Combinations of Metarhizium anisopliae with Chemical Insecticides and their Effectiveness in Mahanarva fimbriolata (Hemiptera: Cercopidae) Control on Sugarcane

Some insecticides can be used jointly with entomopathogenic fungi, and therefore the combination of chemical and biological control measures can be a safe and effective method to control insect pests. The aim of this study was to evaluate the costs and efficacy of combinations of Metarhizium anisopliae (Metschnikoff) Sorokin (Hypocreales: Clavicipitaceae) with thiamethoxam and imidacloprid on spittlebug (Mahanarua fimbriolata (Stål); Hemiptera: Cercopidae) control on sugarcane. The experiment was conducted as a randomized block design (RBD) with 10 treatments and 4 replications. The treatments included a control (untreated), thiamethoxam (250 g ha⁻¹), imidacloprid (700 g ha⁻¹), M. anisopliae (M. a.) (3 × 10¹² conidia ha⁻¹), A1 (3 × 10¹² M. a. conidia ha⁻¹ + 65 g ha⁻¹ of thiamethoxam), A2 (3 × 10¹² M. a. conidia ha⁻¹ + 125 g ha⁻¹ of thiamethoxam), A3 (3 × 10¹² M. a. conidia ha⁻¹ + 187.5 g ha⁻¹ of thiamethoxam), A4 (3 × 10¹² M. a. conidia ha⁻¹ + 175 g ha⁻¹ of imidacloprid), A5 (3 × 10¹² M. a. conidia ha⁻¹ + 350 g ha⁻¹of imidacloprid), and A6 (3 × 10¹² M. a. conidia ha⁻¹ + 525g ha⁻¹ of imidacloprid). The reductions in the numbers of M. fimbriolata nymphs per treatment compared to the control were similar at 15 DAT (days after treatment) in all treatments except combination A5 (M. anisopliae and thiamethoxam). At 30 DAT, the numbers of nymphs were significantly reduced in all treatments except A3, and their effectiveness ranged from 14.28% to 92.85%. At 45 DAT the numbers of M. fimbriolata nymphs per treatment were significantly reduced in the following treatments: imidacloprid alone at 700g ha⁻¹, A1, A2, A3, A4 and A6; and the combinations A1 and A2 caused the lowest M. fimbriolata nymph infestations and effectiveness rates of 77.41 and 87.09 %, respectively. At 75 DAT the 2 best control efficacies occurred in treatments A1 (3 × 10¹² M. a. conidia ha⁻¹ of + 65g ha⁻¹ of thiamethoxam) (82.1%) and A5 (78.6%) (3 × 10¹² M. a. conidia ha⁻¹ + 350 g ha⁻¹of imidacloprid). At 90 DAT the number of nymphs in the control had increased 2.8 fold over the number at 75 DAT. Very good control efficacies at 90 DAT occurred in all treatments with the combination of the fungus with an insecticide. At 105 DAT the numbers of nymphs had surged in all treatments, and no treatment provided effective control. The treatments with the highest earnings per hectare were A1 (3 × 10¹² M. a. conidia ha⁻¹ + 65 g thiamethoxam) and M. anisopliae alone at the recommended dose of 3 × 10 ¹² M. a. conidia ha⁻¹. Our findings demonstrate the effectiveness of using either thiamethoxam or imidacloprid in combination with M. anisopliae to control M. fimbriolata nymphs on sugarcane, but greater net earnings per hectare occurred with the lowest rate of the thiamethoxam combination than with any of the imidacloprid combinations.