Pollinators, including honey bees, wild bees, butterflies and many other insects, are some of the most important creatures on our planet. By pollinating plants, both wild and cultivated, they have an essential role in maintaining wider ecosystems and ensuring our food security. However, we have come to take them for granted, and don’t fully appreciate their function in ensuring our ongoing survival. Insects are declining at a truly alarming rate. Among other factors human activities such as industrial farming and corresponding insecticide and fungicide use over large areas of land to protect food crops against pests and disease are considered to be major contributors. Many different pesticides have also been detected in honeybee colonies. Scientists are attempting to uncover the specific factors involved in insect decline, before it’s too late. Recent research by Sarah Manzer and colleagues in the research groups of Prof. Ricarda Scheiner and Prof. Ingolf Steffan-Dewenter at the Julius Maximilians Universität Würzburg in Germany has shed new light on a potential culprit: a combination of insecticides and fungicides commonly used in agriculture. More
Pesticides, including insecticides that are intended to kill insect pests and fungicides that kill fungal diseases, are widely used to protect valuable agricultural crops. However, they can have unintended consequences for beneficial insects such as honeybees, who interact with treated crops in the course of their normal activities. Neonicotinoids, a class of insecticides, have been particularly controversial due to their harmful effects on pollinators. These chemicals mimic nicotine (the addictive substance in tobacco), binding to receptors in the nervous system of insects, leading to overstimulation, paralysis, and eventually death. Because neonicotinoids are systemic, they permeate the entire plant, including the nectar and pollen, making them difficult for bees to avoid. While some neonicotinoids have been banned in certain regions, others remain in use.
To complicate matters, farmers can treat their crops with a combination of pesticides or pollinators forage on a variety of crops that have been treated with different pesticides. These pesticides can include fungicides, which kill fungal plant pathogens. However, these pesticide cocktails can have effects on pollinators that are different from the effects of their individual pesticide components in isolation. Researching how such pesticide mixtures affect pollinators is important in accurately understanding the full picture of pesticide-mediated pollinator decline.
Consequently, Manzer and her colleagues focused on one commonly used neonicotinoid, acetamiprid, and examined how it interacts with fungicides, specifically boscalid and dimoxystrobin, in its effects on honeybees. Honeybee larvae were a key focus, since adult bees can carry the pesticides back to the hive and expose larvae, which may be particularly vulnerable to the effects of pesticides as they are still developing.
The researchers conducted an in-depth study on honeybee larvae, rearing them in controlled conditions and exposing them to the pesticide combination. Their findings are concerning. As expected, high doses of the acetamiprid insecticide alone significantly reduced the survival of larval and adult bees that received the insecticide during their development. Unexpectedly, a combination of the insecticide with the fungicides did not affect survival when administered at high concentrations. However, alarmingly, when acetamiprid was combined with the fungicides at lower concentrations, it led to a decline in adult survival, even though each pesticide administered alone at these concentrations had no effect on survival. This suggests a troubling synergistic effect. While each pesticide alone might not be fatal, together they become far more dangerous. It also highlights the unpredictable nature of such pesticide combinations, which counterintuitively did not affect survival at high concentrations.
This suggests that even low doses of these chemicals, when mixed, can become more toxic than previously thought. The findings challenge the assumption that fungicides, which are generally considered safe for insects, do not contribute to bee mortality when combined with other agricultural chemicals.
The study also revealed sublethal effects of fungicides. Bees exposed as larvae grew into lighter adults, which could impact their ability to forage effectively. A lighter bee may have a weaker flight capacity, reduced energy reserves, and increased vulnerability to environmental stressors. Weakened bees may struggle to gather nectar, navigate back to their hives, or survive exposure to extreme weather and diseases. Additionally, sublethal effects could make them more susceptible to environmental stressors such as habitat loss, climate change, and disease. If enough bees are weakened in this manner, the survival of the entire colony could be at risk.
Furthermore, pesticide exposure has been linked to behavioral changes in honeybees. Previous research suggests that neonicotinoids can impair learning in bees, which are critical for navigation and efficient foraging. If young bees exposed to pesticides during development suffer from cognitive impairment, they may become ineffective workers within the colony, further weakening overall hive health.
Additionally, exposure to fungicides may disrupt the gut microbiome of honeybees. The gut microbiome plays a crucial role in digestion, immunity, and detoxification. A weakened microbiome may leave bees more susceptible to infections from harmful pathogens such as Nosema, a parasite that affects bee health worldwide.
Manzer and her colleagues stress that we are only beginning to understand the complex interactions between different pesticides. While regulations often assess the safety of individual chemicals, real-world exposure involves mixtures of various agricultural chemicals. The study underscores the urgent need to investigate these combinations more thoroughly to better protect pollinators.
Additionally, it is important to study how different pollinator insects and bee species react to these different pesticides. Honeybees, while an important species, are not the only pollinators affected. Wild bees, such as solitary bees, may be even more vulnerable due to their different foraging behaviors and lack of large colonies to buffer population losses. Understanding the full scope of pesticide impacts requires expanding research beyond managed honeybee populations.
Long-term studies are also needed to examine the effects of repeated pesticide exposure over multiple generations. Chronic exposure to even low doses of pesticides could have cumulative effects on bee colonies, reducing reproductive success and increasing colony collapse risk over time.
As consumers, we can support efforts to protect bees by choosing organic produce when possible and advocating for stricter regulations on pesticide use. Farmers, policymakers, and researchers must work together to develop agricultural practices that balance productivity with environmental sustainability. Simple changes, such as planting pollinator-friendly crops, creating wildflower habitats, and reducing pesticide use during flowering periods, can significantly help bee populations.
Without urgent action, we risk further declines in pollinator populations, an outcome that could have devastating consequences for global food systems and biodiversity. Honeybees, together with wild bees and other pollinators, contribute billions of dollars to agriculture each year through pollination, and their decline threatens the stability of ecosystems that rely on plant reproduction.
Sarah Manzer and her colleagues’ research is a wake-up call: if we want to preserve the vital role of pollinators and other beneficial insects, we must rethink how we use pesticides and prioritize pollinator health in agricultural practices. The health of these incredible insects depends on the choices we make today.
