Bees are crucial pollinators in natural ecosystems and agricultural systems, pollinating numerous crops and wild plants. Their activity is essential for the production of many fruits, vegetables, and nuts, supporting human nutrition and global food security. However, bee populations worldwide face significant threats, with pesticides contributing significantly to their decline.
How Pesticides Harm Bees
Pesticides can harm bees through various routes. Bees encounter these chemicals through direct spray, by consuming contaminated pollen or nectar from treated plants, or by contact with residues on foliage and in water. Systemic pesticides are absorbed by plants and distributed throughout their tissues, including pollen and nectar, making them consistently present in bee food sources.
Pesticides can cause acute toxicity, leading to rapid mortality of individual bees or entire colonies within hours. Chronic exposure to lower, sublethal doses can lead to subtle but damaging effects. These include impaired navigation, reduced foraging efficiency, learning and memory difficulties, compromised immune systems, and adverse reproductive effects. Such effects weaken individual bees and the overall health of a bee colony.
The Seven Most Dangerous Pesticides
Several pesticides are harmful to bees due to their toxicity or widespread use. These chemicals often interfere with bees’ nervous systems or disrupt other biological functions, even at low concentrations.
Imidacloprid (Neonicotinoid)
Imidacloprid, a systemic neonicotinoid, interferes with the central nervous system of insects. It is highly toxic to bees. Even at realistic levels, it can impair bees’ homing abilities, reduce immunocompetence, and increase susceptibility to infections like Nosema. Exposure also decreases survival rates for bees returning to the hive and reduces adult bee populations. Soil-applied imidacloprid can reduce nesting activity and offspring production of ground-nesting bees.
Clothianidin (Neonicotinoid)
Clothianidin, another neonicotinoid, is acutely toxic to bees via contact and oral exposure. It can reduce the lifespan of honey bee workers and impair motor function and walking behavior even at low doses. It can also negatively affect foraging behavior, reducing flower visits and increasing search time between flowers. This pesticide can also interfere with the insect immune system, potentially promoting viral pathogen replication in honey bees.
Thiamethoxam (Neonicotinoid)
Thiamethoxam is a systemic neonicotinoid highly toxic to bees through contact and ingestion. Exposure can prolong larval and pupal developmental periods, reduce bee fecundity, and lower larval and pupal weights. Sublethal exposure can impair honey bee midgut and brain function, reduce lifespan, and make bees more vulnerable to bacterial infections. It also affects bees’ motor responses and foraging behavior, even at doses much lower than the lethal dose.
Fipronil (Phenylpyrazole)
Fipronil, a broad-spectrum phenylpyrazole insecticide, primarily controls soil insects. It is highly toxic to bees, blocking GABA in the central nervous system, leading to hyperexcitation, agitation, seizures, tremors, and paralysis. Even at sublethal doses, fipronil can reduce brood viability, decrease developing larvae and pupae, and cause lethargy in adult bees, sometimes leading to colony abandonment. Fipronil can cause mass mortality in bee colonies due to its capacity to bioaccumulate and exhibit time-reinforced toxicity.
Chlorpyrifos (Organophosphate)
Chlorpyrifos, a neurotoxic organophosphate, can cause learning and memory deficits in honey bees, even at low doses. Exposed bees may experience impaired smell memory and reduced memory recall specificity, hindering foraging success and their ability to return to the hive. Although label precautions aim to reduce direct contact, dietary exposure through pollen and nectar remains a concern. It has been detected in many bee samples, highlighting its widespread environmental presence.
Sulfoxaflor (Sulfoximine)
Sulfoxaflor, a sulfoximine insecticide, disrupts the insect nervous system, similar to neonicotinoids. The EPA classifies sulfoxaflor as “very highly toxic” to bees. Even at low levels, it can impair reproduction and reduce bumblebee colony size. Exposure can lead to reduced worker production and reproductive output in bumblebee colonies. As a systemic chemical, it is absorbed by plants and found in pollen and nectar, posing a continuous threat to foraging bees.
Cypermethrin (Pyrethroid)
Cypermethrin, a synthetic pyrethroid, is highly toxic to bees, even in small doses, by compromising their nerve pathways. It causes neurological effects, leading to disorientation, loss of body control, and rapid death at higher doses. While it binds strongly to leaf waxes, potentially reducing contact toxicity once dry, foraging bees can still be exposed by ingesting dew on treated leaves. Its effects are typically rapid, often occurring within two days of exposure, and its toxicity can increase with bee age.
Broader Impact on Bee Colonies
Pesticide exposure for individual bees has wider consequences for entire colonies and populations. When bees bring contaminated pollen and nectar to the hive, chemicals accumulate in food stores and wax, leading to chronic exposure for the queen, brood, and nurse bees. This contamination can impair queen health, affecting egg-laying and the colony’s reproductive success. Developmental delays and reduced survival rates among larvae and pupae can also occur, impacting the next generation.
The cumulative effects of exposure to multiple pesticides, known as synergistic effects, can amplify harm to bees, with a combined impact greater than individual chemical effects. Such interactions can weaken the colony’s vitality, making it more susceptible to other stressors like parasites, pathogens, and poor nutrition. Ultimately, these combined impacts can lead to a gradual decline in colony strength and, in severe cases, result in colony collapse, where most adult bees disappear, leaving the queen, brood, and food stores.