Supposedly Inert Agrochemicals Impair Learning in Honey Bees

Posted: August 6, 2012

Honey bees exposed to agrochemicals used on farms may develop learning impairments that prevent them from being able to forage or even find their way back to the hive, say researchers at Penn State’s College of Ag Sciences.
Courtesy Maryann Frazier, Penn State

Courtesy Maryann Frazier, Penn State

Former entomology graduate student LTJG Tim Ciarlo, U.S. Navy, professors of entomology Chris Mullin and Jim Frazier, and graduate research assistant Dan Schmehl recently published their research in PLoS ONE, an online open-access journal for the communication of all peer-reviewed scientific and medical research.

 According to their research, pesticide spray adjuvants could be contributing to Colony Collapse Disorder (CCD), which continues to threaten honey populations since its emergence in 2006.  “Previously, active ingredients has been the focus of pesticide-driven CCD research, but not much attention has been placed on the other agrochemicals that are used in addition to and in combination with them,” says Ciarlo, whose research work became the focus of his master’s thesis.

 These other agrochemicals are classified as ‘inert ingredients” and include spray adjuvants that are either included in the pesticide formulations or tank-mixed and sprayed along with the pesticides. Adjuvants are designed to boost the efficacy of active ingredients and are largely assumed to be inert; therefore they are not usually included in risk assessment trials required to register a pesticide or its formulations. Additionally, the specific ingredients that make up spray adjuvants are considered trade secrets of the chemical companies that manufacture them and are not included on the label. “This is the first time the impact of spray adjuvants on honey bee behavior has been studied,” Ciarlo explains.

 To examine the effects of spray adjuvants on honey bees, researchers utilized learning assays much the same as Pavlov’s dog experiments in which he presented dogs with a ringing bell followed by food. The food caused the dogs to salivate, and after repeated bell-food pairings, the bell alone also caused the dogs to salivate. In Ciarlo’s experiments, honey bees were presented with a cinnamon odor (the ‘bell’) while being fed adjuvant-infused sucrose (the ‘food’) to mimic a nectar-feeding event at an adjuvant-sprayed flower.

 Ciarlo says they used sublethal doses of the most widely sprayed adjuvants on almonds in California.  “We chose this crop because hundreds of thousands of pounds of organosilicone adjuvants alone are applied each year to the almonds, and it is the largest pollination event in the world.” Additionally, California requires growers of all important food crops to report their pesticide use, and since adjuvants are considered pesticides in the state and are reported the same way, an abundance of preexisting data was readily available.

Ciarlo says the results of the project were surprising. They found honey bee learning was impaired after ingestion of organosilicone-laced sucrose, but not by most other adjuvants including nonionics and crop oils, indicating agrochemicals previously believed to be benign can be harmful. “Learning is important for foraging honey bees because it allows them to find the most productive floral resources in an area at any given time, which is vital to the success of a honey bee colony.”

 Learning also has other uses besides foraging for food. Bees use visual cues to find their way back to the hive. One of the symptoms of CCD and related bee decline syndromes is the rapid disappearance of adult bees away from the hive without any evidence of dead bees. “One hypothesis for the disappearance is that foragers are becoming disoriented while away on foraging trips and are unable to return to the hive,” Ciarlo explains. “Further study is needed to determine the actual level of adjuvant exposure for colonies in the field and the physiological changes occurring inside the bees.”

 For more information on honey bee research, visit The Penn State Center for Pollinator Research web site at The center is devoted to the study of pollination, pollinators and pollinator management and protection and comprises 26 independent faculty, including researchers, educators, extension specialists and outreach coordinators, spanning multiple departments and colleges at Penn State.