Efficacy of bio-pesticides against major insect pests of okra under field conditions

Introduction

Bio-pesticides are defined as pesticidal substances derived from natural materials such as animals, plants, micro-organisms and certain minerals, and are generally less toxic to nontarget organisms than conventional synthetic pesticides [1] . They fall into three broad classes: microbial pesticides (Bacillus thuringiensis, Beauveria bassiana, Trichoderma species), plant-incorporated protectants, and biochemical pesticides such as neem-based azadirachtin. Over the past decade, the market share of bio-pesticides in Egypt has grown from about 3 per cent of total pesticide sales to roughly 8 per cent, driven by export requirements, residue concerns and the push from the national integrated pest management programme [2,3] . Okra is a major vegetable crop of the country, grown on about 48,000 hectares with an annual production of 0.8 million tonnes. It attracts a complex guild of insect pests throughout its growing season. The shoot and fruit borer Earias vittella, the leafhopper Amrasca biguttula biguttula and the whitefly Bemisia tabaci are the three that shape most of the pest management decisions made by the grower [4,5] . Yield losses from these three alone range between 35 and 63 per cent under unprotected conditions [6] . Continuous use of synthetic pyrethroids and organophosphates has led to documented resistance in Earias populations in several Egyptian governorates, as well as secondary flareups of whitefly due to the destruction of natural predators [7,8] . Bio-pesticides offer an alternative route that carries a slower speed of kill but a better environmental profile and is compatible with natural enemy conservation [9,10] . Several isolated reports have evaluated one or two bio-pesticides on okra, but a side-by-side comparison of the six leading options under farm-representative conditions in the Nile Valley has been missing [11,12] . The present research was undertaken to close that gap.

Material and Methods Material

The Methods: A randomised block design was laid out with seven treatments replicated four times on plot size 4.8 × 3.6 m. Treatments comprised T 1 (untreated check), T 2 (neem oil 1%), T 3 (Beauveria bassiana at 5 g L⁻¹), T 4 (Bacillus thuringiensis at 0.5 kg ha⁻¹), T 5 (NSKE 5%), T 6 (Pongamia oil 2%) and T 7 (azadirachtin 0.03 EC at 2.5 mL L⁻¹). Sprays were given at 40, 55 and 70 days after sowing. Pest counts were recorded on 10 tagged plants per plot at 1, 3, 7 and 14 days after each spray. Fruit infestation was recorded at each picking as the percentage of bored fruits. Marketable yield was the cumulative weight of sound fruits. Natural enemies (coccinellids and spiders) were counted twice before and twice after the third spray. Data were angular-transformed before analysis by ANOVA and treatment means were compared with Tukey's HSD at p = 0.05 using SPSS v26.

Bio-Formulation Quality Checks

Each batch of Bacillus thuringiensis and Beauveria bassiana was checked for viable spore count before field use. A 1 g sub-sample was serially diluted in sterile water and plated on nutrient agar (for Bt) and potato dextrose agar (for Bb).

Plates were incubated at 28 °C for 48-72 hours and colonyforming units were counted. Batches with CFU count below 90 per cent of label claim were rejected. Neem-based products were assayed for azadirachtin content using HPLC on a C18 column with methanol:water (70:30) mobile phase. Only batches with confirmed concentrations within ±5 per cent of label value were used in the research. Statistical analysis confirmed significant treatment differences on every pest measured. Bacillus thuringiensis registered the highest median reduction (76.3%) followed by azadirachtin 0.03 EC (73.1%) and Beauveria bassiana (71.6%). The untreated check showed only a 6.2 per cent apparent reduction, which reflects natural mortality and weather effects. Bacillus thuringiensis was the most effective treatment on fruit borer due to its specific toxicity to lepidopteran larvae, while azadirachtin delivered the best broad-spectrum response averaged across the three pest groups. 2 highlights the place of bio-pesticides as a core rather than isolated component.

Results

Discussion

The main findings indicate that bio-pesticides can deliver pest suppression within 10-20 per cent of the efficacy reported for conventional synthetic insecticides on okra, while retaining strong natural enemy populations. Agreement with published literature is strong for Bacillus thuringiensis. Patel and colleagues [10] recorded 78 per cent reduction in Earias under Bt sprays on okra in Gujarat, a figure almost identical to the 76.3 per cent measured here. Shivaji and Verma [11] reported azadirachtin-driven reductions of 67-72 per cent across the pest complex, again within the range of the present 69.1 per cent mean.

Points of disagreement centre on Beauveria bassiana. Some earlier reports claim reductions above 80 per cent for Beauveria on Earias [13] , whereas the present research recorded 71.6 per cent. The likely explanation is humidity. The summer season of 2022 in Alexandria was drier than the long-term average, with relative humidity averaging 61 per cent during the spray period, below the 80 per cent threshold generally needed for full conidial germination on the insect cuticle. Lower air moisture reduced entomopathogen performance. Broader implications are that bio-pesticides fit well into IPM because they protect natural enemy populations.

Coccinellid and spider counts on bio-pesticide plots were 10-16 per cent higher than in the untreated check, despite the smaller pest base available to them. For export-oriented okra cultivation, where residue limits are strict, the combination of Bt and azadirachtin in rotation provides a workable solution [12,14] .

Conclusion

References

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