Bioactivity of essential oils against insect pests of brinjal

Abstract

The study entitled “Bioactivity of essential oils against insect pests of brinjal” was undertaken in the Department of Agricultural Entomology at College of Agriculture, Vellayani during the period 2019-2021 with an objective of evaluating the bioactivity of essential oils viz., basil oil, citronella oil, eucalyptus oil and orange oil against insect pests of brinjal and characterization of the most effective essential oils. Laboratory screening of essential oils viz., basil oil, eucalyptus oil, citronella oil and orange oil was conducted against one sucking pest (mealy bug, Coccidohystrix insolita (Green)) and one chewing pest (Hadda beetle, Henosepilachna vigintioctopunctata (Fabricius)) in brinjal. Acute toxicity of the essential oils against the test insects was determined by leaf dip bioassay. Preliminary test dose range causing 10 to 100 per cent mortality was fixed and based on these 6 doses including a control treatment was taken. Based on the results of leaf dip bioassay of essential oils against mealy bug, LC50 and LC90 of basil oil, eucalyptus oil, citronella oil and orange oil were (0.49 and 1.33), (1.86 and 4.28), (0.64 and 2.39) and (1.54 and 3.53) per cent respectively at 24 hours after treatment (HAT). The corresponding LC50 and LC90 values of the above four oils at 48 HAT were (0.29 and 0.89), (0.72 and 1.92), (0.28 and 1.22) and (1.06 and 2.17) per cent respectively. Against hadda beetle the LC50 and LC90 values of basil oil, eucalyptus oil, citronella oil and orange oil were (1.25 and 2.85), (3.35 and 5.09), (0.93 and 1.52) and (3.48 and 9.11) per cent respectively at 24 HAT. While the corresponding LC50 and LC90 values at 48 HAT were (0.85 and 1.70), (2.09 and 3.51), (0.63 and 1.14) and (2.02 and 4.05) per cent respectively. Based on toxicity bioassays, basil and citronella oil had lower LC50 and LC90 values than eucalyptus and orange oil against both mealy bug and hadda beetle at both 24 and 48 HAT and these two were selected for further lab and field evaluation. Repellent and antifeedant effect of the selected essential oils from the first experiments viz., basil oil, citronella oil were evaluated against hadda beetle by modified preference method and no choice method respectively. At 0.5 to 1 per cent 76 concentration, both basil and citronella oil showed 90-100 per cent repellence at 30 and 60 minutes of treatment. On other hand, 0.5 to 1 per cent concentration of basil and citronella oil showed antifeedant effect of 100 per cent after 24 hours of treatment. Before conducting field evaluation of essential oils, bloom test and phytotoxicity evaluation on brinjal plants were done. Bloom test was conducted to check the emulsification of essential oil and surfactant formulation in distilled water. Results were showing “Good” bloom rating for basil oil and “Excellent” bloom rating for citronella oil. Phytotoxicity evaluation of basil and citronella oil was carried on brinjal plant as per the protocols of CIBRC (Central Insecticide Board and Registration Committee). Basil oil and citronella oil at 0 to 0.7% were not showing any phytotoxic symptoms on brinjal plants while the higher doses of 1%, 1.5% and 2% were showing slight moderate and severe phytotoxic symptoms. Based on the laboratory and phytotoxicity evaluation, two doses of basil oil and citronella oil (0.5% and 0.7%) were selected for the pot culture experiment along with two chemical checks and an untreated control. The experiments were laid in completely Randomized design with three replications. In field study, spraying of basil oil and citronella oil at 0.7% showed significant reduction in aphid population and it was statistically on par with thiamethoxam 25% WG even at 14 DAT. The leaf webber population was also significantly reduced for basil and citronella oil at 0.7% and it was statistically on par with chlorantraniliprole 18.5% SC even at 14 DAT. Basil oil and citronella oil at 0.7% showed significant reduction in the shoot and fruit borer damage after 14 days of treatment and it was statistically on par with chlorantraniliprole 18.5% WG at 14 days after treatment. Basil oil and citronella oil at 0.7 per cent showed significant reduction in the lady bird beetle population. There was no statistical difference among the different treatments with regard to the biometric observations viz., total number of leaves, damaged leaves, plant 77 height after the field spraying, however the fruit yield was recorded significantly higher in chlorantraniliprole 18.5% WG. GC-FID/GC-MS studies on basil and citronella oil revealed that the predominant component of basil oil is Methyl chavicol (75.73%), followed by Linalool (18.21%) and (8) Cuprenene (1.58%). In citronella oil, Geranial (64.77%) was the most abundant component followed by Citronellyl acetate (7.92%), Geraniol (7.08%), (Z) Iso citral (5.29%) and Neral (3.60%).