Jun 12, 2015
Penn State researches pesticides and pollinators

Researchers believe that long term honey bee declines are a result of a complex set of factors. The primary suspects are: poor nutrition, pesticides, pathogens/ parasites, and poor quality genetic stock. Here we will consider recent research results describing how pesticides might affect pollinators.

Honey bees across North America are exposed to multiple pesticides.

Chemical analysis shows that honey bees and their hive products contain many pesticides derived from various sources, including agricultural crops and horticultural gardens. Beekeepers also apply pesticides and antimicrobial drugs to control pathogens and parasites, including Varroa mites. Chris Mullin, Maryann and Jim Frazier and colleagues analyzed 1300 pollen samples and found that 91% were contaminated with pesticides [31]. The most abundant pesticides in hives were acaricides applied by beekeepers to control Varroa mites, and fungicides that were applied to nearby flowering crops. Acaricides, antimicrobial drugs and fungicides are not highly toxic to bees alone, but in combination there is potential for heightened toxicity due to interactive effects [32].

The cocktail of pesticides that bees are exposed to is most striking. On average, there were six pesticides in each pollen sample, up to 31 pesticides in a single pollen sample, and 39 pesticides in a single wax sample. Think about what the first thing your doctor asks you when you visit the clinic: are you taking any medication? We know that multiple chemicals in the body can react to create toxins. The interactions between these pesticides can be very complicated and difficult to predict.

Lethal and sub lethal effects

Pesticides are now labeled for their direct bee toxicity. Toxicity is measured by the amount which kills 50% of bees or LD50. Look for the symbol on the label insert photo here. If the Acute Contact LD50 is less than or equal to 2 micrograms per bee, the pesticide is classified as Toxicity Category I, “highly toxic to bees.” If the LD50 of the pesticide is greater than 11 micrograms per bee (Toxicity Category III), it is relatively nontoxic, and no bee caution statement is required on the label. Visit The Tree Fruit Production Guide For a listing of Toxicity of insecticides, miticides, fungicides, and blossom- and fruit-thinning agents to honey bees.

Pesticides are also being evaluated for their sub-lethal effects on bees. Impaired learning, impaired foraging and homing ability, and reduced immune response are all possible sub-lethal effects of pesticide exposure. Lifespan is also affected by pesticide exposure; in some cases treated bees live only a few days, rather than their normal 6-week lifespan during the foraging season.

Inert ingredients may impact bee health

Perhaps most surprising is that ‘inert’ ingredients may be impacting bee health. Penn State toxicologist Dr. Mullin explains that there are more than 2,000 additives and 20,000 formulations that farmers work with. These products improve how well insecticides and fungicides work by helping them stick to the plant and helping the active ingredient pass through the outer membrane of the plant, among other functions. The bulk of synthetic chemicals used are formulated ingredients that are unmonitored, un-disclosed, and the residues remain unmonitored. Because the non-active ingredients are assumed to be non-toxic, companies are not required to disclose ingredients or monitor toxicity. However, in some cases, formulations which include ‘inerts’ have a greater impact on bees than the active ingredients by themselves [33]. In one example, the toxicity of the fungicide captan to honey bee brood development was attributed to formulation ingredients other than the active ingredient alone [34]. In another study, even at the lowest concentration of 0.01% the inert ingredient N-methyl-2-pyrrolidone (NMP) caused 50% mortality of bee larvae after four days [35].

Bees explore their environments by smell and what they learn allows them to find and return to locations with many food resources of pollen and nectar. In a recent study, researchers found that some types of spray adjuvants affected bee ability to learn [36].


We can’t talk about pesticides and pollinators without touching on the issue that has been so predominately in the news: neonicotinoids. Imidacloprid and other neonics are “nicotinic acetylcholine receptor agonists,” meaning they bind to and block open nerve receptors in the insect brain, causing paralysis and death. Most neonics are toxic in insects in minute quantities. The LD50 of imidacloprid (the dose that kills 50% of individuals) for honey bees is 5 nanograms per insect. Neonics are systemic, which means the plant absorbs them and moves them throughout their tissues to protect multiple parts of the plant, even parts that were not sprayed directly. Because the neonic is systemic, small amounts can be present in the nectar or pollen that the bees collect for food (usually between < 1 and 8 parts per billion (ppb) in nectar and <1 and 50 ppb in pollen). Although normal field levels from neonic seed treated plants are unlikely to be directly lethal (a 0.1 g bee would have to drink several millilitres of nectar or eat a gram or so of pollen); there is strong evidence of negative sub-lethal effects. Exposure to sub-lethal doses of neonicotinoids is known to reduce learning ability, foraging ability and homing ability in both honey bees and bumble bees [37, 38]. Levels will also be higher in sprayed vs seed treated plants.


Fungicides are often applied to crops in bloom when honey bees are present both because crops in bloom are susceptible to fungal and bacterial diseases and because fungicides are currently deemed safe for bees. Chlorothalonil is a broad spectrum fungicide which is often included in a crop protection program to help reduce the possibility of diseases developing resistance. Recently, researchers have found that some fungicides have direct toxicity to bees at field use rates [39]. They have also learned that fungicides stored in pollen can inhibit the growth of beneficial fungi in the digestive tracks of bees and as a consequence make it harder for larvae to get the full nutritional value from the pollen they digest [40].

Penn State’s pollinator research team fed honey bee larvae royal jelly, fructose and yeast laced with pesticides at levels based on those found in pollen in prior research. Fluvalinate and Coumaphos are common miticides used by beekeepers in the hive. Chlorothalonil a broad spectrum fungicide and chlorpyrifos a broad spectrum insecticide. Not only did the fungicide chlorothalonil directly increase mortality, the probability that larvae would survive four days was only 50% when chlorothlonil was mixed with coumaphos mitacide compared to 90% with just the miticide [35]. This is a brand new area of research and we expect greater clarification as it is explored.

Interactions between multiple factors

The combination of exposure to pesticides and reduced nutrition may make bees more susceptible to the other major suspects in bee decline: parasites and pathogens. “Pesticides plus viruses is the double whammy that kills hives and bees,” says Dr. Cox Foster.

This article is part of a five part series describing pollinators, pollinator threats and on-farm conservation strategies as part of a collaboration between Penn State’s Center for Pollinator Research and Penn State Extension Vegetable and Small Fruit Team.

75 Applewood Drive, Suite A
P.O. Box 128
Sparta, MI 49345
Fruit Growers News August 2020 cover
Get one year of Fruit Growers News in both print and digital editions for only $15.50.

Interested in reading the print edition of Fruit Growers News?

Subscribe Today »

Be sure to check out our sister sites:
website development by deyo designs