Colony Collapse Disorder: Are Pesticides Killing Honey Bees?

Last spring semester I took a class on Enviromental Biology. What follows is my research paper for that class. We were allowed to write about any subject we were interested in, so long as it was somehow related to the environment or biology. I worked so hard on it, and only the teacher reads it! So I thought I’d share it here!


Its common knowledge that honey comes from bees. But bees are much more important to our lifestyle than just the producers of a sweetener for our coffee and teas. Without bees, there would be almost no fruit or flowers, and food not just for us but for other animals in the environment would just disappear. Some estimates say that bees are responsible for 80% of pollination for flowering plants (Moisset and Buchman, 2011). A third of the food we eat is pollinated by bees (bee’s knees). Some plants have multiple ways of spreading seeds and pollen, but many are entirely dependent on insect pollination. Without bees, there would be no more apples, almonds, avocados, blueberries and cranberries (bees knees). tomatoes, squash, and cucumbers, despite being considered vegetables, are also technically fruit (Jacobson, 2008) and therefore need bees and other insects for pollination. There are other pollinators, like the hoverfly, bumblebee, and many kinds of wasps, but not only do these insects not carry nearly as much of the pollination load as the honey bee, but they have begun to experience difficulty finding food and habitat, especially as suburbia’s manicured lawns become more common, replacing both native plants and gardens (Urquhart, 2014)

What is Colony Collapse Disorder?

Within the last 10 years, bees having been dying for reasons which are still not well understood. In 2008, 1/3 of all the bees in the United States died. Colony Collapse Disorder is defined by a lack of bodies near the hive – the bees just fly away and disappear, never to be seen again. Hives full of formerly healthy bees are left empty and abandoned. According to the Apiary Inspectors of America, tens of BILLIONS of bees have vanished or died.

Enemies of the Honey Bee

In 1976, the parasite Varroa destructor, which had previously only infected the Asian honey bee, jumped to Apis mellifera, the western honey bee (Jacobson, 2008). While the Asian bees have developed a balance with the parasite, our bees had no defense. European hives were decimated.  Despite an embargo on bringing foreign bees into the United States, they somehow made it here a decade later. Rowan Jacobson, the author of Fruitless Fall: the Collapse of the Honey Bee and the Coming Agricultural Crisis, points out that all of modern beekeeping must be seen in the light of how Varroa has changed its practice (Jacobson, 2008).  This parasite has killed millions of colonies. Between 1987 and 1997 one quarter of professional beekeepers in America went out of business, entirely due to Varroa. In 1995, in Pennsylvania alone, the number of bee colonies dropped from 85 thousand to 27 thousand. Even in 2006 and 2007, in the midst of Colony Collapse Disorder disaster, as many bees died from Varroa. (Jacobson, 2008) That’s how deadly this infestation can be. Many beekeepers spray their hives to try to kill mites before an infestation kills the whole colony, and most even spray healthy hives every two or three years as a preventive measure. But it has been here for nearly 30 years. Varroa is not the cause of Colony Collapse Disorder. Or it is not the only one. Before it kills bees, it first weakens them and makes them susceptible to any number of pathogens, such as Nosema, which is a fungal infection that attacks the guts of the bees.

The Effect of Plant Pesticides

Systemic neonicotinoids, particularly thiamethoxam and imidacloprid, are suspected of playing a large role in causing Colony Collapse Disorder. Neonicotinoids mimic nicotine, a toxin that tobacco plants evolved to protect themselves from insect predators. These pesticides are called systemic because they are present in every part of a plant. Instead of  just sitting atop the leaves, these toxins are designed to penetrate into the plants circulatory system and are then carried into every flower and bud and pollen, every stem and leaf and root. This is supposed to protect the plant from harmful pests, but it will also come into contact with beneficial insects and pollinators. When honey bees drink the nectar and eat the pollen from these treated crops, they ingest the chemicals as well.

Scientists have found that these pesticides have a “sub-lethal” effect on the bees, meaning that while it doesn’t kill them outright, it does damage them. Some laboratory trials have shown that exposure to imidacloprid in the bee’s diet can reduce that single bee’s foraging success by 6 to 20% (Cresswell and Thompson). Large scale studies with entire colonies have not been done for this particular experiment, but scientists have been able to use computer models to predict its effects. Ingesting both imidaclorid and thiamethoxam is shown in the same study to kill adult foraging honey bees through “navigation failure” – basically, the bees forget how to get back to the hive and die from exposure. However their results do not support that this is enough to cause healthy colonies to collapse in the spring. The scientists who ran these models are quick to point out that the colonies may still be very vulnerable later in the year, if they cannot replace lost worker bees fast enough. In addition, studies also show that neonicotinoids have over 1,000 times their original toxicity when combined with certain fungicides.  (Kimble-Evans, 2009)

Other studies have shown that that pesticides make it difficult for bees to navigate their way back to the hive, and to learn and remember the links between certain floral scents and the nectar and pollen they collect. In order words, their development is so retarded that they don’t know how to feed themselves or find their way home. (Cresswell and Thompson). The evidence is enough that the European Union has issued a temporary ban on these particular pesticides while more study is done (The Guardian, 2013). In America, the Environmental Protection Agency has refused to do so, against the recommendations of its own scientists (Philpott, 2010), an egregious example of corporatism overriding the common good.

But neonicotinoids are not the only class of pesticides, and there are studies saying that cholinergic pesticides can endanger honey bees as well. Cholinergic refers to the type of nerve receptors that these pesticides attack in insects, acetylcholinesterase receptors. An extensive study shows that when used together, both neonicotinoids and organophosphate coumaphous (a common miticide used to fight the aforementioned Varroa infestation) actually block neuronal firing in the honeybee brain (Palmer, et al, 2013). These effects have been shown to be cumulative, so that the more exposure to this type of pesticide, the greater the cognitive impairments (Palmer, et al, 2013). The part of the honey bee’s brain that is most affected is called the mushroom body, and it contains over 40% of the neurons. This is the higher-order part of the brain that integrates the many senses of the honey bee, allowing it to make sense of the world, as well as being related to learning and memory. Essentially it is the part of the brain that allows the bees to work together, and this is the part of the brain that is most damaged (Palmer, et al, 2013).

Scientists Williamson and Wright have used classical conditioning to get honey bees to extend their proboscis when exposed to a scent associated with a reward, essentially what the bees do in nature when learning to forage, and then tested their responses after exposure to the same concentrations found in most agricultural fields. They found that imidacloprid lowered the chance of bees forming a long-term memory, whereas the bees exposed to coumaphos were only less likely to respond during the short-term memory test after conditioning. The combination impaired the bees’ ability to tell the difference between smells that they should already know and new odors.  All the exposed bees were less likely to extend their proboscis to feed, regardless of the type of pesticide they were exposed to. (Williamson and Wright, 2013)

So What Does This All Mean?

When I first started the research for this paper, I was going to argue that pesticides are the single cause of Colony Collapse Disorder. I still believe that pesticides, particularly neonicotinoids such as imidacloprid, bear a significant amount of the blame. But the real problem is that Colony Collapse Disorder appears to have no single cause.  Rather, our entire agricultural system has come together to create a perfect storm. Quite simply, it is human overpopulation, and therefore our overwork of the bees, that is killing them.

Beehives used to be everywhere, on every farm, in every orchard and even in many families’ small, kitchen gardens. But now corporate orchards and farms no longer keep their own bees, but rent them for a short time. It is common practice for beekeepers to have their hives trucked from state to state from March to October to pollinate different monocrops at different times. This is placing undue stress on the bees, so that they are weakened and their immune systems are suppressed, making them even more susceptible to the negative effects of pesticides. In Fruitless Fall, Jacobson described testing the corpses of bees from many failed hives, trying to pinpoint a single culprit. Instead of finding that one virus, fungal infection, or parasite was to blame, these bees seemed to have been infected with ALL of them! Their immune systems were so suppressed that they could be said to have an insect version of AIDS (Jacobson, 2008). The implication of these discoveries are staggering, and its easy to see why the media has suddenly stopped talking about Colony Collapse Disorder. Without a single culprit, no doubt the problem seems insurmountable and simply too big to handle. But handle it we must, for we simply cannot continue in our current system. Our bees cannot take it. We must change the method of production back to small, de-centralized producers of food. If we do not, our options for food will be drastically reduced when the bees inevitably die out from overwork, which would likely lead to massive starvation for humans soon after. If you cannot change things politically, start small by changing them at home. Buy from local farmer’s markets and use local honey. The bees will thank you.


Bees and the European neonicotinoids pesticide ban: Q&A (2013) The Guardian. Retrieved 4/19/2015.
Cresswell, James E. and Thompson, Helen M. (2012) Comment on “A Common Pesticide Decreases Foraging Success and Survival in Honey Bees”. Science. 337
Kimble-Evans, A. (2009). Are Potent Pesticides Killing Honeybees? Mother Earth News. 236. 16.
Jacobson, Rowan. (2008) Fruitless Fall: the Collapse of the Honey Bee and the Coming Agricultural Crisis. Bloomsbury Press.
Moissett, Beatriz and Buchman, Stephen. (2011) Bee Basics: An Introduction to our Native Bees. A USDA-Forest Service Pollinator Partnership Publication. Retrieved 4/19/2015.
Palmer, Mary J., and Moffat, Chistopher and Saranzewa, Nastja and Harvey, Jenni and Wright, Geraldine A. and Connolly, Christopher N (2013). Cholinergic pesticides cause mushroom body neuronal inactivation in honeybees. (2013). Nature Communications. Issue 4.
Philpott, Tom. Leaked document shows EPA allowed bee-toxic pesticide despite own scientists’ red flags. (2010) Grist. Retrieved 3/10/2015.
The Bees’ Knees. (2009). New Internationalist. 425. 8-9.
Williamson, Sally M., Wright, Geraldine A. (2013) Exposure to multiple cholinergic pesticides impairs olfactory learning and memory in honeybees. Journal of Experimental Biology.
Urquhart, Kristina M. (2014) 22 Fruits, Flowers, and Herbs to Attract More Bees to Your Farm. Hobby Farms. Retrieved 3/18/2015.
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