Honeybees, Pesticides and Colony Collapse Disorder
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 (or honey bee) decline. Suggested causes have been 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. However, the evidence that pesticides are responsible for destroying bee colonies 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
A recent study from the Harvard School of Public Health claims that the most likely cause for the honeybee deaths is the use of a pesticide called imidacloprid, which belongs to a group of chemicals called neonicotinoids. These 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.
The Importance of Honeybees
How Does Imidacloprid Kill 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 a 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, paralyzing the insects and eventually killing them. 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.
The Lives of Honeybees
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.
Other Possible Effects of Neonicotinoids on 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.
Why are Bees Disappearing?
The chief manufacturer of imidacloprin, 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.
The final verdict regarding the cause of colony collapse disorder hasn't been reached, but according to the USDA the cause of the bee disappearance is probably due to a combination of factors. Pesticides may well be one of these factors and may perhaps be the major cause of the problem. 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 bees, an explanation and a solution need to be found very soon to protect the bees, our crops and our food supply.
Silence of the Bees
References and Additional Information
© 2012 Linda Crampton