Bio-efficacy of the EPF and neem oil alone and their combinations on okra jassids and whiteflies
Three EPF alone and their combinations with neem oil were found effective against the nefarious sucking pests of okra. L. lecanii was the most promising among the tested EPF under field conditions. Combination of L. lecanii with neem oil at half of their individual concentrations was the best treatment in reducing jassids and whiteflies infesting okra in 2018 and 2019. Raheem and Al-Keridis (2017) observed that L. lecanii, B. bassiana, and M. anisopliae isolates were promising as a fungal biocontrol agent (or pathogens) for whitefly control in the field. They also concluded that among the 3 EPF, L. lecanii was more virulence than others against B. tabaci infesting tomato. In another study, Scorsetti et al. (2008) documented that L. lecanii was highly virulent against B. tabaci and Trialeurodes vaporariorum (Westwood) (Hemiptera: Aleyrodidae) in organic and conventional horticultural crops in greenhouses and open fields in Argentina. White halo fungus at 7 g l−1 gave significantly a high mortality of okra jassid which was in conformity with the present findings (Baladaniya et al. 2010). Maketon et al. (2008) revealed that M. anisopliae (strain CKM-048) at the concentration of 1.25 × 1013 conidia ha-1 showed good controlling efficacy with the 73.33 ± 10% mortality rate of jassids, A. biguttula biguttula in aubergine, Solanum aculeatissimum.
Co-application of fungi like B. bassiana, M. anisopliae, and L. lecanii at suitable sub-lethal concentration of neem oil as two-in-one tank mix successfully employed against various insect pests to reduce the selection pressure in target pests. Compatibility of neem product and Beauveria against B. tabaci was studied (Islam et al. 2010) and highlighted that the highest adult deterrence index (80.15) and oviposition deterrence index (88.25) recorded when neem was combined with of B. bassiana (108 conidia ml-1). Combinations of the EPF and neem oil (1,1) had lower LT50 values than each of their individual indicating the compatibility among them against Epilachna dodecastigmata and Bagrada hilaris under laboratory conditions (Halder et al. 2017). Neem-based formulation nimbecidine has been reported compatible with B. bassiana and L. lecanii (Subbulakshmi et al. 2012). In another in vitro study, Depieri et al. (2005) recorded the compatibility of emulsifiable neem oil, aqueous neem seed extracts, and leaves (0.15 and 1.5%) with B. bassiana and concluded that all the formulations had no effect on the fungus vegetative growth and on conidia production and viability. Many botanical insecticides including azadirachtin is having a diverse mode of action. The apparent enhancement in activity of neem oil and EPF mixtures was attributed to the possible additive, synergistic, and/or stabilizing effect of neem oil (Halder et al. 2012).
Interestingly, bio-efficacy of Imidacloprid 17.8% SL was at par with the different biopesticides, botanicals, and their combinations. This first generation neonicotinoid, Imidacloprid are being used in the region over a decade. Local farmers frequently applied this insecticide more than their recommended concentration. Due to long-term regular use of this neonicotinoid in agri-horticultural ecosystem of the region caused development of resistance among sucking pests. The green peach aphid, Myzus persicae (Sulzer), has developed 5.90-folds resistance against Imidacloprid 17.8% SL during 2010–2018 in Varanasi region (Halder and Rai 2018). Many local farmers also viewed the lower efficacy of Imidacloprid 17.8% SL in recent years. In paradox, microbial insecticides like EPF seldom used for pest management in the area. The reason could be non-availability of suitable biocontrol agents in the local market and lack of knowledge about their usage (Roy et al. 2017). So, using relatively newer control method in the region, i.e., spraying of EPF alone and combinations with neem oil against the sucking pests of okra, might be the reason for superior result.
Bio-efficacy of the EPF and neem oil alone and their combinations against predators
Biopesticides are promising alternatives to chemical pesticides, and they have opened up new avenue in insect pest management to aid in the promotion of safe, eco-friendly pest management (Prithiva et al. 2018). They are relatively host-specific and do not interfere with other living organisms. In the present experiment, all the 3 EPF alone and in combination with neem oil was found relatively safe to the polyphagous predators under field conditions. The present findings were in accordance with Thungrabeab and Tongma (2007), who reported that B. bassiana (Bb.5335) and M. anisopliae (Ma.7965) were relatively safe on non-target natural enemies, viz., Coccinella septempunctata L., Chrysoperla carnea (Stephens) and Dicyphus tamaninii Wagner, and beneficial soil insect Heteromurus nitidus Templeton. Oil formulation of B. bassiana (Bb 112) was found safe to predatory Cryptolaemus montrouzieri adults with the highest adult survival of 92.62% at the highest concentration (108 spores ml-1) and 100% survival at the lowest concentration tested (104 spores ml-1) (Prithiva et al. 2018). In another study, Brown and Khan (2009) reported that M. anisopliae isolate was safe against C. montrouzieri. Toxicity of Imidacloprid to polyphagous predator, C. carnea was studied by Preetha et al. 2009. They recorded that Imidacloprid at the recommended concentration of 0.28 ml/l caused 15.38% egg mortality, 26.67 and 33.33% larval mortality by ingestion and contact, respectively, and 50.00% adult mortality. Imidacloprid was also reported to affect the longevity of C. carnea adults (Mathirajan and Regupathy 2002). Toxicity of Imidacloprid to these predators could be the reason for their low population in the respective treatment.