Field evaluation of insecticides against the fall armyworm, Spodoptera frugiperda in maize

Introduction

The fall armyworm (FAW), Spodoptera frugiperda (J.E. Smith), represents a significant invasive insect pest, posing a serious threat to global food security, particularly in maize production. Originally native to the Neotropical regions of the Americas, FAW was first detected in West Africa in 2016, and has since rapidly spread across sub-Saharan Africa and various countries worldwide (Prasanna et al., 2021; CABI, 2022) [10,5] . In India, the pest was first reported in Karnataka in 2018, leading to considerable maize yield losses, and within a year, it had disseminated to major maize-growing regions (Sharanabasappa et al., 2018; Shylesha et al., 2018; Suby et al., 2020) [11,12,14] . This highly polyphagous insect is capable of infesting over 350 host plant species and completing up to 12 generations per year in tropical climates (Busato et al., 2005) [4] . The larval stages feed on various parts of the plant, leading to substantial yield reductions. FAW infestations in maize have been reported to result in yield losses ranging from 50% to 80%, causing economic losses amounting to millions of dollars (Adhikari et al., 2020) [1] . In Africa, in the absence of effective management strategies, FAW is responsible for annual maize yield reductions of between 8.3 to 20.6 million tonnes, translating to economic losses estimated at $2.5 to $6.2 million (Mendesil et al., 2023) [9] . In India, maize production saw a 3.2% decline in 2017, with a total output of 27.8 million tonnes, largely due to infestations by insect pests, including FAW (Manupriya, 2019) [8] . Preliminary studies have estimated FAW-induced yield losses in India to be between 33% and 36% (Jagdish et al., 2019) [7] . Currently, FAW is established in over 42 countries, with an estimated annual economic impact of $13 billion globally (Stokstad, 2017). In Indian conditions, the FAW control is primarily focused on synthetic insecticides which evolved as effective control options. The overuse and misuse of these synthetic insecticides result in rapid resistance development and control failure. According to the Arthropods Pesticide Resistance Database, there are 42 active ingredients and 182 resistance cases were reported for FAW worldwide (Gutierrez-Moreno et al., 2020) [6] . Despite the cost-intensiveness of chemical usage, documented instances of resistance, and the adoptive practices of Integrated Pest Management (IPM) strategies, chemical control continues to represent a viable and International Journal of Advanced Biochemistry Research https://www.biochemjournal.com efficacious management approach for mitigating fall armyworm infestations, particularly within the context of small-scale and economically marginalized farming communities worldwide. It has been around five years since the pest's initial appearance in India, there exists a variation in the effectiveness of various synthetic insecticides in the field level. In this context, the continuous evaluation of major insecticide's efficacy against the FAW at the field level plays a crucial role in the recommendation of management actions.

Materials and Methods

Experimental design

The study was conducted during the winter season Six different treatments were evaluated and compared the effectiveness along with a control treatment. Recommended package and practices were followed to raise the maize crop except for insecticidal treatment and a good crop stand was maintained. Insecticide treatments were applied twice in the crop period except for the control plot, initially at knee height stage followed by 15 days to the first spray. Spraying was carried during calm and warm evening conditions using a high-volume knapsack sprayer equipped with a hollow cone nozzle. Application rates were set at 400 L per hectare for the first spray and 500 L per hectare for the second.

During the first spray, insecticides were distributed on both leaves and whorls, while the second application targeted only the whorls by adjusting the hollow cone nozzle.

Data collection

Visual assessments of live larvae and damaged leaves (%) were recorded from 10 plants per treatment plot excluding border rows, one day before treatment and at 1, 3, 5, 7, 10, and 14 days after the application of treatments. The final observation data of initial spraying was considered as pretreatment observation for the second round of application. Cob damage (%) was taken once at 90 days of crop growth as the number of damaged cobs to the total number of cobs in each treatment plot. Marketable grain yield was measured for each treatment and expressed in kilograms per hectare.

Statistical analysis

All the observations recorded were subjected to appropriate statistical transformations with subsequent ANOVA analysis. Tukey's test was employed for the mean separation with a significance level set at p≤0.01. IBM SPSS 2.0 statistical software is used for data analysis. Results

Population reduction

Foliar application of different insecticides substantially decreased the FAW incidence on maize evident as a reduction of larval population in standing crop recorded after the initial treatment application in comparison to pretreatment observation (4.17 to 7.50). All the treatments applied showed similar control of larval population 1 day after treatment (1 DAT) ranging from

Leaf damage

The pre-treatment leaf damage in maize taken a day before the application of respective treatments has shown the leaf damage ranging from 38.71% to 41.99% without any significant difference between the treatment plots. Similar to the pre-treatment count the leaf damage recorded from all the treatment plots 1DAT showed no significant difference between the treatments ranging from 39.36% to 41.41%. ) when compared to all the treatments applied (F = 4.41; df = 6, 12; p<0.01). A similar level of damage was reported in the case of Spinosad (20.54%), Chlorantraniliprole (18.69%), Spinetoram (17.97%) and Indoxacarb (20.37%) that are on par with each other. However, the treatment Imidacloprid (26.95%) showed lesser leaf damage than control, it is statistically on par with control (45.18%) plot leaf damage. At, 10 DAT, the Emamectin benzoate (10.08%) has shown the significant lowest level of leaf damage with which the treatment Spinetoram (14.47%) was at par with almost similar levels of damage (F = 13.09; df = 6, 12; p<0.01). Treatments Spinosad (19.87%), Chlorantraniliprole (20.50%) and Indoxacarb (25.61%) on par with each other yet the damage reported in Indoxacarb was comparatively quite high. With 35.95% leaf damage Imidacloprid has been the least effective among the treatments applied. 14 DAT data on leaf damage slightly differed from previous observations taken, with the lowest leaf damage reported in Spinetoram (14.47%) and along with that Emamectin benzoate (19.06%), Chlorantraniliprole (18.29%) and Spinosad (20.41%) shown similar levels of damage which are on par with each other (F = 5.78; df = 6, 12; p<0.01). The average level of leaf damage was recorded in Indoxacarb (23.39%) and the highest damage among the treatments was in Imidacloprid (36.16%) which is on par with control plot (46.69%) (Table No. 4). The mean data of leaf damage in maize influenced by different insecticidal treatments after the second application exhibited a moderate deviation from the result obtained from the first round of treatment application. The effective treatment in controlling the leaf damage in maize due to FAW larvae was Emamectin benzoate (15.89%) with a 66.91% reduction over control (F = 85.53; df = 6, 30; p<0.01). The Spinetoram (16.95%) with a 64.70% reduction over control stands as the next best treatment with which the Chlorantraniliprole (19.45%) was at par statistically. A mediocre range of effectiveness with 22.16% and 23.98% leaf damage was reported in Spinosad and Indoxacarb treated plots respectively. A constant peek of leaf damage was continued in the Imidacloprid (32.97%) with a damage reduction of 31.34%. Control plot leaf damage was significantly high reporting 48.02% (Fig No . 2).

Cob damage and Yield

All the insecticidal treatments significantly reduced cob damage when compared to the control (F = 30.23; df = 6, 12; p<0.0001). The extent of cob damage varied across the treatments, ranging from the lowest 10.28% in the case of Spinetoram and 36.95% in the control plot. Among the insecticides, Imidacloprid was the least effective with a damage percentage of 22.78% (Table No. 5). Furthermore, the assessment of yield (kg/ha) revealed that the highest yield of 2480 kg/ha was obtained from plots treated with Spinetoram, exhibiting a significant superiority over all other treatments (F = 15.67; df = 6, 12; p<0.0001). Comparable yields were also observed for Emamectin benzoate (2213.33 kg/ha) and Chlorantraniliprole (2360 kg/ha). Additionally, the yields from Spinosad (2133.33 kg/ha) and Indoxacarb (2186.67 kg/ha) were found to be statistically similar (Table No.

5). Discussion

The field trial aimed to assess the efficacy of various insecticides against the fall armyworm in maize. The results of the trial demonstrated that all tested insecticides outperformed the control plot in terms of reducing fall armyworm larval populations and minimizing maize damage. Among the insecticides evaluated Spinetoram and Emamectin benzoate exhibited the most favourable control outcome in reducing the larval population of FAW in maize. However, after the second round of treatment application, the final larval load was minimal in Spinetoram and Chlorantraniliprole. Regarding leaf damage reduction after the first application, treatments of Spinetoram and Chlorantraniliprole have shown the maximum reduction. In contrast, the Emamectin benzoate treated plots have shown the maximum reduction over control followed by Spinetoram after the second round of application. The efficacy of Spinetoram was consistent throughout the crop period and, Emamectin benzoate as well as Chlorantraniliprole also served the purpose with a similar level of control. The above three treatments can be best incorporated into IPM modules for effective management of FAW. Since resistance development in FAW is a major bottleneck in the chemical control of FAW, the above insecticides can be rotated in a single crop season. Our results follow similar trends of Bharadwaj et al., 2020 [3] ; Thumar et al., 2020 [16] ; Tayde and Ramesh, 2022 [15] .

Conclusion

Overall, the six different frequently used insecticides for FAW control were evaluated at field level for their efficacy in controlling FAW on maize crop. Spinetoram showed the best results among all, followed by Emamectini benzoate and Chlorantraniliprole. These three chemicals can be best utilized for FAW control and can be included in the IPM package of FAW of the Chhattisgarh region.

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