Novel insecticides for the management of shoot fly, Atherigona approximata Malloch (Diptera: Muscidae): an emerging insect pest of wheat in India

Shoot flies (Atherigona spp.) are the members of muscidae family which have got economic importance as pest of several crops of Gramineae family mainly cereals and millets. One of the most effective management strategies for controlling shoot fly is the use of novel insecticides either as seed dressers, soil applicators or as foliar treatment. Under the field condition, seed treatment with thiamethoxam 30 FS @ 5 ml/kg seed followed by a spray of cypermethrin 10 EC @ 0.5 ml/l has recorded least shoot fly oviposition (0.84 and 0.94 eggs/plant), dead heart (8.90% and 9.96%), more per cent productive tillers (79.68% and 79.50%), grain yield (32.22 and 31.27 q/ha) and BC ratio (2.05 and 1.99) as compared to the rest of the combinations during the investigation in rabi, 2019–20 and 2020–21, respectively. Under the changing climate where, the minor insect pests attaining major pest status, the present investigation would help the policy makers to reduce the losses caused by shoot fly infestation significantly by using an effective above-mentioned chemical treatment combination that can be integrated in future IPM modules.


Introduction
Wheat (Triticum aestivum L.) is one of the major food grain consumed directly by human beings. It is relatively more important as a human food than any other food grains due to its unique baking quality. It is a key crop for green revolution which made India as the second highest producer of wheat next to China (Anonymous 2020). The low productivity of wheat in Karnataka, India (1193 kg/ha) as analysed analogue with national average of 3368 kg/ha is attributed by the several factors including distribution of majority of cropped area (60%) under rainfed condition, poor adaptability of improved technologies and due to the attack of many insect pests and diseases (Anonymous 2019). Insect pests and diseases together reported to cause 20 to 37 per cent yield loss in wheat (Pimentel 1997).
In global perspective, 26 insect pests are reported to damage wheat crop (Miller and Pike 2002), while 12 species have been reported from Indian Sub-continent which infest from planting to till harvest of the crop (Anonymous 2013).
In recent past fifty years, there has been considerable change in pest complex due to pest succession and dynamic use of inorganic chemicals in order to increase grain yield (Gill and Garg 2014). Introduction of high yielding varieties to the Indian sub-continent has altered the wheat ecosystem to become more conducive for growth, development and multiplication of certain insect pest species (Jambagi et al. 2021). This lead to the emergence of new insect pests and also accelerated the incidence of existed minor pests.
Shoot flies (Atherigona spp.) are known to cause 'dead heart' damage in a number of tropical grass species (Deeming 1971;Pont 1972) and wheat (Pont and Deeming 2001). During the recent past shoot fly incidence has become a major threat to wheat cultivation in Karnataka as more than 26 per cent of dead heart incidence was reported (Anonymous 2013). Shoot fly incidence in wheat initiates at 7-10 Days After Emergence (DAE) and continues up to a month. The adult fly deposits single, elongated, cigar shaped white eggs on abaxial leaf surface, parallel to midrib (Raina 1981;Padmaja et al. 2010). Early instar maggots enter the plant whorl by moving downward through folds 1 3 of the younger leaves. After reaching the base, maggot incise the growing point which results in drying of the central shoot and produce 'dead heart' symptom with foul smell (Raina 1981;Shiang-Lin et al. 1981;Sharma et al. 2003). In most of the cereals and millets timely planting, exploitation of local patterns (cultural practices) and need-based insecticide usage are the familiar management practices which are being in practice to tackle shoot fly incidence (Balikai 2012). Use of insecticides for the management of insect pests in wheat is meagre as it is one of the low input crop (Gahukar 1992). But in the changed pest scenario, it is essential to incorporate the chemical management tool for the better harvest of the grain yield. There are possibilities to use insecticides as seed dressers, soil applicators, foliar treatment or use of combination of either of the tactics to achieve better results for the management of seedling shoot fly pest. In this context, the present investigation was framed to study the effect of selected insecticides either as sole or in combination of either of the practices.

Material and methods
The present investigation on shoot fly management with insecticides was carried out during rabi 2019-20 and 2020-21 at Main Agricultural Research Station (MARS), University of Agricultural Sciences, Dharwad, Karnataka state-India by using wheat cultivar UAS-304 on medium black soil with maize as the previous crop under irrigated condition. The experiment was laid out in Randomised Block Design (RBD) with three replications of nine treatments including untreated check (UTC). The crop was sown with a row spacing of 23 cm apart in a plot size of 3 m × 2.7 m. All the crop production technologies were adopted as per the package of practice to get good crop except plant protection measures against insect pests. Prophylactic application of insecticides either as seed treatment or soil application has been done at the time of sowing whereas, the other foliar treatments were imposed at 15 Days After Emergence (DAE) by using knapsack sprayer with 450 l/ha spray fluid.

Observations
The observation on shoot fly oviposition was made by selecting five plants randomly from each treatment and observed for the egg load, same has been expressed as number of eggs per plant. Dead-heart incidence due to shoot fly was recorded by selecting 100 plants randomly from each treatment and expressed in percentage by using the formula below. The observations on both parameters were noted a day before spraying and at five, ten and fifteen days after spraying.

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The total number of tillers per plant and number of productive tillers were recorded from five randomly selected plants in each treatment at the time of harvest. The data was subjected to suitable transformations. Analysis was done by using RBD design and values were analysed using Duncan's Multiple Range Test (DMRT) in M-STAT software (Duncan 1955). Later, the grain and straw weight of each plot was recorded by using electronic weighing balance. The yield was then converted in to quintal per hectare. Based on market price of wheat grain and straw, gross return (GR) was calculated. The cost of labour and cost of other inputs, the total cost of cultivation (COC) was worked out based on prevailing market prices of insecticides. By utilising these data, we have worked out net returns (NR) and BC ratio for each treatment by using below formulas.

Results and discussions
The results obtained during both the year of investigation is described herewith consideration of different damage parameters as sub-headings as follows.

Egg load (number/plant)
The pooled mean of shoot fly egg load recorded during 5, 10 and 15 Days After Spraying (DAS)in both the year of investigation (2019-20 and 2020-21) endorsed the least oviposition of 0.84 and 0.94 eggs per plant, respectively in the plot characterized with seed treatment of thiamethoxam 30 FS (5 ml/kg seed) along with a spray of cypermethrin 10 EC (0.5 ml/l) (T 7 ), which was in close proximity with T 5 (Seed treatment with thiamethoxam 30 FS @ 5 ml/kg of seed followed by a spray of emamectin benzoate 5 SG @ 0.2 g/l) (0.88 and 0.98 eggs/plant) (Tables 1 and 2). The present findings are in agreement with the reports of Taneja and Henry (1993) who observed least oviposition (20%) in the plots treated with cypermethrin 10 EC (45 g a.i/ha) once or twice. This implies that the chemical application either acted as oviposition deterrent or the adult females were killed by the chemical action before they could lay the eggs. Malschi (2003), Prem et al. (2004) and Dawareet al. (2011) reported seed treatment with thiamethoxam 70 WS at different concentrations reduced the shoot fly infestation to an acceptable level whereas, the soil application of carbofuran 3 G @ 30 kg/ha also showed on par results with it (Shrinivas 2006;Hari and Jawala 2008;Ambarish et al. 2017).

Dead-heart (%)
The incidence of shoot fly (dead-heart) in the study has followed increasing trend from 5 to 15 DAS in all the treatments during both the year investigation. However, in the best treatments (T 7 -seed treatment of thiamethoxam 30 FS @ 5 ml/kg of seed along with a spray of cypermethrin 10 EC @ 0.5 ml/l and T 5 -seed treatment with thiamethoxam 30 FS @ 5 ml/kg of seed followed by a spray of emamectin benzoate 5 SG @ 0.2 g/l), though there was increase in damage from 5 to 15 DAS but the increase was at a very gradual rate. This implies that these treatments have not allowed the pest to cause heavy damage. The observations recorded in different treatments revealed that the mean shoot fly incidence ranged between 8.90 to 26.05 per cent and 9.96 to 27.96 per cent during 2019-20 and 2020-21, respectively (Tables 3 and 4). The treatment T 7 found superior with least incidence of 8.90 and 9.96 per cent dead heart followed by T 5 (9.25 and 9.99%) and T 8 (12.50 and 14.41%) in both the years, respectively. The per cent reduction of shoot fly infestation (oviposition and dead-heart) over control in different insecticidal treatments is depicted in Figs. 1 and 2. Due to the scanty literature pertaining to combination of seed treatment with foliar spray of investigated insecticides, the information available on usage of chemicals as a sole application also has been considered here for discussion. Our present findings are substantiated by Taneja and Henry (1993) who recorded least shoot fly incidence of 19, 21 and 20 per cent dead-heart at different dosages of cypermethrin 10 EC (22.5, 33.75 and 45 g a.i./ha) and proved as superior than endosulfan 35 EC and carbofuran 3G. Balikai (2006) and Sandhu (2016) also recorded the application of cypermethrin 10 EC @ 0.05 kg a.i./ha at 6 and 12 DAE provided best control of shoot fly. Another synthetic pyrethroid, deltamethrin 2.8 EC @ 1 ml/l during 28 DAE was found to be effective for the control of shoot fly in sorghum (Patil and Bagde 2017). In another study (Biradar and Sajjan 2018)   of carbofuran 3 G @ 30 kg/ha. However, the earlier workers (Kulkarni et al. 1973;Gupta et al. 1980;Borad et al. 1995) also opined that soil application of carbofuran 3 G @ 1 kg/ha or 3 g/m row length was found to be alternative for endosulfan 35 EC for control of sorghum shoot fly. But in later days, our pesticide industry blessed with a novel neonicotinoid, thiamethoxam which was dominated over other chemicals due to its efficacy as seed dresser and foliar insecticide to combat seedling insect pest like shoot fly in different agricultural crops.

DEAD HEART
2019-20 2020-21 So far, the largest and most popular application in crop protection is the prophylactic seed dressing. During germination and growing, the active substance in the seed coating is taken up by the roots and translocated to all parts of the crop, making the crop toxic to insects that attempt to feed upon it (Van der Sluijs et al. 2013). As shoot fly is a seedling pest, the successful control can be achieved by combining a foliar spray with such seed treatment chemical which is being practiced in sorghum against Atherigona soccata. The treatments where the shoot fly infestation was low have recorded higher grain yield, straw yield (Tables 7 and 8) and a greater number of productive tillers (Tables 5 and 6) while, there was no significant difference observed between the treatments with respect to total number of tillers in both the year of investigation(Tables 5 and 6) as the meagre shoot fly infestation supports the production of side tillers, but they were not productive as they produce chaffy earheads.

Yield economics
The higher grain yield (32.22 and 31.27 q/ha) and straw yield (76.30 and 74.78 q/ha) was obtained in the plots characterised with seed treatment of thiamethoxam 30 FS (5 ml/kg seed) followed by a spray of cypermethrin 10 EC (0.5 ml/l) (T 7 ) compared to other treatments during 2019-20 and 2020-21, respectively. The economic analysis on the feasibility of various treatments indicated that higher net returns of Rs.45557.49 and Rs. 43,012.71 per hectare was achieved in T 7 with highest BC ratio of 2.05 and 1.99 during both the year of experimentation, respectively. The results of the present investigation are supported by the findings of Ningaraj (2015) who also reported that seed treatment with thiamethoxam 70 WS followed by foliar spray of thiamethoxam 25 WG provided an excellent efficacy against shoot fly in wheat with highest net returns of Rs 56,303.72/ ha compared to rest of the treatments with a grain yield of 41.71 q/ha and B: C ratio of 2.60.
With the above foregone discussion, it is evident that the management of early insect pests such as shoot fly with novel insecticides either as sole or in combination with one another may contribute significantly for higher grain yield. Meanwhile, it is also essential to investigate the crop loss estimation due to A. approximata in wheat as it is one of the new emerging major insect pest of wheat in subtropical countries such as India. Under the changing climate where, the minor insect pests attaining major pest status, the present investigation would help the policy makers to formulate chemical management strategies with seed treatment alone or in combination with foliar spray, to reduce the losses caused by shoot fly infestation significantly.