Exposure to Sublethal Doses of Fipronil and Thiacloprid Highly Increases Mortality of Honeybees Previously Infected by Nosema ceranae

Cyril Vidau,Catherine Texier,Nicolas Blot,Frédéric Delbac,Bernard Viguès,Marie Diogon,Julie Aufauvre,Luc P Belzunces,David G Biron,Hicham El Alaoui,Jean-Luc Brunet,Régis Fontbonne

doi:10.1371/journal.pone.0021550

Abstract

BackgroundThe honeybee, Apis mellifera, is undergoing a worldwide decline whose origin is still in debate. Studies performed for twenty years suggest that this decline may involve both infectious diseases and exposure to pesticides. Joint action of pathogens and chemicals are known to threaten several organisms but the combined effects of these stressors were poorly investigated in honeybees. Our study was designed to explore the effect of Nosema ceranae infection on honeybee sensitivity to sublethal doses of the insecticides fipronil and thiacloprid.Methodology/FindingFive days after their emergence, honeybees were divided in 6 experimental groups: (i) uninfected controls, (ii) infected with N. ceranae, (iii) uninfected and exposed to fipronil, (iv) uninfected and exposed to thiacloprid, (v) infected with N. ceranae and exposed 10 days post-infection (p.i.) to fipronil, and (vi) infected with N. ceranae and exposed 10 days p.i. to thiacloprid. Honeybee mortality and insecticide consumption were analyzed daily and the intestinal spore content was evaluated 20 days after infection. A significant increase in honeybee mortality was observed when N. ceranae-infected honeybees were exposed to sublethal doses of insecticides. Surprisingly, exposures to fipronil and thiacloprid had opposite effects on microsporidian spore production. Analysis of the honeybee detoxification system 10 days p.i. showed that N. ceranae infection induced an increase in glutathione-S-transferase activity in midgut and fat body but not in 7-ethoxycoumarin-O-deethylase activity.Conclusions/SignificanceAfter exposure to sublethal doses of fipronil or thiacloprid a higher mortality was observed in N. ceranae-infected honeybees than in uninfected ones. The synergistic effect of N. ceranae and insecticide on honeybee mortality, however, did not appear strongly linked to a decrease of the insect detoxification system. These data support the hypothesis that the combination of the increasing prevalence of N. ceranae with high pesticide content in beehives may contribute to colony depopulation.

Highlights

Honeybee pollination contributes to agriculture productivity and biodiversity
Honeybee infection The success of N. ceranae infection was monitored by measuring sucrose consumption of honeybees and counting spores present in their digestive tract 10 days p.i
We showed that sublethal doses of a neonicotinoid and of a phenylpyrazole highly increased mortality of honeybees previously infected by the microsporidian parasite N. ceranae

Summary

Introduction

Honeybee pollination contributes to agriculture productivity and biodiversity. One-third of food consumed in the world is linked to the pollination activity of honeybees, representing a global economic worth of 153 billion euros in 2005 [1]. The lack of knowledge about their toxicological profile has prevented drawing conclusions about a causal link between exposure to insecticides and the honeybee decline [4]. This is partly due to the fact that the assessment of the risk posed by pesticides is mainly based on the determination of acute toxicity using LD50 as the critical toxicological value [5]. Our study was designed to explore the effect of Nosema ceranae infection on honeybee sensitivity to sublethal doses of the insecticides fipronil and thiacloprid

Methods

Results

Conclusion