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Author: search.filters.author.Naveena Unnikrishnan.Subject: search.filters.subject.Agricultural Entomology

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    Mechanisms of host plant resistance in Tomato (Solanum lycopersicum L.) genotypes to cotton whitefly (Bemisia tabaci Genn.)
    (Department of Agricultural Entomology, College of Agriculture, Vellanikkara, 2025) Naveena Unnikrishnan.
    Tomato (Solanum lycopersicum) is one of the most widely cultivated crops worldwide, with India ranked second in global production (Ministry of Agriculture and Farmers Welfare, MOAFW, 2023). The cotton whitefly (Bemisia tabaci) is a major pest of tomato, inflicting direct damage by feeding on phloem sap and serving as a vector for the tomato leaf curl virus (ToLCV). The extensive use of synthetic insecticides has led to resistance in whitefly populations, alongside concerns about environmental and health risks. As a result, developing host plant resistance has emerged as a sustainable alternative for pest management. Building on a previous study at the Department of Agricultural Entomology, College of Agriculture, Vellanikkara, which evaluated fifty tomato genotypes for whitefly resistance under polyhouse conditions (Tejaswee, 2023), the current study, titled "Mechanisms of host plant resistance in tomato genotypes to cotton whitefly," was conducted during 2023-2024. The primary objectives were to evaluate the resistance of different tomato genotypes to cotton whitefly under field conditions and to investigate the mechanisms behind host plant resistance. Twenty tomato genotypes selected from the previous research (Tejaswee, 2023) were evaluated for their resistance to whitefly under field conditions. The genotypes exhibited significant variation in the density of all life stages of Bemisia tabaci. The number of whitefly eggs ranged from 1.33 ± 0.30 (LC Palakkad) to 11.80 ± 0.22 (EC 617060) on the top leaves, 0.90 ± 0.40 (LC Palakkad) to 11.03 ± 0.29 (EC 617060) on the middle leaves, and 0.73 ± 0.24 (LC Palakkad) to 7.63 ± 0.39 (EC 617060) on the bottom leaves. Nymph populations varied from 1.27 ± 0.21(LC Palakkad) to 11.23 ± 0.37 (EC 617060) on the top leaves, 0.93 ± 0.16 (LC Palakkad) to 8.73 ± 0.37 (EC 617060) on the middle leaves, and 0.70 ± 0.21 (LC Palakkad) to 6.47 ± 0.24 (EC 635520) on the bottom leaves. Adult whitefly populations ranged from 0.73 ± 0.30 (LC Palakkad) to 12.50 ± 0.37 (EC 617060) on the top leaves, 0.50 ± 0.35 (LC Palakkad) to 10.33 ± 0.55 (EC 638522) on the middle leaves, and 0.43 ± 0.15 (LC Palakkad) to 8.03 ± 0.27 (EC 617060) on the bottom leaves. The pooled mean counts for all life stages were highest on the top leaves, followed by the middle and bottom leaves. Three genotypes viz., LC Idukki, LC Palakkad, and EC 519806 had mean whitefly populations under 2.62 per leaf, classifying them as resistant. Six genotypes with a mean population between 2.62 and 5.31 were moderately resistant, while six others were moderately susceptible (5.31–8), and five genotypes with >8 were highly susceptible. Additionally, the screening of genotypes for the incidence of Tomato leaf curl virus (ToLCV) and the coefficient of infection (CI) revealed that LC Palakkad, LC Idukki, and EC 519806 were highly resistant to the leaf curl disease. In the free-choice assays, whitefly settling and oviposition preferences were monitored at regular intervals in the customised experimental arenas. EC 617060 had the highest whitefly count, with 9.33 ± 0.577 per leaf at 24 hours and 7 per leaf at 48 hours. No whiteflies were observed on LC Palakkad and LC Idukki after 48 hours. LC Palakkad and LC Idukki had the lowest oviposition preference, with only 0.67 eggs/cm², while the highest oviposition (7.33 eggs/cm²) was recorded on EC 635520 and EC 638522. The epicuticular wax content of the tomato genotypes varied from 0.032 ± 0.002 mg/cm² (EC 617060) to 0.197 ± 0.002 mg/cm² (LC Palakkad). A significant negative correlation was observed between wax content and both oviposition and settling preferences of B. tabaci. Additionally, settling and oviposition preferences were negatively correlated with glandular trichome density, but positively correlated with non-glandular trichome density and trichome length. In the no-choice experiment, the developmental parameters of B. tabaci were observed in clip-cages. Oviposition rates (OR) on the tomato genotypes ranged from 0.31 ± 0.10 to 4.24 ± 0.58 eggs per day. Pre-adult survival (PS) rates varied from 0.57 ± 0.07 to 0.90 ± 0.01, while adult survival (AS) rates ranged from 0.72 to 0.98 ± 0.02. The developmental period (DP) of B. tabaci ranged from 21.95 ± 0.03 to 31.97 ± 2.77 days. Oviposition, pre-adult survival, and adult survival showed negative correlations with phenol, flavonoid, and alkaloid content, but positive correlations with free amino acid content and relative water content. In contrast, the developmental period was positively correlated with phenol, flavonoid, and alkaloid content, and negatively correlated with free amino acid and relative water content. The present study identified LC Idukki, LC Palakkad, and EC 519806 as highly resistant to whitefly, B. tabaci, exhibiting low level of whitefly population and ToLCV incidence, reduced settling preference, oviposition, survival rates, and longer developmental periods. While glandular trichomes and epicuticular wax contribute to resistance through antixenosis, the study highlights the crucial role of antibiosis, especially through phenolic compounds, in disrupting the development of B. tabaci. These results emphasize antibiosis as the primary resistance mechanism against B. tabaci in the evaluated tomato genotypes. Thus, the present study offers valuable insights for further research into host plant resistance mechanisms, aiding the development of sustainable and economically viable integrated pest management strategies for tomato cultivation.
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    Mechanisms of host plant resistance in Tomato (Solanum lycopersicum L.) genotypes to cotton whitefly (Bemisia tabaci Genn.)
    (Department of Agricultural Entomology, College of Agriculture, Vellanikkara, 2024) Naveena Unnikrishnan.; Ranjith, M T
    Tomato (Solanum lycopersicum) is one of the most widely cultivated crops worldwide, with India ranked second in global production (Ministry of Agriculture and Farmers Welfare, MOAFW, 2023). The cotton whitefly (Bemisia tabaci) is a major pest of tomato, inflicting direct damage by feeding on phloem sap and serving as a vector for the tomato leaf curl virus (ToLCV). The extensive use of synthetic insecticides has led to resistance in whitefly populations, alongside concerns about environmental and health risks. As a result, developing host plant resistance has emerged as a sustainable alternative for pest management. Building on a previous study at the Department of Agricultural Entomology, College of Agriculture, Vellanikkara, which evaluated fifty tomato genotypes for whitefly resistance under polyhouse conditions (Tejaswee, 2023), the current study, titled "Mechanisms of host plant resistance in tomato genotypes to cotton whitefly," was conducted during 2023-2024. The primary objectives were to evaluate the resistance of different tomato genotypes to cotton whitefly under field conditions and to investigate the mechanisms behind host plant resistance. Twenty tomato genotypes selected from the previous research (Tejaswee, 2023) were evaluated for their resistance to whitefly under field conditions. The genotypes exhibited significant variation in the density of all life stages of Bemisia tabaci. The number of whitefly eggs ranged from 1.33 ± 0.30 (LC Palakkad) to 11.80 ± 0.22 (EC 617060) on the top leaves, 0.90 ± 0.40 (LC Palakkad) to 11.03 ± 0.29 (EC 617060) on the middle leaves, and 0.73 ± 0.24 (LC Palakkad) to 7.63 ± 0.39 (EC 617060) on the bottom leaves. Nymph populations varied from 1.27 ± 0.21(LC Palakkad) to 11.23 ± 0.37 (EC 617060) on the top leaves, 0.93 ± 0.16 (LC Palakkad) to 8.73 ± 0.37 (EC 617060) on the middle leaves, and 0.70 ± 0.21 (LC Palakkad) to 6.47 ± 0.24 (EC 635520) on the bottom leaves. Adult whitefly populations ranged from 0.73 ± 0.30 (LC Palakkad) to 12.50 ± 0.37 (EC 617060) on the top leaves, 0.50 ± 0.35 (LC Palakkad) to 10.33 ± 0.55 (EC 638522) on the middle leaves, and 0.43 ± 0.15 (LC Palakkad) to 8.03 ± 0.27 (EC 617060) on the bottom leaves. The pooled mean counts for all life stages were highest on the top leaves, followed by the middle and bottom leaves. Three genotypes viz., LC Idukki, LC Palakkad, and EC 519806 had mean whitefly populations under 2.62 per leaf, classifying them as resistant. Six genotypes with a mean population between 2.62 and 5.31 were moderately resistant, while six others were moderately susceptible (5.31–8), and five genotypes with >8 were highly susceptible. Additionally, the screening of genotypes for the incidence of Tomato leaf curl virus (ToLCV) and the coefficient of infection (CI) revealed that LC Palakkad, LC Idukki, and EC 519806 were highly resistant to the leaf curl disease. In the free-choice assays, whitefly settling and oviposition preferences were monitored at regular intervals in the customised experimental arenas. EC 617060 had the highest whitefly count, with 9.33 ± 0.577 per leaf at 24 hours and 7 per leaf at 48 hours. No whiteflies were observed on LC Palakkad and LC Idukki after 48 hours. LC Palakkad and LC Idukki had the lowest oviposition preference, with only 0.67 eggs/cm², while the highest oviposition (7.33 eggs/cm²) was recorded on EC 635520 and EC 638522. The epicuticular wax content of the tomato genotypes varied from 0.032 ± 0.002 mg/cm² (EC 617060) to 0.197 ± 0.002 mg/cm² (LC Palakkad). A significant negative correlation was observed between wax content and both oviposition and settling preferences of B. tabaci. Additionally, settling and oviposition preferences were negatively correlated with glandular trichome density, but positively correlated with non-glandular trichome density and trichome length. In the no-choice experiment, the developmental parameters of B. tabaci were observed in clip-cages. Oviposition rates (OR) on the tomato genotypes ranged from 0.31 ± 0.10 to 4.24 ± 0.58 eggs per day. Pre-adult survival (PS) rates varied from 0.57 ± 0.07 to 0.90 ± 0.01, while adult survival (AS) rates ranged from 0.72 to 0.98 ± 0.02. The developmental period (DP) of B. tabaci ranged from 21.95 ± 0.03 to 31.97 ± 2.77 days. Oviposition, pre-adult survival, and adult survival showed negative correlations with phenol, flavonoid, and alkaloid content, but positive correlations with free amino acid content and relative water content. In contrast, the developmental period was positively correlated with phenol, flavonoid, and alkaloid content, and negatively correlated with free amino acid and relative water content. The present study identified LC Idukki, LC Palakkad, and EC 519806 as highly resistant to whitefly, B. tabaci, exhibiting low level of whitefly population and ToLCV incidence, reduced settling preference, oviposition, survival rates, and longer developmental periods. While glandular trichomes and epicuticular wax contribute to resistance through antixenosis, the study highlights the crucial role of antibiosis, especially through phenolic compounds, in disrupting the development of B. tabaci. These results emphasize antibiosis as the primary resistance mechanism against B. tabaci in the evaluated tomato genotypes. Thus, the present study offers valuable insights for further research into host plant resistance mechanisms, aiding the development of sustainable and economically viable integrated pest management strategies for tomato cultivation.