Field-based bio-efficacy assessment of sequential biopesticidal schedules against whitefly (Bemisia tabaci Gennadius) on okra (Abelmoschus esculentus L. moench)

Introduction

Bemisia tabaci Gennadius ranks among the most economically damaging sap-sucking pests confronting okra [Abelmoschus esculentus (L.) Moench] cultivation worldwide, and its impact extends well beyond the phloem sap withdrawn directly by its nymphal and adult stages. Colonies settled on the lower leaf surface deposit copious honeydew, and the sooty mould that develops on it visibly darkens the canopy and cuts photosynthetic output, ultimately depressing pod yield and quality (Meena and Kanwat, 2005) [1] . A further complication peculiar to this pest is its capacity to transmit Bhendi Yellow Vein Mosaic Virus, a disease able to inflict severe crop loss once vector populations cross a threshold density. The prospect of using entomopathogenic fungi against whitefly is not a recent one: glasshouse trials conducted several decades ago on Verticillium lecanii were the first to establish that hyphomycete fungi could infect and kill this insect under controlled conditions (Quinden, 1984) [2] . That early evidence has since carried over into open-field research across Indian vegetable systems, where botanical extracts and microbial formulations, applied singly or in combination, have repeatedly lowered B. tabaci populations on okra (Anitha, 2007; Mahmood et al., 2014; Sridharan et al., 2015) [3,6,7] . Insecticide sprays nonetheless remain the default recourse for most growers, a habit that hastens resistance build-up within whitefly populations and depletes naturally occurring predators and parasitoids, strengthening the case for fungal-and plant-based alternatives (Janghel and Singh Rajput, 2016; Sarkar et al., 2016) [8,9] . What remains comparatively unexamined is whether these tools, rather than being deployed in isolation, can be combined in a planned sequence, a botanical spray to blunt early population build-up, a targeted fungal spray to follow through, and a broader fungal consortium to sustain control later in the season, so as to outperform any single intervention used alone. The present field trial was designed to test this sequential approach against whitefly infesting okra.

Materials and Methods

Experimental site and treatments

The 1 details the make-up of each schedule: a botanical spray thirty days post-sowing, a fungal spray ten days on, and a three-fungus bioinoculant mixture (2×10 8 CFU/ml) at the fifty-day mark, with a ten-day gap separating consecutive sprays; schedule M7 received plain water at every stage.

Recording of whitefly population

Whitefly numbers were tallied on leaves selected from the lower, middle and upper canopy of five tagged plants within every plot. Counts were logged a single time before the opening spray and then repeated nine days after each of the three subsequent applications.

Statistical analysis

Prior to running the analysis of variance, all counts underwent square-root transformation [√(x+0.5)]. Comparisons among treatment means relied on standard error and critical difference values worked out at the 5% probability level, with both raw and transformed figures reported together in Table 2.

Results

One day ahead of the first spray, whitefly numbers across the seven plots varied only slightly, from 2.53 to 2.84 insects per leaf, with no statistically meaningful differences, evidence that the pest load was evenly spread before treatments began (Table 2). By the ninth day after the first spray, schedule M3 had already brought the count down to its lowest point (1.98 insects/leaf), a figure on par with M2 (2.02) and M1 (2.11); the untreated check, meanwhile, had climbed to 4.40 insects/leaf. This lead widened with each subsequent spray: nine days after the third application, M3's population stood at just 1.13 whiteflies/leaf against 5.60/leaf in the check, with M6 (1.29), M2 (1.47) and M5 (1.47) trailing not far behind. Pooling data across all three sprays (Table 2; Fig. 1) confirmed M3 as the top-performing schedule at 1.59 whiteflies/leaf, ahead of M6 (1.78) and M2 (1.79); the untreated check averaged 5.07 insects/leaf over the same period. Every biopesticide schedule proved significantly superior to the check (CD at 5% = 0.28). On a cumulative basis, schedule M3 recorded the highest percent reduction in whitefly population over control (68.64%), followed by M6 (64.89%) and M2 (64.69%), whereas M4 recorded the lowest reduction (58.97%) among the treated schedules (Table 3). 0.00 0.00 0.00 0.00 Per cent reduction over control was worked out as [(Control-Treatment)/Control] × 100, using untransformed mean whitefly population values.

Discussion

M3's edge over the other schedules can plausibly be traced to how its two active components complement one another through the season: Azadirachtin's antifeedant action likely curbs population build-up early on, leaving a smaller whitefly cohort for L. lecanii, sprayed ten days afterward, to act against. Whitefly nymphs, once settled, remain stationary on the underside of the leaf, which may keep them in prolonged contact with fungal spores deposited there, a condition that could favour infection more than it would for a mobile insect, though this point was not tested directly within the present trial. The ability of Verticillium (now reclassified as Lecanicillium) lecanii to suppress glasshouse whitefly was first shown decades ago by Quinden (1984) [2] , and later research pairing L. lecanii with other entomopathogenic fungi against B. tabaci on okra has similarly reported robust suppression (Palthiya, 2014; Janghel and Singh Rajput, 2016) [5,8] . Broader evidence of Beauveria bassiana and Metarhizium anisopliae acting against sap-feeding pests, documented for aphids by Ekesi et al. (2000) [4] , reinforces the case for a fungus-led strategy. Botanical oils have also proven effective against whitefly in separate studies: Sridharan et al. (2015) [7] achieved good control using mineral-oil blends with neem or pongamia, while Patel et al. (2017) [10] and Sarkar et al. (2016) [9] both recorded substantial whitefly reduction using bio-rational treatments on okra. The consortium spray given at fifty days post-sowing seems to have carried control through the later crop stages, aligning with findings that microbial biopesticide blends can restrain the broader sucking-pest complex, whitefly included, in crops such as Bt cotton (Raghunandan et al., 2018; Wawdhane et al., 2020) [11,13] . The neem oil-based schedules (M4 to M6) generally trailed those built around Azadirachtin, although M6, pairing neem oil with L. lecanii, came close to matching the leading Azadirachtin-based schedules, again pointing to the fungal element as the more decisive factor within the sequence. Similar patterns, with L. lecanii or combined fungal-botanical schedules outperforming other treatments against whitefly, have also been noted by Naik et al. (2019) [12] and Halder et al. (2021) [14] . Read alongside the comparative work of Jain et al. (2021) [15] , the present findings add weight to sequential, fungus-anchored biopesticide schedules as a practical route to whitefly control in okra.

Conclusion

Taken together, results from the three sprays identify the Azadirachtin-L. lecanii-consortium sequence (Module M3) as the most dependable option among those tested for controlling Bemisia tabaci on okra, holding the cumulative population to 1.59 whiteflies/leaf against 5.07 whiteflies/leaf in the untreated check, a cumulative reduction over control of 68.64%. Since this schedule relies exclusively on botanical and microbial inputs, it offers a practical starting point for eco-friendly whitefly management in okra, though testing across further seasons and locations would help confirm its consistency.

Acknowledgement

AcknowledgementThe authors wish to thank the Head, Entomology Section, and the Associate Dean, College of Agriculture, Nagpur, for extending the field and laboratory support needed for this work.

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