Background

Potato (Solanum tuberosum L.) is an important tuber crop and is the fourth most essential food crop after rice, wheat, and maize in the world, producing the highest dry matter per unit area and time (Thamburaj and Singh 2003). The low productivity of potatoes is mainly due to the heavy economic losses induced by pests and diseases which resulted in 40 per cent yield losses.

The considerable economic losses of the potato crop are due to various insect pests, out of which cutworms and white grubs are major ones causing extensive damage to the potato crop (Singh 2003). Amongst the major phytophagous white grubs, the genus Lepidiota commonly known as cane grub is a pest of considerable economic importance in several countries of the world. Species within this genus are the most destructive pests of sugarcane confronting the sugar industry as well as infesting potato, maize, colocasia, green gram, groundnut, coconut, etc. Over the past decades, research, especially on bioecology and management, has revealed that Lepidiota spp. are characterized by their long life cycle (usually 1 year or more), subterranean larval habit (making sampling and insecticide treatments difficult), ability to survive in wet soil conditions of riverine and sea islands and to withstand prolonged submergence or flood, etc. which make them difficult to be managed. Of late, an endemic biennial white grub species, Lepidiota mansueta had emerged as a severe key pest in India. It has a biennial life cycle, which is the first of its kind from North East India and this species can be regarded as a rare species, because it spends its entire life cycle under the ground, except a short period during which adults come out of the ground for mating and oviposition. There was no evidence showing that the adults fed on plants in both field and laboratory and hence this species got the unique distinction of first Indian phytophagous white grub species with non-feeding adults (Bhattacharyya et al. 2015). Second and third instar grubs are the most destructive instars, found throughout the year due to overlapping generations and they feed on the fibrous roots of young plants and tubers making shallow, circular cavities (Bhattacharyya et al. 2015). The infestation was found to vary from year to year and from place to place and it was impossible to predict the spotty and localized damage as the late damage was only visible when the plant dries up (Bhattacharyya and Dutta 2014). Practically very little work has been conducted so far to manage this important pest in different crops in India and elsewhere, excepting a few insecticidal trials. The persistent use of synthetic insecticides against white grubs in potato and other economically important crops may initially give some levels of control against this pest, but in long term it will pose a threat to the ecosystem causing the resurgence of other pest species, adverse effects on beneficial organisms as well as long residual action leading to various health hazards. The need to develop and implement organic pest management strategies instead of chemocentric approaches has also become an exigency since this North Easter region is tagged as the “Organic Hub” of India. Keeping these facts in the backdrop, the present study was undertaken to find out a holistic and comprehensive eco-friendly management against L. mansueta grubs in potato crop at Majuli river island, Assam, India.

Methods

Evaluation of the efficacy of eco-friendly management modules against L. mansueta grubs in potato was conducted in the farmer’s field at Maharichuk village, Majuli district (93°39 E to 94°35 E Longitude, 26°45 N to 27°12 N Latitude and an altitude of 84.50 m above mean sea level), Assam during October to February 2019–2021 using seven treatments and three replications in Randomized Block Design using potato variety, Kufri Jyoti. The details of various eco-friendly management modules evaluated against L. mansueta grubs are presented (Table 1).

Table 1 Details of eco-friendly management modules examined against Lepidiota mansueta grubs in potato
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Observations recorded

During the time of harvest, both healthy and L. mansueta grub infested tubers were separated. The efficiency of each module was evaluated in terms of tuber yield (q/ha) of the crop, per cent tuber damage resulted due to L. mansueta grubs on both weight and number basis along with the number of L. mansueta grubs per square metre during the time of harvest and Benefit Cost Ratio (BCR). The per cent tuber damage based on weight and number basis was calculated, using the following formulae (Sharma 2013).

$${\text{TD}}\left( \% \right) = \left( {{\text{Wd}}/{\text{Wt}}} \right) \times {1}00$$

where TD-Tuber damage (by wt.), Wd-weight of damaged tubers in a plot, Wt-Total weight of tubers in the same plot

$${\text{TD}}\left( \% \right) = \left( {{\text{Nd}}/{\text{Nt}}} \right) \times {1}00$$

where TD-Tuber damage (by number), Nd-Number of damaged tubers in a plot, Nt-Total number of tubers in the same plot.

Statistical analysis

Field data recorded during the investigation were analysed using standard statistical procedures. The per cent tuber infestation data on weight and number basis was transformed into angular values, before Analysis of Variance (ANOVA) for Randomized Block Design (Gomez and Gomez 1984).

Economics of cultivation

BCR (Benefit Cost Ratio) was computed for the value of total expenditure and net return.

$${\text{Net}}\;{\text{return}} = {\text{Gross}}\;{\text{return - Cost}}\;{\text{of}}\;{\text{Cultivation}}$$
$${\text{BCR}} = \frac{{{\text{Net}}\;{\text{return}}\left( {{\text{Rs}}.\;{\text{ha}}^{{ - {1}}} } \right)}}{{{\text{Cost}}\;{\text{of}}\;{\text{cultivation}}\left( {{\text{Rs}}.\;{\text{ha}}^{{ - {1}}} } \right)}}$$

Results

Per cent tuber damage on weight and number basis

All the modules were significantly superior over the untreated control in reducing tuber damage per cent (weight and number basis) by L. mansueta (Table 2, Figs. 1, 2, 3, 4). However, the pooled analysis of two years of data (2019–2020 and 2020–2021) revealed that the lowest per cent of tuber damage on weight and number basis was observed in Module-VI (11.78 and 14.65%, respectively, in 2019–2020; 10.86 and 13.50%, respectively, in 2020–2021) and Module-III (12.40 and 15.53%, respectively, in 2019–2020; 11.23 and 14.02%, respectively, in 2020–2021) that showed statistical parity and significant superiority over the other treatments. The next effective modules were Module-II (17.68 and 14.98%), followed by Module-IV (20.72 and 16.83%), Module-V (23.79 and 19.86%) and Module-I (25.78 and 22.10%) in recording tuber damage per cent on weight basis during the two consecutive years, respectively. These sequences of modules were regarded to the tuber damage on a number basis, the per cent of infestation and followed by the same trend as observed on a weight basis. The tuber damage on a number basis in Module-II (20.39 and 18.55%), followed by Module-IV (23.44 and 20.64%), Module-V (25.42 and 24.35%) and Module-I (27.74 and 26.71%) were found to be significantly superior to untreated control during 2019–2020 and 2020–2021, respectively. Out of all tested treatments, the highest per cent of tuber damage on weight (32.84% in 2019–2020; 30.85% in 2020–2021) and number basis (35.30% in 2019–2020; 33.65% in 2020–2021) was recorded in the untreated control. While considering the mean L. mansueta grub’s population in different treatments, it was found that the population ranged from 2.17 to 8.00 individuals at the time of harvesting.

Fig. 1
figure 1

Lepidiota mansueta grubs infested potato tubers

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Fig. 2
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Effect of different modules on per cent of tuber damage caused by Lepidiota mansueta in potato crop during 2019–2020

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Fig. 3
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Effect of different modules on per cent of tuber damage caused by Lepidiota mansueta in potato crop during 2020–2021

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Fig. 4
figure 4

Effect of different modules on per cent of tuber damage caused by Lepidiota mansueta in potato crop (pooled data)

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Table 2 Effect of different eco-friendly management modules in reducing Lepidiota mansueta grub infestation in potato
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Tuber yield and Benefit Cost Ratio (BCR)

As regards tuber yield, all the modules were found to be significantly superior over the untreated control (Table 2 and Fig. 5). Based on two consecutive tested years of data, the highest yield was recorded in Module-VI (121.39 and 124.03 q/ha) and Module-III (120.64 and 122.36 q/ha) that showed statistical parity and the next best modules were Module-II (114.17 and 116.67 q/ha), Module-IV (109.72 and 111.18 q/ha), Module-V (102.24 and 104.24 q/ha) and Module-I (91.31 and 94.31 q/ha). However, all the treatments were significantly superior to the untreated control (83.82 q/ha in 2019–2020 and 85.28 q/ha in 2020–2021) in recording high yields.

Fig. 5
figure 5

Effect of different modules on yield of potato

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The BCR was found to be superior in Module-VI (2.82), followed by Module-III (2.77), Module-II (2.53), Module-IV (2.40), Module-V (2.18) and Module-I (1.92) during 2019–2020. During 2020–2021, the highest BCR was 2.90 in Module-VI, 2.82 in Module-III, followed by 2.61 in Module-II, 2.44 in Module-lV, 2.24 in Module-V and 2.02 in Module-I.

Discussion

The present study indicates that Metarhizium anisopliae (Module-VI) and jatropha oil (Module-III)-based modules provided significant protection against L. mansueta grubs over the untreated control. The effectiveness of M. anisopliae against different white grub species had already been documented by various earlier workers (Saharwat et al. 2021). The potential of Metarhizium against a broad range of scarabs (dynastids, rutelines and melolonthids), and other coleopteran pests was extensively studied worldwide as they act as biocontrol agents, plant-pathogen antagonists, symbiotic endophytes, rhizosphere colonizers and plant-growth promoters (Faria and Wraight 2007). Metarhizium spp. also acts as plant rhizosphere associates and is known to enhance faster plant growth with increased plant root hair density. The mycelia of Metarhizium grow within the cortical cells of the roots as well as in between intercellular spaces to promote the proliferation of root hairs and colonize plant roots endophytically without causing any damage to the plant, thus showing potential as a plant endosymbiont (Sasan and Bidochka 2012). Wheat seeds treated with M. brunneum resulted in the promotion of plant growth and various plant-growth parameters such as plant height, root length, and root or shoot weight along with their endophytic establishment within the plants (Jaber 2018). Metarhizium can transfer essential nutrients like nitrogen, phosphorous, and nonlimiting soil nutrients to their host plants and are also able to mitigate high salt induced oxidative stress conditions and improve the quantity and quality of crops (Behie and Bidochka 2014). As the entomopathogenic fungus has regenerative capability in the natural environment, there is no chance of resistance build up by the insect pests, like in chemical pesticides (Saharwat et al. 2021). Since the myco-pathogens persist for a longer period in the soil than chemicals, M. anisopliae can be an epitome for the white grub’s management in endemic areas. It can be one of the best options for organic potato growers as an alternative against synthetic soil insecticides.

In jatropha oil, the major toxic constituents are phorbol esters that are effective against many insects by antifeedant, ovipositional deterrent and ovicidal properties (Gopalakrishnan et al. 2014). The present findings agree with Kashyap et al. (2020) who studied the superiority of different botanicals against Agrotis ipsilon and Dorylus orientalis in potato. The highest per cent reduction of cutworm population/pit was observed in neem oil @ 5 per cent drenching (64.23%), followed by karanj oil drenching (61.54%) and jatropha oil drenching (56.54%) at 15 days after treatment. They recorded the per cent tuber damage caused in neem oil drenched (14.93 and 12.21%) and jatropha oil sprayed plots (19.79 and 17.13%) on both weight and number basis, respectively. The present findings are in confirmatory with the earlier work carried out by Rani et al. (2009) who reported that Azadirachta indica and Jatropha sp. kernel extracts recorded minimum per cent tuber damage by the white grub, Brahmina coriacea in potato. Similarly, Meshram and Homkar (2011) concluded that jatropha cake was found to be statistically significant over the untreated control in minimizing seedling damage due to Holotrichia serrata. Devi (2018) recorded the least per cent of tuber damage caused by D. orientalis with soil drenching with neem oil @ 5 ml/l (8.65 and 10.70%), followed by jatropha oil (14.79 and 16.89%) and pongamia oil (16.92 and 18.63%) compared to untreated control (25.93 and 28.70%) on weight and number basis, respectively.

The potential of “panchagavya” that acts as both biopesticide and bio-enhancer might be due to the presence of chemolithotrophs, autotrophic nitrifiers (ammonifiers and nitrifiers) and naturally occurring beneficial microorganisms viz. lactic acid bacteria (Lacto bacillus), yeast (Saccharomyces), actinomyces (Streptomyces), photosynthetic bacteria (Rhodopseudomonas), and fungi (Aspergillus and Penicillium), macro and micronutrients and plant-growth hormones (Indole-3-acetic acid and Gibberellic acid) that enhance the disease-free growth and yield (Pagar et al. 2015). Sarkar et al. (2014) observed an induced defence mechanism and linear growth of both shoot and root systems in tomato, chilli and cowpea when sprayed with panchagavya during their seedling, which was quantified due to the presence of polyphenol oxidase in panchagavya. Mustard Oil Cake (MOC) is used as an organic fertilizer for flowering and vegetable plants because of its higher protein and micronutrient contents. It also prevents many diseases by keeping plants healthy. The efficacy of wood ash might be due to potash, which contains potassium (Hume 2006). The ash acts like diatomaceous silica-based dust and the thick layers of ash act as a physical barrier limiting colonization by beetles. The ash destroys the insect cuticle by absorbing the protective wax layer and prevents body water loss and eventually leads to the death of the insect by desiccation. Initially, the application of ash reduced the feeding and activity levels of Colorado potato beetle adults (Hakbijl 2002). Ulrichs et al. (2006) stated that the formulations of silica with smaller particle sizes usually have high insecticidal efficacy due to their greater ability to absorb lipids from the epicuticle of the insect’s exoskeleton. Wood ash is being considered as an alternative fertilizer for potato (Porter and Ocaya 2009).

Conclusions

The results showed that all the management modules were indubitably effective, however, Module-VI (Metarhizium anisopliae based) and Module-III (Jatropha curcas seed oil) were found to be significantly superior in recording the least tuber damage on weight and number basis, and the highest tuber yield. Hence, botanical pesticides and bio control agents can be effective means to replace synthetic chemical pesticides in pest management programmes against white grubs and other soil dwelling insect pests. The stress on organic farming and residue free commodities would certainly warrant increased adoption of biopesticides by the farmers. Although the modules were effective against L. mansueta grubs under field conditions, it is extremely necessary to conduct On-Farm Trials and Front-Line Demonstrations to popularize the effective and eco-friendly grub management module in different agro-ecological zones of India.