Sunday, February 16, 2014

Are Pesticides the Primary Contributor to the Decline of Honeybees?

The danger of losing one of the world’s most efficient pollinators is imminent- honeybees, (Apis mellifera), are on the decline in many parts of the world.
During the winter of 2006-2007, unusually high losses of 30-90% of bee hives were reported by bee farmers.  Farmers noted that the worker bee population had vanished, leaving the queen and young behind.  Oddly, the missing bees are never found dead around the hive- they disappear.  Without the worker bees in a colony, the hive eventually dies.  This sequence of events is known as colony collapse disorder (CCD).
CCD is now described as “a serious problem threatening the health of bees and the economic stability of commercial beekeeping and pollination operations”.  Bees are integral to sustaining the world’s food supply as one-third of the food we eat is directly or indirectly affected by pollination.  It is estimated that bee pollination is responsible for more than $15 billion in increased crop value each year. 
With bees on the decline, several studies over the past eight years have been trying to determine the cause of CCD.  The New York Times reports on a USDA publication which describes CCD as having several factors to the decline of honeybees.  A complex set of stressors and pathogens are thought to contribute to CCD, which are described in the following figure.  Pesticides, especially the insecticide class of neonicotinoids, are thought to be a primary driver for CCD.

Neonicotinoids are systemic insecticides, as they are taken up by the plant’s vascular system and distributed throughout the plant.  Generally, neonicotinoids are applied to the seeds of crops, but crops can also receive spray applications.  Pests ingest the toxin as they consume parts of the plant, and die as the central nervous system is affected, causing paralysis and death.  Although bees are not the target pest, the pollen and nectar of the plants contain the insecticide, which is then transported to the rest of the hive, which may induce CCD.  Other effects may include disruptions in bee mobility, navigation, feeding behavior, foraging activity, memory and learning, and overall hive activity. 
Discussions of placing a ban on neonicotinoids are not uncommon.  A recent report stated that the European Union (EU) has discussed placing a ban on these insecticides in hopes of boosting bee populations.  Although a majority of the EU (15 countries) voted for banning the insecticides, 8 countries voted against the ban and 4 countries abstained, because of the lacking scientific data.
John Atkins, Chief Operating Officer of Syngenta, the world’s foremost producer of neonicotinoids agrees that more scientific data is needed before a ban on neonicotinoids is instated. He states, “…a ban on neonicotinoids would simply close the door to understanding the problem’ [of bee populations declining]’’.  Syngenta also states on its page concerning decreasing bee populations, “Neonicotinoids are some of the most effective forms of crop protection technology available and have been used safely across millions of hectares of European crops. Many years of independent monitoring prove that when used properly – as they consistently are – neonicotinoids do not damage the health of bee populations”.  A YouTube video by Syngenta outlining the benefits of neonicotinoid use and potential problems with a ban is located here.
On the contrary, several studies indicate that the effect of neonicotinoids on bees is devastating in more ways than one.  In addition to possible CCD effects, a French study noted that exposure makes it harder for bees to return to their hive as the neonicotinoids cause confusion.  A British study found that bees do not collect enough food to sustain their hive, therefore compromising the health of the colony.  Another study linked neonicotinoid exposure to depressed immune system function.  With a depressed immune system, honeybees are more susceptible to viral and bacterial attack.
Not only do neonicotinoid exposure affect bee health, but a myriad of other pesticides do as well, according to this EcoNews article.  The study’s findings supplement the USDA’s report on several factors affecting bee populations.  In addition to the USDA’s findings, the researchers found 35 different pesticides in pollen with samples averaging 9 different pesticides ranging from neonicotinoids, pyrethroids, carbamates, organophosphates, and more.  The study suggests that neonicotinoids are not just to blame- there are other chemicals present that may cause just as much harm to bee populations. 
Over the years, studies have confirmed that there are several factors that contribute to CCD.  Pesticides like neonicotinoids are currently to blame for the rapid decrease in bee populations among the other causes of CCD.  Is the banning of neonicotinoids or other pesticides the best option to save the honeybee?  Or will CCD eliminate our crucial pollinators by increasing the incidence of biological elements beyond human control?


  1. One of the BBC articles that you linked to ("voted") shows a map of EU countries and their honeybee availability versus need. These data seem to indicate that crop yields are lower than they could be...maybe that's why the EU Commission went ahead with the proposed 2-year ban? ( their motive, it will be very interesting to see if there is a detectable effect. How quickly do neonicotinoids get processed out of an ecosystem? Will two years be enough for this and the potential beginning of honeybee population recovery?

  2. Dr. Saunders, I found a wonderful paper giving half-lives of neonics in soil and some data on water, and the likely exposure to honeybees as well as some other non-target pests. Here is the link to the paper:

    "Given the oral LC50 value for imidacloprid in honeybees of 5 ng/bee (Suchail, Guez & Belzunces 2000), and taking the mean values for seed-treated crops calculated here, a bee would need to consume nearly 1 g of pollen or 2 6 ml of nectar to obtain an LC50 dose. This seems unlikely
    in the short term for a honeybee, but could easily be accumulated over a number of days or weeks, so the actual effect of field exposure on mortality is likely to depend on the rate at which neonicotinoids are metabolized or excreted."