Honeybee Disappearance: Pesticides and Colony Collapse Disorder
The Death of Honeybees
Honeybees around the world have been dying in frightening numbers since 2006. This observation is very significant for agriculture, since honeybees not only produce honey but also pollinate flowers, allowing for fruit development. It’s been estimated that one third of agricultural crops in the United States are pollinated by bees.
There has been a great deal of speculation about the reason for the honeybee decline. Suggested causes have included infections, the presence of pests, environmental changes, and the use of pesticides. Some researchers feel that a combination of factors is causing the honeybee deaths. The evidence that pesticides are at least partly responsible for killing bees is growing.
A honeybee colony contains a fertile bee called the queen. She lays eggs and is fed by the workers. Worker bees are sterile females that collect pollen and nectar and care for the colony. Male bees are called drones. Their sole function is to mate with a queen. They die soon after this job is finished.
Colony Collapse Disorder
The unexpected and unexplained death of a honeybee colony is known as colony collapse disorder. When a colony is experiencing this disorder, a strange observation is that the worker bees abandon the colony and disappear instead of dying in the hive. The living queen bee is found in the hive, as well as some young bees, but there are no worker bees present, either dead or alive. The workers have left the colony in their search for nectar and pollen and haven't returned. This is very different from the usual results when a bee colony is destroyed. Virus infections and pest invasions result in dead bees being found in and around the hive and bees of all types are killed.
Neonicotinoids and Imadacloprid
Researchers at the Harvard School of Public Health think that the most likely cause of honeybee deaths in colony collapse disorder is the use of a pesticide called imidacloprid. This belongs to a group of chemicals feferred to as neonicotinoids. The chemicals have a structure that is based on the nicotine molecule.
Bees are exposed to imidacloprid or another pesticide in the neonicotinoid family when they collect nectar from flowers or when they eat high fructose corn syrup. This syrup is often fed to bees by beekeepers. Corn in the United States is generally treated with a neonicotinoid pesticide, which contaminates the syrup made from the corn.
How May Imidacloprid Harm Insects?
Imidacloprid affects the central nervous systems of insects. It blocks the transmission of nerve impulses in nicotinergic neuronal pathways, which are very common in insects but much less common in humans and other mammals.
The word "neuron" means nerve cell. There is a small gap between one neuron and the next. When a nerve impulse reaches the end of a neuron, it's transmitted via a chemical called an excitatory neurotransmitter to the next neuron. The neurotransmitter is released from the end of the first neuron, travels through the gap between the two neurons, and binds to a receptor on the second neuron. When the binding takes place, a new nerve impulse is generated in the second neuron.
Acetylcholine is a common neurotransmitter and binds to both nicotinergic and muscarinic receptors. Imidacloprid also binds to nicotinergic receptors, thereby blocking the action of acetylcholine, but it can't bind to muscarinic receptors. Since insects have a lot of nicotinergic receptors, imidacloprid interferes with the action of acetylchoine in their bodies. if the dose is high enough, the insects may be paralyzed by the pesticide and eventually die. Mammals have more muscarinic receptors than nicotinergic receptors. Imidacloprid is therefore less toxic to mammals, including humans, than to insects.
Uses of Imidacloprid
Imidacloprid is used to protect crops and garden plants from insect pests, to control insects in homes, and to control fleas on animals when applied to the back of the animal's neck. It's usually given a trade name when it's sold, so a buyer would need to check the ingredient list to see if imidacloprid is present in a product.
When imidacloprid is applied to soil, it's absorbed by the plant roots and travels throughout the plant, reaching the nectar and the pollen. Imidacloprid is said to be a systemic pesticide because it spreads through the plant's body. Adding pesticides to a plant so that they can kill insects throughout the growing season instead of spraying the pesticides on the insects directly is a relatively new technique. The dose of pesticide received by foraging bees is not enough to kill them immediately (a lethal dose) but is instead classified as a sublethal dose.
Genetically modified crops have sometimes been suggested as a cause of bee death. The reason why these crops may kill bees is believed to be the fact that the seeds of the plants are soaked in insecticide, which end up in the adult plant, rather than the fact that the crops are genetically modified.
Effects of Neonicotinoids on Honeybee Colonies
Imidacloprid and other popular neonicotinoids such as clothianidin kill insects. Since bees are insects, the pesticides have long been suspected to be an agent in their disappearance.
In 2012, a Harvard School of Public Health study tested hives with different concentrations of imidacloprid in high fructose corn syrup, including a concentration that the researchers claim was lower than that normally encountered by bees. The researchers found that even low levels of pesticide hurt the bee populations. Death wasn't immediate, but several months after the first pesticide exposure the hives were found to be empty, apart from some young bees. The researchers didn't find any evidence of a viral infection in the hives. They also pointed out that empty hives are a characteristic feature of colony collapse disorder.
In 2014, the Harvard School of Public Health completed another study involving the effects of neonicotinoid pesticides on bees and found similar results to their first experiment. This time they also found that the colony collapse disorder was not correlated with the presence of parasites in the colony. Colonies exposed to pesticides and those that weren't contained about the same level of parasites. Only the colonies exposed to the pesticide underwent collapse.
The chief manufacturer of imidacloprid, Bayer CropScience, strongly denies that the pesticide is dangerous. The company claims that the doses used in the 2012 Harvard experiment were unrealistically high and that the experiment was flawed. However, some researchers say that they are using doses that would be found in the environment in their experiments and that their results show that neonicotinoid exposure is detrimental to bees.
Other Possible Effects of Neonicotinoids on Bees
Even sublethal doses of pesticides may be harmful for bees. Researchers in France and the United Kingdom have also found evidence that a neonicotinoid pesticide affects bees. The French scientists found that the pesticide-treated bees found it more difficult to navigate back to the hive after a foraging expedition, while the British scientists found that the pesticide made bumblebee colonies less successful in producing queen bees.
Neonicotinoid pesticides may weaken the bees' immune system. Scientists working for the United States Department of Agriculture (USDA)—and other scientists—report that bees exposed to sublethal doses of imidacloprid have an increased level of a gut parasite called Nosema in their bodies. The 2014 Harvard experiment didn't find any evidence that supported this idea, however. Nosema is one of the parasites suspected of causing colony collapse disorder.
In 2017, the results of four large-scale research projects were published. A Canadian experiment and three European ones studied the effect of a neonicotinoid on honeybees. Two of the experiments clearly support the idea that the neonicotinoid is harming bees. A third provides weaker support. The fourth provides no support.
A Canadian Experiment
A research team from York University in Toronto studied honeybee colonies close to cornfields as well as colonies so far away from the fields that they would never have visited them. According to one scientist from the university, almost all corn is treated with neonicotinoids. The team collected samples of pollen and nectar from the hives every few weeks.
The researchers found neonicotinoids in the hive samples collected near the cornfields. The most abundant kind was clothianidin. Interestingly, the scientists found that the contaminated product collected by the bees came mostly from flowers around the cornfields rather than from the field itself.
The team then fed some test bees pollen containing the same concentration of clothianidin discovered in the first part of the experiment. Other bees were given uncontaminated pollen. The researchers also attached tracking devices to the bees. The bees given the contaminated pollen had a 23% shorter lifespan and took up to 45 minutes longer to return to the hive after a foraging expedition. The researchers speculate that the bees were having trouble remembering where their hive was. The contaminated bees also took longer to remove sick bees from the hive.
The European Experiments
A team of European researchers placed some bees near a field of rapeseed plants that had been treated with clothianidin. (Rapeseed or oilseed rape plants are grown for their oil-rich seeds.) The researchers placed other bees far away from the plants. The experiment was performed in three countries. After the winter, around 24% of the test bees in Hungary had died. The test bee population in Britain decreased as well, though to a lesser extent. Unexpectedly, the population of test bees in Germany wasn't harmed and even increased.
The scientists discovered that food from rapeseed fields formed 15% of the diet of the German bees. It formed 40-50% of the diet of the Hungarian and British bees. The lower percentage may have enabled the German bees to survive. The bees may also have been healthier at the start of the experiment or may have had genetic resistance to the pesticide. It's also possible that other components of their diet gave them resistance.
In October 2017, Swiss scientists reported their analysis of honey sold to humans in different countries. Overall, 75% of the 198 samples contained a measurable amount of neonicotinoids. The percentage of contaminated North American honey was 86% (the highest result).
2018 Research and a New Regulation
In 2018, scientists published their analysis of research related to the effects of pesticides (neonicotinoids and other types) on honeybee and bumblebee memory. Researchers from the Royal Holland University of London looked at 23 studies involving a total of 100 experiments. The studies showed that whether the bees were exposed to a high dose of pesticide at one time or a small dose repeated over a long time, their memory was impaired.
On May 30th, 2018, the European Union banned the use of imidacloprid, clothianidin, and thiamethoxam outdoors. The pesticides may only be used inside permanent greenhouses. The Union considers another neonicotinoid—acetamiprid—to be of low risk to bees. This pesticide may still be used outdoors.
Why Are Bees Disappearing?
The final verdict regarding the cause of colony collapse disorder or a general decrease in bee population hasn't been reached. According to the USDA, the cause of the bee disappearance is probably due to a combination of factors. Some other researchers agree with this assessment. Many scientists believe that pesticides are likely at least one of the factors affecting the bees. The pesticides may be affecting the behaviour of the bees and/or some other aspect or aspects of their biology.
Whatever the cause—or causes—of the disappearing honeybees, an explanation and a solution need to be found very soon to protect the bees, our crops, and our food supply.
Neonicotinoids and colony collapse from the Harvard School of Public Health
Bees and pesticide information from the EPA (Environmental Protection Agency)
Information about honeybee health and colony collapse disorder from the USDA
Imidacloprid facts from the National Pesticide Information Center
Canadian and European research from Scientific American
Pesticides can sometimes kill bees from Science (an American Association for the Advancement of Science Publication)
Honey contaminated by pesticides from The Guardian
Pesticides and bee memory from Popular Science
Causes of colony collapse disorder from The Conversation
Neonicotinoids from the European Commission
Questions & Answers
© 2012 Linda Crampton