Aphicidal effects of terpenoids present in Citrus limon on Macrosiphum roseiformis and two generalist insect predators

- Lemon fruit peel aqueous extract (LFPAE) showed high toxicity towards aphids.
- MS/MS analysis confirmed presence of 11 insecticidal components including limonene.
- Among all the terpenoids tested, γ-terpinene was most toxic with least LC50 values.
- Field bioassays proved affectivity up to 9 days after treatment against aphids.
- Extracts are safe to two insect predators.

The current study is the first report that revealed insecticidal effects of lemon (Citrus limon) peel aqueous extract (LFPAE) against common insect pest of rose, aphids (Macrosiphum roseiformis) and for its influence on useful non-target predators viz., Coccinella septempunctata and Orius laevigatus on rose plants. Effects of LFPAE seemed promising concerning pest and predators of Rosa spp. The LC ESI Q-TOF MS/MS analysis of extract revealed the presence of (l-pipecolic acid, scopoletin, pulegone, γ-terpinene, carvone, menthone, myrcene, limonene, (−)-trans-isopiperitenol, geranial and linalool as active components that may be ascribed to observed effects. Also, some major terpenoids α-pinene, β-pinene, γ-terpinene, menthone, linalool, limonene which are known to be present in lemon were tested for their efficacy. LFPAE did not show any toxicity towards C. septempunctata. However, it was moderately toxic to O. laevigatus. Food choice assays and odor choice assay revealed that there was no significant difference obtained between the percentage of pests/predators that reached a particular choice (LFPAE treated, or untreated rose leaves). According to LC50 values of LFPAE for aphids, the highest toxicity and least value (6.68 mg/ml), was obtained at 72 h which was almost similar to the lowest concentration tested (× 6 = 6.25 mg/ml). Besides at 24 and 48 h LC50 values were somewhere around × 2 (100 mg/ml) and × 3 (50 mg/ml) concentrations. Among all the terpenoids tested at 24-72 h, γ-terpinene was most toxic with LC50 value ranging from 0.18 to 0.004 mg/ml followed by menthone (0.8-0.008), β-pinene (5.8-0.004), limonene (5.8-0.009), mixture (8.5-0.003), linalool (12.3-0.13) and α-pinene (17.8-0.02), respectively. Field data showed that LFPAE at × 1 (200 mg/ml), × 2 and × 3 concentration was effective until nine days after treatment (DAT) and lowest concentration × 4 did not show affectivity beyond seven days. However, terpenoids mixture at × 1 (10 mg/ml each) showed affectivity till 11DAT and lower two concentrations (× 2 and × 3) caused significant mortality up to 7DAT.

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