Bijensterfte, oorzaken en gevolgen

Abstract: Bumble bees are important pollinators whose populations have declined over recent years, raising widespread concern. One conspicuous threat to bumble bees is their unintended exposure to trace residues of systemic neonicotinoid pesticides, such as imidacloprid, which are ingested when bees forage on the nectar and pollen of treated crops. However, the demographic consequences for bumble bees of exposure to dietary neonicotinoids have yet to be fully established. To determine whether environmentally realistic levels of imidacloprid are capable of making a demographic impact on bumble bees, we exposed queenless microcolonies of worker bumble bees, Bombus terrestris, to a range of dosages of dietary imidacloprid between zero and 125 μg/L and examined the effects on ovary development and fecundity. Microcolonies showed a dose-dependent decline in fecundity, with environmentally realistic dosages in the range of 1 μg/L capable of reducing brood production by one third. In contrast, ovary development was unimpaired by dietary imidacloprid except at the highest dosage. Imidacloprid reduced feeding on both syrup and pollen but, after controlling statistically for dosage, microcolonies that consumed more syrup and pollen produced more brood. We therefore speculate that the detrimental effects of imidacloprid on fecundity emerge principally from nutrient limitation imposed by the failure of individuals to feed. Our findings raise concern about the impact of neonicotinoids on wild bumble bee populations. However, we recognize that to fully evaluate impacts on wild colonies it will be necessary to establish the effect of dietary neonicotinoids on the fecundity of bumble bee queens.

On 30 March 2012 Science published 2 studies and a comment on neonicotinoid insecticides and pollinator decline:

- A Common Pesticide Decreases Foraging Success and Survival in Honey Bees
- Neonicotinoid Pesticide Reduces Bumble Bee Colony Growth and Queen Production
- Field Research on Bees Raises Concern About Low-Dose Pesticides

Abstract: Bumble bees (Bombus) are vitally important pollinators of wild plants and agricultural crops worldwide. Fragmentary observations, however, have suggested population declines in several North American species. Despite rising concern over these observations in the United States, highlighted in a recent National Academy of Sciences report, a national assessment of the geographic scope and possible causal factors of bumble bee decline is lacking. Here, we report results of a 3-y interdisciplinary study of changing distributions, population genetic structure, and levels of pathogen infection in bumble bee populations across the United States. We compare current and historical distributions of eight species, compiling a database of >73,000 museum records for comparison with data from intensive nationwide surveys of >16,000 specimens. We show that the relative abundances of four species have declined by up to 96% and that their surveyed geographic ranges have contracted by 23–87%, some within the last 20 y. We also show that declining populations have significantly higher infection levels of the microsporidian pathogen Nosema bombi and lower genetic diversity compared with co-occurring populations of the stable (nondeclining) species. Higher pathogen prevalence and reduced genetic diversity are, thus, realistic predictors of these alarming patterns of decline in North America, although cause and effect remain uncertain.

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.

Research conducted using only honey bees as the indicator species may not adequately reflect the risk posed by insecticides to wild bees because of their differential susceptibility and unique biology.

C.D. Scott-Dupree, L. Conroy, and C.R. Harris

ABSTRACT Pest management practices may be contributing to a decline in wild bee populations in or near canola (Brassica napus L.) agroecosystems. The objective of this study was to investigate the direct contact toxicity of five technical grade insecticides - imidacloprid, clothianidin, deltamethrin, spinosad, and novaluron - currently used, or with potential for use in canola integrated pest management on bees that may forage in canola: common eastern bumble bees [Bombus impatiens (Cresson); hereafter bumble bees], alfalfa leafcutting bees [Megachile rotundata (F.)], and Osmia lignaria Cresson. Clothianidin and to a lesser extent imidacloprid were highly toxic to all three species, deltamethrin and spinosad were intermediate in toxicity, and novaluron was nontoxic. Bumble bees were generally more tolerant to the direct contact applications > O. lignaria > leafcutting bees.
However, differences in relative toxicities between the three species were not consistent, e.g., whereas clothianidin was only 4.9 and 1.3x more toxic, deltamethrin was 53 and 68x more toxic to leafcutting bees than to bumble bees and O. lignaria, respectively. Laboratory assessment of direct contact toxicity, although useful, is only one measure of potential impact, and mortality under Þeld conditions may differ greatly depending on management practices. Research conducted using only honey bees as the indicator species may not adequately reßect the risk posed by insecticides to wild bees because of their unique biology and differential susceptibility. Research programs focused on determining nontarget impact on pollinators should be expanded to include not only the honey bee but also wild bee species representative of the agricultural system under investigation.