Tablet formulation of entomopathogenic fungus and its bioefficacy in mosquito control

Abstract

The research work entitled ―Tablet formulation of entomopathogenic fungus and its bioefficacy in mosquito control‖ was conducted at Biocontrol Laboratory for Crop Pest Management, Department of Agricultural Entomology, College of Agriculture, Vellayani, Thiruvananthapuram, during the year 2019-2021 with an objective to develop water dispersible tablets of entomopathogenic fungus and to test their effectiveness in managing mosquitoes. Pathogenicity studies carried out using 108 spores mL-1 of Metarhizium anisopliae (Metsch.) Sorokin NBAIR isolate Ma4, Beauveria bassiana (Bals.) Vuillemin NBAIR isolate Bb5, B. bassiana KAU isolate ITCC 6063 and 107 spores mL-1 of Lecanicillium lecanii (Zimmerman) Zare and Gams NBAIR isolate Vl 8 and Lecanicillium saksenae (Kushwaha) Kurihara and Sukarno KAU isolate ITCC – 7714, revealed that all the fungi tested were infective to Anopheles, Aedes and Culex at varying levels. Infected larvae were less active at 12 hour after treatment (HAT) with sinking movement and colour change from dark grey to white. Death occurred within 24 h, with degeneration of gut. Copious amount of mucus was noticed around the body, except in L. saksenae. Infected adults were inactive and died within 24 h. The cadavers were mummified and found attached to the walls of the container. In Ma4 treated adults, white mycelial growth which turned green upon sporulation was noted 72 h after death. Observations on mortality taken at 24 h interval revealed that, M. anisopliae was the most effective fungus for mosquito control, followed by B. bassiana isolates. Lecanicillium spp. were less effective to mosquitoes. M. aniospliae was found to be superior to the biocontrol check, Bti and equivalent to the chemical check malathion 50 EC based on larval mortality. Mortality recorded by M. anisopliae was 93.33, 96.66 and 100 per cent in Anopheles, Aedes and Culex respectively, 5 days after treatment (DAT), while in Bti it was 73.33, 66.66 and 70 per cent respectively. The corresponding mortality in malathion 50 EC was 100 per cent. Based on mortality recorded in adults, M. anisopliae (83.33 and 86.66 per cent mortality) was equally effective as Bti to Anopheles and Aedes (66.66 and 76.66 per cent mortality) but inferior to malathion 50 EC that recorded complete mortality. The mortality recorded 112 in Culex was 96.66 per cent, which was on par with that recorded in malathion 50 EC (100 per cent). Dose-mortality studies of M. anisopliae on 4th instar larvae, revealed that 108 spores mL-1 was the effective dose. The LC90 values for M. anisopliae was 106 in Anopheles, Aedes and Culex. The LT90 values were 90.33, 79.46 and 55.9 h on Anopheles, Aedes and Culex, revealing that Culex is the most susceptible species. Larva was found to be the most susceptible stage for Culex, while for Anopheles and Aedes, both larva and adult were equally susceptible, whereas pupa was the least susceptible stage for all the three species. Experiment to standardize the carrier material for M. anisopliae tablets, revealed that talc + chitosan (90:10) was superior to bran, talc and bran + chitosan, as there was 71.16 per cent conidial germination and 95 per cent larval mortality in Culex on the 5th week after storage (WAS). The corresponding viability and virulence in talc were 66.33 per cent and 87.5 per cent respectively, while in bran it was 43.16 and 70 per cent and in bran + chitosan, it was 47.58 and 80 per cent, respectively. Among the binding agents tested, Carboxy Methyl Cellulose (CMC) 7% was superior to Microcrystalline Cellulose (MCC) 7%, Polyvinyl Pyrrolidone (PVP) 5% and Acacia Gum Arabic (AG) 5% as it maintained 62.66 per cent germination and 95 per cent mortality on 5 WAS. The tablets of M. anisopliae @ 1010 spores mL-1, formulated using talc + chitosan + CMC 7% at varying moisture levels of 8, 10 and 15 % when subjected to shelf life studies revealed that tablets formulated at 15 % were superior in viability (65.06 per cent) and virulence (88 per cent mortality) at 8 WAS. Thereafter, though there was a decrease in germination rate below 60 per cent, the virulence could be maintained up to 72 per cent till three months of storage. Extent of contamination noticed was significantly high (105 cfu mL-1) in 15 % moisture compared to 104 in 8 and 10 % moisture levels. However, 105 being the permitted level of contaminants as per CIBRC standards, 15% was fixed as the ideal moisture content for formulating tablets. The effective shelf life was therefore determined as three months under ambient conditions. 113 The tablets when tested for their bioefficacy to Culex larvae revealed that 3 and 4 tablets L-1 were equally effective causing 82 and 83 per cent mortality within 9 DAT, under laboratory conditions. The adult emergence from the treated larvae was 17 and 16 per cent for 3 and 4 tablets L-1, which was significantly lower than that from control (81 per cent). Under field conditions, 9 and 10 tablets 10 L-1 exhibited similar level of mortality (83.97 and 84.1 per cent) compared to 7 and 8 (75.03 and 79.59 per cent). Therefore 9 tablet 10 L-1 was fixed as the effective dosage for treating stagnant water bodies. It is concluded that water dispersible tablets of M. anisopliae formulated at 1010 spores mL-1 with talc + chitosan + CMC (7%) at 15% moisture is effective for the management of mosquito larvae in stagnant water bodies @ 9 tablets 10 L-1. They can be stored effectively for three months with 72 per cent virulence, under ambient conditions.