the main components of plant essential oils. The insecticidal potential of terpenoids is reported for many agricultural pests, but few studies are focused on the toxicity of these compounds on non-target arthropods such as natural enemies. Once plant compounds can give rise to the development of new classes of insecticides, the assessment of their toxicity to non-target arthropods is important. Given this, this work aimed to evaluate the acute toxicity of seven terpenoids to the green lacewing Chrysoperla externa, an important predator of pests in many crops. Third-instar larvae were topically treated on the dorsum with 0.5μL of an acetone solution of each terpenoid. A range of five to seven concentrations were used for each terpenoid, and for each concentration, approximately 24 larvae were used. The mortality was measured 72 hours after application and data were used to calculate the LD₅₀ values. Terpenoid toxicity was compared with natural pyrethrin. Citral, s (-) limonene and nootkatone showed no toxicity to third-instar larvae (LD₅₀ > 104,895 µg/g), while the ketone thujone and the hydrocarbon r (+) limonene presented intermediate toxicity, with LD₅₀ of 62,189 µg/g and 103,241µg/g, respectively. The phenolic monoterpenoids thymol (LD_{50 =} 19,115 µg/g) and carvacrol (LD_{50 =} 11,556 µg/g) were the most toxic ones. Nevertheless, the acute toxicity of terpenoids was low compared to natural pyrethrin (LD_{50 =} 175 µg/g), which shows that these compounds tend to be relatively selective to the predator Chrysoperla externa.