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K. DEVAKI*, T. MURALIKRISHNA, K.V. HARI PRASAD, R. SARADA JAYALAKSHMI DEVI AND G. MOHAN NAIDU
Department of Entomology, Institute of Frontier Technology, RARS, Tirupati-517 502, Chittoor Dt., Andhra Pradesh
A total of 61 Bacillus thuringiensis strains were collected from 264 soil samples representing different forest ecosystem in Southern zone of Andhra Pradesh in Chittoor, Kadapa and Nellore districts during 2014-2016. These strains were tested for their efficacy against third instar larvae of Spodoptera litura in laboratory bioassay studies. A series of 0.00 to 53.33; 0.00 to 20.00; 0.00 to 30.00; 0.00 to 30.00 and 0.00 to 23.33 per cent larval mortality was observed at 72, 96, 120, 144 and 168 hours after treatment, respectively. A cumulative mortality of 0 to 100 per cent was observed among the 61 strains collected from forest ecosystem. The Bt strain F493 recorded highest mortality of 100 per cent, followed by F468 (86.67%) which were comparable with standard strain HD-1 for their efficacy. The other strains F287 (76.67%), F504 (76.67%) were proved next best to F493, HD-1 and F468 for their efficacy which were statistically on par with each other.
Bacillus thuringiensis, bioassay studies, forest soils, Spodoptera litura.
Spodoptera litura (Fabricius) is a serious insect pest on groundnut during rabi in groundnut growing regions of Andhra Pradesh particularly in Chittoor, Kadapa, Nellore and Anantapur districts. Application of synthetic insecticides is one of the major strategies used by the farmers for managing this pest. The farmers of Chittoor district apply around 4-5 rounds of insecticides including combination insecticides for managing S. litura. Brooke and Hines (1999) reported that, 40 per cent of the broad spectrum chemical insecticides have been targeted for the control of lepidopteran pests.
Microbial pesticides are considered as an important component of IPM, among which Bt based biopesticides occupies the pivotal position. Bt is a Gram-positive bacterium, stands out representing approximately 95 per cent of microorganisms used in biological control of agricultural pests in different countries, which accounts for 1.3 per cent of total pesticides (Ramanujam et al., 2014). Though commercial Bt strains have been used since ages for managing the lepidopteran pests, there is a need to isolate and test the efficacy of locally available Bt strains that can fit into local agro ecosystem.
The studies were carried out at Regional Agricultural Research Station, Tirupati during 2014-15 and 2015-16. A total of 264 soil samples were collected from forest ecosystem in Southern zone of Andhra Pradesh representing three districts viz., Chittoor, Kadapa and Nellore. Out of 264 soil samples, 61 B.thuringiensis strains were isolated and these strains along with standard strain HD-1 were used to determine their activity against S. litura in laboratory bioassay. All the 61 strains along with standard strain was inoculated on Luria Bertani agar media and incubated overnight at 370C. One loop of overnight cultures was inoculated in Luria broth and kept for sporulation under shaking condition at 280 C for 24h. S. litura culture was reared till III instar for conducting bioassay studies. The bioassay experiment had 63 treatments with three replications.
Bioassay was followed by leaf dip bioassay method developed by Shelton et al. (1993). Groundnut compound leaf containing four leaflets was dipped for 10 minutes into Bt culture broth (5 × 108 CFU 1 mL-1) containing 0.2 per cent Triton X-100, then kept leaf for air drying till leaf surface free from moisture. After drying, the petiole of leaf was swabbed with wet cotton to maintain leaf
succulence and turgidity. Two compound leaves were used for one replication, which was placed in a Petri plate of 9 cm diameter. Ten larvae were released per one replication. HD-1 served as a reference strain. The leaf dipped in distilled water served as control. The larval mortality was assessed after 72h at regular intervals. The data subjected to ANOVA in one way analysis using GENSTAT.
A total of 61 strains which were native to forest ecosystem of southern zone of Andhra Pradesh were assessed for their efficacy against third instar larvae of S. litura. A series of 0.00 to 53.33; 0.00 to 20.00; 0.00 to 30.00; 0.00 to 30.00 and 0.00 to 23.33 per cent larval mortality was observed at 72, 96, 120, 144 and 168 hours after bioassay. A cumulative mortality of 0.00 to 100 per cent was observed in different strains after 168 hours of bioassay.
Among the 61 strains, F468, with larval mortality of 53.33 per cent was found as effective as standard check HD-1, which was superior over all other strains at 72 hours after bioassay. Next best strains, in the efficacy order were F437 (43.33%), F504 (40.00%), F493 (40.00%), F487 (40.00%) and these strains were statistically on par with each other in their efficacy. The larval mortality after 96 hours after bioassay was 36.67 per cent in F493, followed by F399 (30.00%) which were comparable with standard check HD-1 (36.67%). The other strains, F261, F323, F435 recorded a larval mortality of 20.00 per cent, followed by F468 (16.67%), F500 (13.33%), F462 (13.33%), F493 (13.33%), F399 (13.33%), F347 (13.33%) which were statistically on par with each other at 120h. F262 (30.00%), F268 (30.00%) and F307 (23.33%) recorded higher mortality (on par with each other) at 144h and 168h after bioassay F440 recorded recorded 26.67 per cent larval mortality followed by F254 (23.33%), F258 (20.00%), F281 (23.33%), F287 (20.00%), F307 (20.00%), F486 (20.00%) and F491 (23.33%) which were statistically on par with each other.
A cumulative mortality of 0 to 100 per cent was observed among the 61 strains collected from different forest ecosystem. Bt strain F493 recorded a higher mortality of 100 per cent, followed by F468 (86.67%) which were comparable with standard strain HD-1 for their efficacy. The other strains F287 (76.67%), F504 (76.67%) were also next best to F493, HD-1 and F468 for their efficacy which were statistically on par with each other (Table.1).
The present results of 100 per cent larval mortality of S. litura with locally available Bt strains was in accordance with that of Nariman et al. (2009) who reported a higher mortality of 100 per cent and 90 per cent of second instar Spodoptera littoralis with two Bt strains Ts-5 and As-3 collected from seven governorates of Egypt. Meadows (1993) also reported elevated levels of insecticidal activity against Galleria mellonella with B. thuringiensis strains isolated from cultivated environments, which are preferred by lepidopteran insects.
Further, Meihiar et al. (2015) reported that, forest, beach and cultivated soils had more B. thuringiensis strains than uncultivated and interior arid soils. The frequency of B. thuringiensis was partially dependant on organic matter and pH content of the soil. A total of 65 per cent of the strains were found to be toxic to Galleria mellonella. The most toxic isolate of B. thuringiensis was obtained from cultivated area and produced bipyramidal, cuboidal and rectangular inclusions.
In present study, only four strains out of 61 strains recorded mortality in the range of 76-100 per cent against larvae of S. litura (Table 2). Similarly, Merdan et al., (2010) reported that, among ten B. thuringiensis strains they tested two were potent against S. littoralis (Boisduval), another two strains against Helicoverpa armigera (Hubner) and six strains were potent against Pectinophora gossypiella (Saunders). Contrary to this, Valicente and Barreto (2003) reported a higher frequency of 62 per cent of Bt strains recorded mortality between 81 to 100 per cent.
