Scientific Studies Examining Reasons For Bee Decline

Author: Mosquito Squad of Greater Washington DC

To help people understand the science behind the problem of honey bee dies off, we’re continuing our bee spotlight series. This week, we’re taking a look at two notable studies that both aim to determine the cause of pollinator death.

Assessment of Chronic Sublethal Effects of Imidacloprid on Honey Bee Colony Health

In March of last year, a study on the effects of a specific pesticide on bee colony health was published in PLOS ONE journal. The study was conducted by three researchers based in Maryland from the University of Maryland, the U.S. EPA, and the USDA’s Agricultural Research Service.

This study evaluated the effect of imidacloprid, a type of neonicotinoid. The Pesticide Action Network explains that neonicotinoids are a relatively new type of systemic insecticide. Systemic pesticides are not sprayed on the surface of a plant, but rather are applied at the root or seed so that the plant can absorb the neonics, which help it fight pests for the whole growing season.

There have been numerous studies in the past that have looked at the effects of certain pesticides, but some of these used extremely high doses of the pesticide in question, which is not true to what a bee would typically encounter in its normal life.

So, for the PLOS ONE study, the researchers fed bees realistic doses of the pesticide, representative of what a bee might be exposed to during foraging. These doses were 5, 20, and 100 µg/kg. The pesticides were applied over multiple brood cycles, and the effects were measured at various points.

The three-year study found that there were no significant effects on foraging activity or colony performance during or shortly after the exposure, but infestations of Varroa mites were significantly higher in those colonies that had been exposed to the pesticide. Observations also indicated that bees might have been avoiding contaminated food.

However, the more noticeable effects of the pesticide were delayed. During the summer after the exposure, the researchers saw that colonies gave 20 and 100 µg/kg doses of imidacloprid “experienced higher rates of queen failure and broodless periods, which led to weaker colonies going into the winter.

The researchers also looked at winter survival rates over two years and found a significant difference in success for those colonies that were given 20 and 100 µg/kg doses. The winter survival rate for colonies that were not exposed averaged 85.7%, while those given 20 and 100 µg/kg doses were much lower, averaging 61.2% and 59.2% respectively.

They concluded that chronic exposure to the pesticide at levels of 20 and 100 µg/kg could cause negative health impacts and reduce the winter survival rates. However, for the smallest and most commonly encountered dose (5 µg/kg), there were negligible effects on colony health. The researchers added that this lowest level of exposure is “unlikely a sole cause of colony declines.”

Bee declines are driven by combined stress from parasites, pesticides, and lack of flowers

Also in March of last year, a study in Science journal evaluated a number of the factors that are thought to be causing bees to die off. The team of researchers was from the University of Sussex in the U.K.

They based their study on the fact that stressors interact with one another and have varying effects on wildlife, and the specific interactions can be difficult to predict. For instance, a bee colony that is stressed by factors such as high pesticide exposure or lack of food can become more susceptible to parasites.

They state that “chronic exposure to multiple interacting stressors is driving honey bee colony losses and declines of wild pollinators, but the precise combination differs from place to place.” In other words, it’s not one single factor that is causing bees to die off, but rather a combination of many factors interacting in ways that don’t always remain constant.

They go on to explain that even though the cause of die-off is complicated, the solutions can be simple: removing any stress factors or harmful inputs is likely to improve colony health. Some techniques they mention include: growing bee-friendly plants and flowers, increasing natural habitat, and practicing integrated pest management.

We at DC Mosquito Squad are big proponents of integrated pest management (IPM). We use this technique to protect natural populations while keeping you and your backyard free of mosquitoes and ticks. Contact DC Mosquito Squad to find out what we can do for you!

Next week, we’ll explain how our IPM practices work, and what precautions we take with our pest control services to protect wildlife, pets, and kids.