Thiamethoxam-induced changes in honey bee colony dynamics: A study in mustard fields

Introduction

Honey bees (Apis mellifera) are among the most important pollinators in both natural and agricultural ecosystems, contributing significantly to global food security and biodiversity (Potts et al., 2010) [13] . They play a pivotal role in the pollination of a wide variety of crops, including fruits, vegetables, and nuts, which are essential for human nutrition and economic stability (Klein et al., 2007) [10] . However, in recent decades, honey bee populations have faced unprecedented declines, with colony losses reported across North America, Europe, and other regions (VanEngelsdorp et al., 2008) [16] . This phenomenon, often referred to as Colony Collapse Disorder (CCD), has been attributed to a combination of factors, including habitat loss, climate change, pathogens, parasites, and exposure to pesticides (Goulson et al., 2015) [3,8] . Among the various stressors contributing to bee declines, the widespread use of neonicotinoid insecticides has garnered significant attention due to their potential sublethal and lethal effects on pollinators (Godfray et al., 2014) [7] . Neonicotinoids, such as Thiamethoxam, are systemic insecticides that are absorbed by plants and distributed throughout their tissues, including pollen and nectar, which are primary food sources for bees (Bonmatin et al., 2015) [3] . While these chemicals are effective in controlling agricultural pests, their persistence in the environment and their high toxicity to non-target organisms, including bees, have raised concerns about their ecological impact (Pisa et al., 2017) [12] . Thiamethoxam, a second-generation neonicotinoid, is widely used in agriculture due to its broad-spectrum efficacy against insect pests. However, studies have shown that even sublethal doses of Thiamethoxam can impair honey bee behavior, physiology, and colony health (Henry et al., 2012) [9] . For instance, exposure to Thiamethoxam has been linked to reduced foraging efficiency, impaired navigation, and increased mortality in honey bees (Aliouane et al., 2009; Fischer et al., 2014) [1,5] [17,11] . Despite the growing body of evidence on the adverse effects of neonicotinoids, there is still a need for comprehensive studies that examine the longterm impacts of Thiamethoxam on honey bee colonies under realistic field conditions. Most previous research has focused on individual bees or short-term laboratory experiments, which may not fully capture the complex interactions between pesticide exposure, colony dynamics, and environmental factors (Blacquière et al., 2012) [2] . Furthermore, the mechanisms by which Thiamethoxam affects honey bee development, reproduction, and resource management remain poorly understood. This study aims to address these gaps by investigating the effects of Thiamethoxam on honey bee biology and colony health, with a focus on developmental stages, fecundity, brood production, and resource storage. By examining these parameters, we seek to provide a more holistic understanding of how Thiamethoxam exposure impacts honey bee colonies and to contribute to the ongoing debate on the risks posed by neonicotinoid insecticides to pollinators. The findings of this study have important implications for the development of sustainable pest management practices and the conservation of pollinator populations, which are vital for maintaining ecosystem health and agricultural productivity.

Materials and Methods

The study was conducted in a mustard field at the Apiary of Dr. Rajendra Prasad Central Agricultural University (RPCAU), Pusa, Bihar, India, a site dedicated to honeybee domestication and research. Two disease-free honeybee (Apis mellifera) colonies, each containing all stages of bees (eggs, larvae, pupae, and adults), were selected and placed in separate plots within the same mustard field to ensure uniform environmental conditions. One colony served as the control and received a 50% sucrose solution and water, while the other was treated with Thiamethoxam at 0.1 g/L, administered in a 50% sucrose solution. Both colonies were provided with fresh water and their respective food sources daily. Developmental parameters (egg, larval, and pupal periods; fecundity; larval and pupal weights), colony growth metrics (brood area, pollen and nectar storage, colony weight), and behavioral observations (foraging activity, worker bee mortality) were monitored weekly over two consecutive seasons.

Results

Table The egg period remained unaffected, showing no significant difference between the treated (2.80±0.42 days) and control (3.00±0.00 days) groups. However, significant differences were observed in other parameters. The larval period increased in the Thiamethoxam-treated group (5.70±0.67 days) compared to the control (4.80±0.63 days), while the pupal period was also extended from 8.30±0.48 days in the control to 9.30±0.94 days in the treated group. Fecundity was notably reduced in the treated bees (847±52.45 eggs) compared to the control (1199±38.71 eggs). Additionally, both larval and pupal weights were significantly lower in the treated group, with larval weight decreasing from 114.90±6.13 mg in the control to 88.60±7.93 mg in the treated group, and pupal weight dropping from 113.90±3.63 mg to 90.90±3.60 mg is detailed in Table 1. From Table 2 the egg period showed no significant difference between the control (2.90±0.31 days) and the Thiamethoxam-treated group (2.70±0.48 days). However, the larval period was significantly prolonged in the treated group (6.00±0.66 days) compared to the control (5.10±0.31 days). The pupal period was also extended from 7.80±0.42 days in the control to 10.10±0.73 days in the treated group. Fecundity was notably Similarly, in the second season, control colonies showed an increase in brood area from 1035.00 cm² before spraying to 1108.33 cm² by day 28. However, Thiamethoxam-treated colonies exhibited a marked decline from 1023.33 cm² to 775.00 cm² by day 7. Though a partial recovery was noted by day 28 (925.00 cm²), the brood area in treated colonies remained lower than in the control group. From Table 5 we can know that in the first season, control colonies exhibited a steady increase in pollen storage, from 58.66 cm² before spraying to 121.00 cm² by day 28. However, Thiamethoxam-treated colonies showed a significant reduction in pollen storage, declining from 70.66 cm² before spraying to 57.33 cm² by day 7, and further decreasing to 27.33 cm² by day 21. No significant recovery was observed by day 28, with pollen storage remaining at 27.33 cm². A similar trend was observed in the second season. Control colonies showed a consistent increase in pollen storage from 62.33 cm² before spraying to 126.33 cm² by day 28. In contrast, Thiamethoxam-treated colonies experienced a sharp decline from 67.00 cm² before spraying to 52.66 cm² by day 7, followed by a continuous decrease to 24.66 cm² by day 21. A slight increase was noted by day 28 (25.00 cm²), but pollen storage remained significantly lower than in the control colonies. In the first season, control colonies exhibited a steady increase in nectar storage, rising from 120 cm² before spraying to 200 cm² by day 28. In contrast, Thiamethoxamtreated colonies showed a sharp decline in nectar storage, decreasing from 130 cm² before spraying to 100 cm² by day 7, and further reducing to only 20 cm² by day 28.A similar trend was observed in the second season, where control colonies increased nectar storage from 125 cm² before spraying to 205 cm² by day 28. However, Thiamethoxamtreated colonies showed a drastic reduction from 135 cm² before spraying to 105 cm² by day 7, with storage continuously declining to 25 cm² by day 28 this is detailed in Table 6.

Table 7 shows in the first season, control colonies showed a continuous increase in weight, from 2540 g before spraying to 2840 g by day 28. However, Thiamethoxam-treated colonies exhibited a progressive decline, dropping from 2510 g before spraying to 2430 g by day 7, and further decreasing to 2280 g by day 28. A similar pattern was observed in the second season. Control colonies steadily increased in weight from 2560 g before spraying to 2860 g by day 28. In contrast, Thiamethoxam-treated colonies experienced a gradual decline from 2530 g before spraying to 2450 g by day 7, eventually reaching 2300 g by day 28.

Discussion

The findings of this study provide robust evidence that Thiamethoxam, a widely used neonicotinoid insecticide, has significant adverse effects on the biology, development, and overall health of honey bee (Apis mellifera) colonies. These results are consistent with a growing body of literature that highlights the detrimental impacts of neonicotinoids on pollinators, particularly honey bees, which play a critical role in global ecosystems and agriculture. The observed effects on larval and pupal development, fecundity, colony growth, and resource storage underscore the multifaceted ways in which Thiamethoxam disrupts honey bee biology and colony dynamics.

Developmental and Reproductive Impacts

One of the most striking findings of this study is the significant prolongation of larval and pupal periods in honey bees exposed to Thiamethoxam, coupled with reductions in larval weight, pupal weight, and fecundity. These results align with previous studies that have documented the developmental toxicity of neonicotinoids. For instance, Wu-Smart and Spivak (2016) [17] demonstrated that sublethal exposure to neonicotinoids delays larval development and reduces the body weight of emerging adult bees, which can compromise their ability to perform essential tasks within the colony. Similarly, Henry et al. (2012) [9] found that neonicotinoid exposure impairs the foraging behavior of honey bees, which indirectly affects larval development by reducing the availability of high-quality nutrition. The reduction in fecundity observed in this study is particularly concerning, as it suggests that Thiamethoxam may impair the reproductive capacity of queen bees. This finding is consistent with the work of Pettis et al. (2013) [11] , who reported that neonicotinoid exposure reduces the sperm viability of drone bees and the egg-laying capacity of queen bees, ultimately threatening colony sustainability. The lack of significant effects on the egg period, however, indicates that Thiamethoxam may not directly interfere with the earliest stages of development. Instead, its primary impacts appear to manifest during the larval and pupal stages, potentially through neurotoxic mechanisms or disruption of hormonal regulation, as suggested by Aliouane et al. (2009) [1] .

Brood Area and Colony Growth

The decline in brood area following Thiamethoxam exposure is a critical indicator of the insecticide's impact on colony growth and vitality. The initial reduction in brood area, followed by partial recovery, suggests that Thiamethoxam may impair the queen's ability to lay eggs or the workers' ability to care for the brood. This observation is supported by Schneider et al. (2012) [14] , who found that neonicotinoids reduce brood production and delay colony development, leading to smaller and less resilient colonies. The partial recovery observed in this study may reflect a compensatory mechanism within the colony, such as increased egg-laying by the queen or improved brood care by workers. However, the fact that brood area in treated colonies remained significantly lower than in controls indicates that these compensatory mechanisms are insufficient to fully mitigate the negative effects of Thiamethoxam.

Pollen and Nectar Storage

The significant reduction in pollen and nectar storage in Thiamethoxam-treated colonies highlights the insecticide's impact on foraging behavior and resource collection. This finding is consistent with Gill et al. (2012) [6] , who demonstrated that neonicotinoids impair the foraging efficiency of honey bees, leading to reduced pollen and nectar collection. The sharp decline in storage observed in this study suggests that Thiamethoxam may affect the bees' ability to locate and retrieve resources, or it may alter their motivation to forage. This could be due to the neurotoxic effects of neonicotinoids, which disrupt the bees' ability to navigate and communicate, as reported by Fischer et al.

(2014) [5] . The lack of recovery in pollen and nectar storage over time further underscores the long-term negative effects of Thiamethoxam on colony resource management, which can have cascading effects on colony survival, especially during periods of resource scarcity.

Colony Weight

The progressive decline in colony weight in Thiamethoxamtreated colonies is a clear indicator of the insecticide's detrimental impact on overall colony health. This finding aligns with Tsvetkov et al. (2017) [15] , who reported that chronic exposure to neonicotinoids leads to reduced colony weight and increased colony mortality. The decline in weight may be attributed to a combination of factors, including reduced brood production, impaired foraging efficiency, and increased worker mortality. These factors collectively contribute to a weakened colony structure, making it more susceptible to additional stressors such as pathogens, parasites, and environmental changes (Doublet et al., 2015) [4] .

Implications for Bee Health and Ecosystem Services

The results of this study have significant implications for bee health and the ecosystem services they provide. Honey bees are essential pollinators of both agricultural crops and wild plants, and their decline due to pesticide exposure poses a serious threat to biodiversity and food security (Potts et al., 2010) [13] . The findings of this study add to the growing body of evidence that neonicotinoids, including Thiamethoxam, are a major contributing factor to pollinator decline. This underscores the urgent need for stricter regulations on the use of neonicotinoids and the development of alternative pest management strategies that minimize harm to pollinators. Moreover, the sublethal effects of Thiamethoxam observed in this study, such as reduced fecundity and impaired foraging, may have long-term consequences for colony survival and resilience. These effects are particularly concerning given the increasing prevalence of multiple stressors, such as habitat loss, climate change, and disease, which can interact synergistically with pesticide exposure to further exacerbate bee declines (Goulson et al., 2015) [3,8] .

Future Implications

This study demonstrates the adverse effects of Thiamethoxam on honeybee colony performance, including delayed development, reduced fecundity, and impaired resource management. Future research should investigate the long-term and cumulative impacts of neonicotinoids under field conditions, considering interactions with other stressors like pathogens and climate change. Exploring the mechanisms of Thiamethoxam's toxicity, its effects on other bee species, and the potential of lower, sublethal doses is essential. Developing alternative pest management strategies, such as integrated pest management (IPM) and biopesticides, is critical to minimize harm to pollinators.

Policymakers should consider stricter regulations on neonicotinoid use, especially in pollinator-dependent crops, while promoting pollinator-friendly practices through education and awareness. This research underscores the need for sustainable agricultural practices to protect pollinators, ensuring ecosystem stability and global food security.

Conclusion

This study provides compelling evidence that Thiamethoxam adversely affects honey bee biology, colony growth, and resource management. The findings are consistent with previous research and highlight the urgent need for action to protect pollinators from the harmful effects of neonicotinoid insecticides. Future research should focus on understanding the mechanisms underlying these effects and developing strategies to mitigate the impacts of pesticides on pollinators. Policymakers and agricultural stakeholders must also prioritize the adoption of sustainable practices that balance the need for pest control with the preservation of pollinator health and ecosystem services.

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