The Curious Beekeeper

August 2014


Running the Risk, Part II: The Higher Tiers of

Honey Bee Risk Assessment and Comparisons

Between the U.S., Canada and Europe

Pesticide Research Institute


In the last installment of The Curious Beekeeper, we provided an overview of the basics of pesticide risk assessment for pollinators. In this article, we introduce you to the higher tier risk assessment methods that are being used to assess risks to entire colonies rather than just individual bees. We also compare the new US EPA pollinator risk assessment guidance published in June 20141 to the methods currently used in Canada2, 3, and the European Union, and provide recommendations for minimizing the current uncertainties in assessing the risk of pesticide exposure to bees.

As described in Part I of this series, the first step in a pollinator risk assessment is the determination of potency of the pesticide for killing individual adult bees outright. This is called a Tier 1 assessment.

The standard of measure is the LD50, the Lethal Dose for 50 percent of the test bees in a laboratory test. The contact LD50 is a measure of toxicity for foragers who may actually contact treated foliage. The oral LD50 provides more relevant information for nurse bees, the queen, and others inside the hive that may eat contaminated nectar and pollen; however, fewer oral toxicity studies have been done, and substantial data gaps remain.
If risks are identified in the Tier I assessment, the regulatory authority may require additional “higher tier” studies.

Higher Tier Studies
Semi-field (Tier II) and full field (Tier III) studies are conducted to further refine the risk assessment and clarify uncertainty related to colony level impacts of a pesticide. The semi-field (or “tunnel”) study evaluates the effects of known field exposure by confining the bees with a mesh tunnel to a single treated crop. Tier II studies may also include studies in which bees are fed pesticide-spiked syrup or pollen, but are also allowed to forage freely. The effects on larval development and brood hatching success rate, queen survival and/or overall colony health are then observed as a function of administered dose. Procedures for conducting these kinds of studies are currently in development.

Challenges with Tier II Tunnel Studies
Tunnel studies provide a controlled way to limit foragers to pesticide treated or untreated plants. These studies are often cited as “worst-case” exposure scenarios; however, the inherent limitations of tunnel studies typically preclude an actual worst-case exposure. Some challenges include:

  1. Significantly reduced forage distance. Much less energy is required to forage in a tunnel, which in turn reduces the daily food needs of the foragers and their corresponding intake of pesticide residues. This will result in an underestimate of the adverse effects of the pesticide.
  2. Too little forage for the colony. The confined space limits the amount of contaminated nectar or pollen available, thereby reducing the impact of the pesticide on the colony.
  3. Ensuring that colonies utilize the resources brought in from the treated plants. For accurate assessment of pesticide impacts on the colonies, it is essential that they are placed in the tunnel with minimal other food reserves to draw on in the hive. There is some evidence that bees faced with a choice of eating “clean” food or pesticide-contaminated food will choose the “clean” food preferentially, although the magnitude of this effect is likely dependent on the pesticide and the level of contamination.
  4. Ensuring that bees can return to the hive after foraging in the tunnel. Hive placement is critical. Commericial beekeepers doing tent pollination have discovered that hives placed on the ground of the enclosure quickly lose ...