Terpenes and Phenylpropanoids with Potent Toxicity on Two Tephritid Fruit Flies and Reduced Impact to a Non-Target Parasitoid

Tephritid flies, highlighting Anastrepha fraterculus and Ceratitis capitata, cause substantial economic losses and trigger strict quarantine measures worldwide. In response, research is looking for eco-friendly alternatives and, in this study, we evaluated 7 terpenes/terpenoids (α-pinene, β-pinene, β-caryophyllene, γ-terpinene, ρ-cymene, citronellal and geropine) and 3 phenylpropanoids (isoeugenol, methyl cinnamate and coumarin) commonly found in a diverse group of aromatic plants tested against A. fraterculus and C. capitata adults. Subsequently, their selectivity for the egg–larvae endoparasitoid Doryctobracon areolatus was also assessed. Ingestion and topical application bioassays (range = 100–6000 mg L−1) revealed that isoeugenol caused the highest mortality (≥75%) in both fruit fly species. Concentration–response indicated a similar toxicity of this compound to A. fraterculus (ingestion: LC50 ≅ 34.2 mg L−1, LC90 ≅ 105.7 mg L−1; topical: LC50 ≅ 33.7 mg L−1, LC90 ≅ 121.3 mg L−1) and C. capitata (ingestion: LC50 ≅ 33.2 mg L−1, LC90 ≅ 125.5 mg L−1; topical: LC50 ≅ 31.4 mg L−1, LC90 ≅ 129.2 mg L−1). None of the tested compounds (α-pinene, β-pinene, ρ-cymene, β-caryophyllene, isoeugenol, γ-terpinene, coumarin, citronellal, geropine and methyl cinnamate) reduced the number of oviposition punctures on grape berries by either fruit fly species. In contrast, all compounds caused lower mortality in D. aerolatus (≤17%) and did not impair parasitism, with a range of 43.21–62.13% among the tested compounds. Overall, our findings indicate that selected phytocompounds, particularly isoeugenol and citronellal, show strong insecticidal activity against A. fraterculus and C. capitata while having low toxicity to D. areolatus, supporting their use into sustainable pest management programs.