Varietal susceptibility of selected potato cultivars to Dorylus orientalis Westwood under organic pest management regimes

Introduction

Organic potato production in the eastern Himalayan highlands faces an unresolved problem: D. orientalis damage remains high even when organic pest management inputs are applied, because cultivar selection is still based on yield potential and consumer preference rather than resistance to subterranean ant feeding [1] . Without varietal resistance as a first line of defence, organic pest management modules must compensate entirely through biological and cultural means, which raises costs and reduces reliability in seasons with high ant pressure [2] . Bhutan has commited to becoming a fully organic nation, and potato is the country's most important cash crop in the temperate highland zones of Paro, Thimphu, and Bumthang, where it occupies over 3,200 hectares and generates annual revenues exceeding USD 8 million [3] . D. orientalis is the principal subterranean pest, with damage rates of 25-45% in untreated fields across western Bhutan [4] . Organic-compliant control measures, including entomopathogenic fungi, botanical soil drenches, and cultural practices, have shown 60-80% damage reduction in recent trials, but residual damage on susceptible cultivars still renders 15-30% of tubers unmarketable [5] . Host plant resistance against soil-dwelling insects in potato is governed largely by tuber periderm characteristics. Periderm thickness, suberin lamellae density, and the concentration of antimicrobial phenolic compounds in the outer cell layers influence the ability of insects to penetrate and feed on tuber tissue [6] . Cultivars with thick, heavily suberised periderm present a physical barrier that slows ant mandible penetration and may deter sustained feeding, while thinskinned cultivars are breached more easily and sustain deeper tissue damage [7] . These traits are genetically controlled and reasonably stable across environments, making them useful selection criteria for breeding programmes targeting ant resistance [8] .

Despite this understanding, no published research has ranked commercially grown Bhutanese and South Asian potato cultivars for their susceptibility to D. orientalis under field conditions where organic pest management is the sole protection strategy. Most varietal screening work has been conducted either without any pest management (pure susceptibility) or under chemical protection (which masks varietal diferences by suppressing ant populations to near zero) [9] . Neither approach gives organic growers the information they need: which cultivars perform best when paired with the biological and botanical tools available to them? This research addressed that question by evaluating six commercially available potato cultivars under a standardised organic pest management regime at the National Institute of Agriculture, Thimphu, Bhutan. Tuber damage, periderm characteristics, marketable yield, and wound recovery capacity were measured to identify cultivars suited for organic production in D. orientalis-prone zones and to examine the relationship between physical tuber traits and ant feeding resistance.

Material and Methods

Climatic and Environmental Conditions

The research was conducted at the highland research station of the National Institute of Agriculture, Thimphu district, Bhutan (27° 28′N , 89° 38′E , elevation 2,320 m above sea level). The site has a cool temperate climate with a monsoon-influenced wet season (June-September) and a dry winter growing season for potato (October-March). During the trial period, mean monthly temperatures ranged from 4.8 Ω °C (January) to 17.6 °C (October), with total rainfall of 126 mm. Soils were classified as sandy clay loam (Dystric Cambisols) with pH 5.4, organic carbon 2.63%, and available phosphorus 12.8 kg ha⁻¹. Pre-planting soil excavation confirmed D. orientalis infestation at an average density of 3.7 active colonies per 10m² across the experimental area [4] .

Material

Six potato cultivars were selected to represent a range of maturity groups, skin types, and geographic origins: Desiree (red-skinned, medium maturity, European origin), Khangma Kaap (local Bhutanese landrace, white-skinned), Yusikaap (local landrace, thin-skinned), Kufri Jyoti (white-skinned, medium maturity, South Asian), Lady Roseta (red-skinned, early-medium maturity, European), and Janakdev (white-skinned, early maturity, South Asian). Certified seed tubers were sourced from the National Seed Centre, Paro. M. anisopliae and B. bassiana formulations (2×10 ⁹ conidia g⁻¹ each) were obtained from the National Plant Protection Centre, Semtokha. Neem kernel extract (5% w/v) and garlic bulb extract (10% w/v) were prepared following standardised protocols [5] .

Field Experimental Design

A randomized complete block design with six cultivar treatments and four replications was used, giving 24 plots. Each plot measured 4 m × 3 m (12 m²) with a 1 m bufer between plots. All cultivars were planted at 0.60: m×0.25 m spacing on 3 October 2022. A uniform organic pest management regime was applied to all plots: combined M. anisopliae + B. bassiana soil drench (1×10 ⁸ conidia mL⁻¹ each) at tuber initiation (40 DAP) and repeated twice at 15-day intervals, plus neem + garlic soil drench (400 mL per plant base) applied concurrently with the second and third fungal applications. No synthetic pesticides or fertilisers were used; compost at 10 t ha⁻¹ was the sole nutrient source [3] .

Methods

Tuber damage was assessed at harvest (120-140 DAP, depending on cultivar maturity) on 30 randomly selected tubers per plot. Damage severity was rated on a 1-5 scale: 1 = no damage, 2 = superficial scratches, 3 = shallow scars (< 3 mm depth), 4 = deep scars (3-6 mm), 5 = tunnelling or secondary infection. Colony counts were taken at harvest by excavating three 50×50×50 cm soil blocks per plot. Periderm thickness was measured on five tubers per plot using a digital micrometer on hand-cut cross sections at the tuber equator. Periderm suberin content was determined by the alkaline hydrolysis method on excised periderm discs and expressed as mg cm⁻² [6] . Marketable yield comprised tubers > 40 g with damage severity ≤ 2. Recovery rate was calculated as the percentage of tubers scored 3 that showed wound periderm formation by 14 days postharvest under ambient storage (8-12 °C). Data were analysed by ANOVA and means separated by DMRT at mathsfp<0.05 . Pearson correlation coeficients were calculated between periderm traits and damage parameters [7] .

Results

Cultivar diferences in D. orientalis damage were large and statistically significant (p<0.001) Lady Roseta and Desiree consistently ranked as the most resistant, while Yusikaap and Kufri Jyoti were the most susceptible. Periderm thickness and suberin content showed strong negative correlations with tuber damage, confirming the role of physical tuber traits in ant resistance. The heatmap (Figure 1) provides a visual ranking of cultivars across six parameters simultaneously. Lady Roseta and Desiree show consistently dark shading (high resistance) across all metrics, while Yusikaap displays the palest cells throughout.

The gradient from resistant to susceptible cultivars maps closely to the periderm thickness and suberin content columns, confirming that these physical traits drive the observed diferences in ant feeding damage.

Comprehensive Interpretation

The data consistently show that cultivar choice is the single largest determinant of D. orientalis damage under organic management. Lady Roseta and Desiree, classified as resistant, had damage rates 69-74% lower than the most susceptible cultivar Yusikaap even when all cultivars received identical biological and botanical protection. The process diagram (Figure 3) positions cultivar selection as the foundation of the organic management framework: starting with a resistant cultivar means that the subsequent biological and botanical steps need only suppress residual ant pressure rather than compensate for inherent susceptibility. The strong correlation between periderm traits and damage (r = -0.94 for skin thickness, r = -0.96 for suberin) ofers breeders a quantitative screening criterion that can be measured in the laboratory before field evaluation.

Discussion

Lady Roseta recorded the lowest tuber damage (9.7%) among all cultivars tested, a value 69.1% below the most susceptible cultivar Yusikaap (31.4%). This cultivar also had the thickest periderm (2.14 mm) and the highest suberin content (3.78 mg cm⁻²). The strong inverse relationship between periderm traits and ant damage (r=-0.94t0-0.96) supports the hypothesis that physical barrier thickness governs D. orientalis feeding success more than any chemical deterrent mechanism in the cultivars tested. Lulai and Freeman [6] demonstrated that suberin lamellae act as the primary physical barrier to wound invasion in potato periderm, and the present findings extend this principle to insect feeding resistance under field conditions. Desiree ranked second in resistance with 12.3% damage, which aligns with reports from Nepal where this cultivar showed moderate tolerance to red ant damage in mid-hill trials [9] . The two local Bhutanese landraces, Khangma Kaap and Yusikaap, showed contrasting responses: Khangma Kaap fell in the moderate category (18.7%) while Yusikaap was clearly susceptible (31.4%). The thin periderm of Yusikaap (1.23 mm) may reflect selection for smooth skin texture preferred by local consumers rather than for pest resistance, a trade-of that organic growers must consider when choosing planting material [10] . The recovery rate, which measured the ability of damaged tubers to form wound periderm and heal feeding scars, followed the same cultivar ranking as initial damage resistance. Lady Roseta recovered 91.2% of superficially damaged tubers to marketable condition, compared with only 62.8% for Yusikaap. This trait has not been previously linked to D. orientalis resistance but has practical value: even resistant cultivars sustain some damage, and the ability to heal minor wounds determines how many of those tubers can ultimately be sold [11] . The uniform organic management regime applied across all cultivars ensures that the observed diferences reflect true varietal susceptibility rather than variable pest pressure or treatment inconsistency. By equalising the management input, this design isolates the cultivar efect more clearly than mixedmanagement trials where treatment and cultivar efects can be confounded [12] . The resulting cultivar ranking directly answers the practical question faced by organic growers in Bhutan: which varieties give the best combination of ant resistance and yield when only non-chemical tools are available?

Conclusion

The main contribution of this research is the first field-based ranking of commercially available potato cultivars for their susceptibility to D. orientalis under organic pest management conditions in Bhutan. By testing six cultivars under identical biological and botanical protection, the research isolated the cultivar efect and identified Lady Roseta and Desiree as resistant, Khangma Kaap and Janakdev as moderately susceptible, and Kufri Jyoti and Yusikaap as susceptible to ant feeding damage. The significance of these findings lies in their direct applicability to Bhutan's national organic agriculture strategy. With cultivar selection as the foundation of the management framework (Figure 3), organic growers can reduce residual D. orientalis damage by 60-70% simply by switching from susceptible to resistant varieties, without any additional cost. This approach complements the biological and botanical treatments already available, creating a layered defence system in which each component reduces pest pressure progressively. For Bhutan's organic certification programme, which prohibits synthetic pesticides entirely, the identification of naturally resistant cultivars removes a major barrier to profitable organic potato production in ant-prone highland zones.

Several open questions warrant further investigation. The genetic basis of periderm thickness and suberin deposition in the resistant cultivars should be explored through quantitative trait locus mapping to accelerate breeding for ant resistance.

Whether the observed cultivar rankings hold across diferent agro-ecological zones within Bhutan, particularly the warmer eastern districts where D. orientalis pressure may difer, needs verification through multilocation trials. The interaction between cultivar resistance and diferent organic management intensities, ranging from minimal (cultural practices only) to comprehensive (biological + botanical + cultural), could reveal whether resistant cultivars allow reduced input levels without sacrificing yield protection. And consumer acceptability testing should accompany varietal recommendations, since cultivars like Yusikaap remain popular for their cooking qualities despite their pest vulnerability, and growers will adopt resistant varieties only if market demand follows.

Acknowledgement

Acknowledgements Institutional SupportThe National Institute of Agriculture, Thimphu, provided field plots, laboratory facilities, and climate data. The National Plant Protection Centre, Semtokha, supplied entomopathogenic fungal cultures and assisted with formulation quality testing.

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