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Abstract

Phenacoccus solenopsis Tinsley, commonly known as the invasive cotton mealybug, is a serious pest that suppresses the growth and physiology of tomato (Solanum lycopersicum). This study assessed the role of an entomopathogenic fungal endophyte (Trichoderma harzianum) in alleviating mealybug-induced stress under controlled growth-chamber conditions. Four treatments were compared, which included control, mealybug infestation, T. harzianum inoculation, and mealybug + T. harzianum. Mealybug infestation significantly decreased the height of plants (29.8 cm), root depth (18.1 cm), shoot dry weight (2.76 g), root dry weight (1.03 g), and total pigment content (1.20 mg g−1 fresh weight, FW) and increased proline (70.65 µmol g−1 FW), glycine betaine (13.43 µg g−1 FW), malondialdehyde (MDA, 7.43 µmol g−1 FW) and hydrogen peroxide (H2O2, 7.74 µmol g−1 FW). In contrast, T. harzianum treatment alone maximized growth (plant height: 86.83 cm; root depth: 48.87 cm; shoot dry weight: 7.68 g; root dry weight: 3.38 g), pigment content (2.14 mg g−1 FW), and relative water content (92.21%), with minimal oxidative stress (MDA: 4.05 µmol g−1 FW; H₂O₂: 3.34 µmol g−1 FW). Co-application partially restored growth (plant height: 52.67 cm; shoot dry weight: 4.80 g), and decreased stress markers (MDA: 5.17 µmol g−1 FW; H2O2: 4.90 µmol g−1 FW), while further raising proline (88.04 µmol g−1 FW) and glycine betaine (18.05 µg g−1 FW). Infestation also raised the activities of indole-3-acetic acid oxidase (IAA oxidase, +45%) and polyphenol oxidase (PPO, +142%), whereas co-inoculation lowered both and produced the highest activities of superoxide dismutase (16.80 U mg−1 protein), catalase (10.27 U mg−1 protein), ascorbate peroxidase (13.94 U mg−1 protein) and glutathione reductase (2.25 U mg−1 protein), 17 to 26% above those of infested plants. One-way analysis of variance (ANOVA) with Fisher's least significant difference (LSD) test and principal component analysis (PCA) indicated significant treatment effects (p<0.001) and clear separation of the four treatments, each represented by three biological replicates. These findings indicate that T. harzianum can help tomato plants tolerate mealybug attack by sustaining growth, photosynthetic pigments, water status and redox balance, in association with a stronger antioxidant enzyme system and restrained auxin-degrading activity. Because mealybug population size, survival and fecundity were not recorded, these results describe improved host tolerance to infestation rather than demonstrated suppression of the pest, and any contribution of this endophyte to integrated pest management remains to be established in insect-performance and field studies

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