Bijensterfte, oorzaken en gevolgen

New study in Nature - Scientific Reports finds strong evidence for pesticide + pathogen hypothesis as key explanation for bee disorders.

ABSTRACT: In ecosystems, a variety of biological, chemical and physical stressors may act in combination to induce illness in populations of living organisms. While recent surveys reported that parasite-insecticide interactions can synergistically and negatively affect honeybee survival, the importance of sequence in exposure to stressors has hardly received any attention. In this work, Western honeybees (Apis mellifera) were sequentially or simultaneously infected by the microsporidian parasite Nosema ceranae and chronically exposed to a sublethal dose of the insecticide fipronil, respectively chosen as biological and chemical stressors. Interestingly, every combination tested led to a synergistic effect on honeybee survival, with the most significant impacts when stressors were applied at the emergence of honeybees. Our study presents significant outcomes on beekeeping management but also points out the potential risks incurred by any living organism frequently exposed to both pathogens and insecticides in their habitat.

Abstract: Nosema ceranae and pesticide exposure can contribute to honey bee health decline. Bees reared from brood comb containing high or low levels of pesticide residues were placed in two common colony environments. One colony was inoculated weekly with N. ceranae spores in sugar syrup and the other colony received sugar syrup only. Worker honey bees were sampled weekly from the treatment and control colonies and analyzed for Nosema spore levels. Regardless of the colony environment (spores+syrup added or syrup only added), a higher proportion of bees reared from the high pesticide residue brood comb became infected with N. ceranae, and at a younger age, compared to those reared in low residue brood combs. These data suggest that developmental exposure to pesticides in brood comb increases the susceptibility of bees to N. ceranae infection.

Abstract: Global pollinator declines have been attributed to habitat destruction, pesticide use, and climate change or some combination of these factors, and managed honey bees, Apis mellifera, are part of worldwide pollinator declines. Here we exposed honey bee colonies during three brood generations to sub-lethal doses of a widely used pesticide, imidacloprid, and then subsequently challenged newly emerged bees with the gut parasite, Nosema spp. The pesticide dosages used were below levels demonstrated to cause effects on longevity or foraging in adult honey bees. Nosema infections increased significantly in the bees from pesticide-treated hives when compared to bees from control hives demonstrating an indirect effect of pesticides on pathogen growth in honey bees. We clearly demonstrate an increase in pathogen growth within individual bees reared in colonies exposed to one of the most widely used pesticides worldwide, imidacloprid, at below levels considered harmful to bees. The finding that individual bees with undetectable levels of the target pesticide, after being reared in a sub-lethal pesticide environment within the colony, had higher Nosema is significant. Interactions between pesticides and pathogens could be a major contributor to increased mortality of honey bee colonies, including colony collapse disorder, and other pollinator declines worldwide.

Scientists in France have discovered that honeybees are at a higher risk of dying from infection by Nosema ceranae (N. ceranae) when they are exposed to low doses of insecticides. The results, presented in the journal PLoS ONE, support the theory that combining more N. ceranae with a high pesticide content in beehives could contribute to colony depopulation.

ABSTRACT: The 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.

ABSTRACT
Global pollinators, like honeybees, are declining in abundance and diversity, which can adversely affect natural ecosystems and agriculture. Therefore, we tested the current hypotheses describing honeybee losses as a multifactorial syndrome, by investigating integrative effects of an infectious organism and an insecticide on honeybee health. We demonstrated that the interaction between the microsporidia Nosema and a neonicotinoid (imidacloprid) significantly weakened honeybees. In the short term, the combination of both agents caused the highest individual mortality rates and energetic stress. By quantifying the strength of immunity at both the individual and social levels, we showed that neither the haemocyte number nor the phenoloxidase activity of individuals was affected by the different treatments. However, the activity of glucose oxidase, enabling bees to sterilize colony and brood food, was significantly decreased only by the combination of both factors compared with control, Nosema or imidacloprid groups, suggesting a synergistic interaction and in the long term a higher susceptibility of the colony to pathogens. This provides the first evidences that interaction between an infectious organism and a chemical can also threaten pollinators, interactions that are widely used to eliminate insect pests in integrative pest management.

Prof. Joe Cummins presents evidence that parasitic fungi can kill insects when low, otherwise non-lethal concentrations of pesticides are present

The neonicotinoid insecticides used to dress seeds are systematic, and accumulate in plant parts including the flowers. Hence honeybees collecting pollen will become exposed to the pesticide, and become more susceptible to fungal pathogens. The parasitic fungus, Nosema ceranae, a single celled parasite was indeed found in CCD-affected bee hives from around the USA.

1. Inleiding
Over bijensterfte verschenen de afgelopen maanden talloze publicaties in de media, informeerde de minister van LNV de kamer, discussieerden imkers op het imkerforum van Bijenhouden.nl. , en kon de omvang vanaf april real-time gevolgd worden op www.beefriends.org. Niet alleen het sterftecijfer bleek van belang, maar ook hoe dit geïnterpreteerd moet worden. Is een hoge sterfte een natuurlijk verschijnsel dat zich zo af en toe voordoet en dat bijen zelf oplossen in de zwermperiode of is er sprake van een structureel probleem waarbij bijensterfte een indicator is van problemen die een grotere reikwijdte hebben? Daarbij werden door de media met name klimaatveranderingen en straling door zendmasten opgepakt. Vanuit imkerzijde werd gewezen op het gebruik van pesticiden in de landbouw, door gemeentelijke diensten en door particulieren.
Naast het leveren van zo goed mogelijke cijfers en argumenten voor deze publieke discussie is onze invalshoek het vaststellen van de dynamiek van bijensterfte in omvang en verspreiding om te kunnen evalueren of maatregelen die worden genomen zinvol zijn, en of er verschillen tussen dode volken te onderscheiden zijn die kunnen verwijzen naar verschillende ziekteverwekkers . In de vragenlijsten van 2007 en 2008 hebben wij met name onderzocht of sterfte met kenmerken van CCD (Colony Collapse Disorder) te onderscheiden viel van andere bijensterfte. CCD wordt in Nederland vaak omschreven met ‘verdwijnziekte’. In dit artikel gebruiken wij verder de term CCD.