Performance of microbial biopesticides in managing red ant injury and enhancing potato tuber quality under varying climatic conditions

Introduction

In contrast to the predictable dose-response relationships observed in laboratory bioassays, microbial biopesticide performance under field conditions exhibits considerable variability attributable to environmental factors, creating a persistent gap between demonstrated potential and realized efficacy. Potato (Solanum tuberosum L.) cultivation in Thailand occupies approximately 35,000 hectares, concentrated in the northern highland provinces where cooler temperatures permit viable production. The crop serves both domestic fresh market and processing demands, with increasing importance as dietary diversification reduces rice dependence. Red ant (Solenopsis spp.) infestations constitute a significant pest challenge in Thai potato production, with warm baseline soil temperatures accelerating colony growth rates and foraging intensity relative to temperate production environments. Temperature represents arguably the most influential abiotic factor governing the efficacy of microbial pest control agents, affecting pathogen viability, germination rate, host immune response, and infection progression kinetics. For entomopathogenic fungi, optimal germination and growth typically occur within 25-30 °C, while thermal stress above 35 °C can denature enzymatic machinery and reduce conidial viability. Bacterial biopesticides including B. thuringiensis maintain crystal protein stability across a broader temperature range but require host ingestion, which is behaviorally modulated by ambient thermal conditions. Viral biopesticides including nucleopolyhedroviruses exhibit persistence characteristics influenced by UV exposure and thermal degradation of polyhedrin matrices.

Despite the recognized importance of temperature in modulating biopesticide efficacy, systematic field investigations comparing performance across defined temperature regimes within the same experimental framework are scarce, particularly for soil-applied formulations targeting subterranean pest complexes. The present research addressed this knowledge gap by evaluating three microbial biopesticide formulations across three distinct temperature regimes within a single experimental design at an altitude-variable research station in northern Thailand.

Material and Methods

Material

The research conducted at the Chiang Mai College of Applied Agriculture highland research station, Chiang Mai Province, Thailand (18.7883° N, 98.9853° E) during two sequential growing periods in 2023-2024. Three experimental zones at different altitudes within the research station provided the temperature regimes: cool zone (altitude 1,200 m, soil temperature 15-20 °C), moderate zone (altitude 800 m, soil temperature 25-30 °C), and warm zone (altitude 400 m, soil temperature 30-35 °C). Soil at all zones was classified as clay loam with pH ranging from 5.4 to 5.8 and organic matter from 2.0 to 2.6%. Certified seed tubers of potato variety Atlantic were used. Four biopesticide treatments were evaluated: NPV formulation (Spodovir at 2×10⁹ polyhedral inclusion bodies per ml), combined Bt (DiPel at 1×10⁶ CFU/ml) + NPV (1×10⁹ PIB/ml), commercial B. bassiana (Mycotrol O at 1×10⁸ conidia/ml), and untreated control. The research protocol received approval from the university ethics committee (Protocol: CCAA-EC/2023-042).

Methods

A split-plot design with three temperature regimes as main plots and four biopesticide treatments as subplots was employed with four replications, yielding 48 experimental plots (3 zones × 4 treatments × 4 reps). Individual plot dimensions were 4 m × 3 m with 65 cm row and 25 cm plant spacing. Planting depth was 10-12 cm. Fertilization followed regional guidelines at 120:80:160 kg/ha NPK. Biopesticide applications were made as soil drenches at 450 ml per plant at 12-day intervals from tuber initiation through bulking, totaling six applications per growing period. Soil temperature was continuously monitored using data loggers at 15 cm depth. Red ant populations were assessed through pitfall trapping (three per plot) and quadrat sampling (1 m²) at biweekly intervals. Growth parameters were measured biweekly. Post-harvest tuber quality was evaluated using a composite 10-point scale incorporating firmness (penetrometer reading), dry matter content, specific gravity, visual surface quality, and absence of ant-related scarring. Storage evaluation assessed quality retention at 30 and 60 days under ambient conditions. Data were analyzed using PROC MIXED in SAS with temperature, treatment, and their interaction as fixed effects, and block nested within zone as random.

Influence of temperature and humidity on biopesticide efficacy

The mechanistic basis for temperature-dependent biopesticide efficacy involves multiple interacting physiological processes in both the microbial agent and the target pest organism. For B. bassiana, conidial germination rate increases from 15% at 15 °C to 92% at 25 °C and declines to 68% at 35 °C, with thermal inactivation of germination enzymes above 37 °C. The Bt crystal protein dissolution rate in alkaline gut environments is temperatureindependent, but host feeding activity, which governs toxin ingestion, increases with temperature up to behavioral thermal limits. NPV polyhedrin matrix stability decreases at temperatures above 32 °C, reducing the effective infectious dose under warm conditions. Relative humidity at the soil surface interacts with temperature to modulate conidial germination for fungal agents, with optimal germination requiring relative humidity above 90% -a condition more readily achieved in soil microhabitats than on exposed surfaces, partially explaining why soil-applied fungal biopesticides outperform foliar applications against subterranean pests.

Results

Acknowledgement

Acknowledgements S.T.R. conceptualized and designed the experimental framework, N.K.S. managed pest monitoring and biopesticide applications across the three altitude zones, P.V.W. conducted growth and yield assessments, and K.S.T. performed the post-harvest quality evaluations. The Chiang Mai College of Applied Agriculture is acknowledged for providing access to the multi-altitude research station and laboratory facilities. Financial support was provided by the Thailand Research Fund through the Research Career Development.

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