Bijensterfte, oorzaken en gevolgen

Op 13 september 2009 verscheen in het wetenschappelijk tijdschrift Ecotoxicology een nieuwe studie van onderzoekers van de Vrije Universiteit Brussel naar de gevolgen voor hommels van langdurige blootstelling aan imidacloprid. De studie toont aan dat lage concentraties die niet acuut dodelijk zijn op lange termijn wel tot volksterfte leiden. Bij afnemende concentratie van imidacloprid in de voeding vonden de onderzoekers 100% sterfte in het hommelnest na respectievelijk enkele uren (bij 200 ppm), 14 dagen (bij 20 ppm), 28 dagen (bij 2 ppm) en 49 dagen (bij 0,2 ppm). Wat opvalt is dat de totale hoeveelheid gif die 100% sterfte in het nest veroorzaakt minder wordt als die hoeveelheid over een langere periode wordt uitgesmeerd. Dit is typerend voor CT-giffen die onomkeerbare schade geven die zich bij herhaalde blootstelling opstapelt (regel van Haber, zie ook www.bijensterfte.nl/nl/node/102)

Hieronder de abstract van de studie en enkele citaten uit het artikel.

Veerle Mommaerts, Sofie Reynders, Jana Boulet, Linde Besard, Guido Sterk, Guy Smagghe (Free University of Brussels) in Ecotoxicology.

Abstract Bombus terrestris bumblebees are important pollinators of wild flowers, and in modern agriculture they are used to guarantee pollination of vegetables and fruits. In the field it is likely that worker bees are exposed to pesticides during foraging. To date, several tests exist to assess lethal and sublethal side-effects of pesticides on bee survival, growth/development and reproduction. Within the context of ecotoxicology and insect physiology, we report the development of a new bioassay to assess the impact of sublethal concentrations on the bumblebee foraging behavior under laboratory conditions. In brief, the experimental setup of this behavior test consists of two artificial nests connected with a tube of about 20 cm and use of queenless micro-colonies of 5 workers. In one nest the worker bees constructed brood, and in the other food (sugar and pollen) was provided. Before exposure, the worker bees were allowed a training to forage for untreated food; afterwards this was replaced by treated food. Using this setup we investigated the effects of sublethal concentrations of the neonicotinoid insecticide imidacloprid, known to negatively affect the foraging behavior of bees. For comparison within the family of neonicotinoid insecticides, we also tested different concentrations of two other neonicotinoids: thiamethoxam and thiacloprid, in the laboratory with the new bioassay. Finally to evaluate the new bioassay, we also tested sublethal concentrations of imidacloprid in the greenhouse with use of queenright colonies of B. terrestris, and here worker bees needed to forage/fly for food that was placed at a distance of 3 m from their hives.
In general, the experiments showed that concentrations that may be considered safe for bumblebees can have a negative influence on their foraging behavior. Therefore it is recommended that behavior tests should be included in risk assessment tests for highly toxic pesticides because impairment of the foraging behavior can result in a decreased pollination, lower reproduction and finally in colony mortality due to a lack of food.

Results of the study include:
"Chronic toxicity assay not including foraging behavior
For imidacloprid at 200, 20, 2 and 0.2 ppm, 100% mortality (IOBC class 4 of highly toxic) was observed in the nests and this was after a few hours, 14, 28 and 49 days, respectively. In contrast, at 20 and 10 ppb worker mortality was much lower with 15% and 0%, respectively. Probit analyses of the data resulted in a LC50 value for imidacloprid of 59 ppb (95% CI: 52–68 ppb; R^2 = 0.99) which corresponds to 1/3390 of the MFRC"

(Note: MFRC is the maximum field recommended concentration)

"Imidacloprid at 20 and 10 ppb affected all (100%) the workers in the treated hives after 2 weeks of exposure via treated sugar water, representing class T (harmful) in accordance with the IOBC classification. In detail, we scored an effective kill of workers (62 ± 1 and 92 ± 5% mortality with 20 and 10 ppb, respectively), and all the other workers from these hives were totally apathic with any signs of movement and foraging."

Source: Mommaerts et al (in press) Risk assessment for side-effects of neonicotinoids against bumblebees with and without impairing foraging behavior, Ecotoxicology