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For many years environmental groups and beekeepers´ organizations have been pushing for a ban on neonicotinoid pesticides which are linked to bee decline across the world. In a recent study, The toxicity of neonicotinoid insecticides to arthropods is reinforced by exposure time, the Dutch toxicologist Henk Tennekes demonstrates that the long-term risks associated with the insecticides imidacloprid and thiacloprid are far greater than hitherto thought. Honeybees, bumblebees and many other insects are being slowly poisoned to death by these persistent insecticides. Small doses accumulate over time, meaning that there is no safe level of exposure. The study was published on the 23rd of July 2010 in the journal Toxicology (online).

There is a very important shift in our understanding of the risk of neonicotinoid insecticides. Their risk may increase exponentially over time, rendering even very small amounts of neonicotinoids much more toxic than previously realized. Dutch researcher Dr. Henk Tennekes, with Dr. Francisco Sanchez-Bayo of Australia, have recently published a review article in the Journal of Environmental & Analytical Toxicology: "Time-Dependent Toxicity of Neonicotinoids and Other Toxicants: Implications for a New Approach to Risk Assessment" (attached). Their starting point is the Druckrey–Küpfmüller equation dt^n = constant (where d = daily dose and t = exposure time-to-effect, with n>1) for chemical carcinogens. The Druckrey–Küpfmüller equation established in the early 1960s explains why toxicity may occur after prolonged exposure to very low toxicant levels. In essence, this equation states that the total dose required to produce the same effect decreases with decreasing exposure levels, even though the exposure times required to produce the same effect increase with decreasing exposure levels. Druckrey and Küpfmüller inferred in the late 1940s that if both receptor binding and the effect are irreversible, exposure time would reinforce the effect. Recently, similar dose–response characteristics have been established for the toxicity of the neonicotinoid insecticides imidacloprid and thiacloprid to arthropods. Imidacloprid was the first highly effective insecticide whose mode of action has been found to derive from almost complete and virtually irreversible blockage of postsynaptic nicotinic acetylcholine receptors (nAChRs) in the central nervous system of insects. Imidacloprid mimics the action of acetylcholine, but unlike acetylcholine, imidacloprid is not deactivated by acetylcholinesterase and thus persistently activates nAChRs. Chronic exposure of insects to imidacloprid therefore leads to cumulative and virtually irreversible blockage of nAChRs in their central nervous system, which play roles in many cognitive processes.
An example of the consequences for insects in the case of imidacloprid is given in Table 5 (attached). Since imidacloprid and other neonicotinoid insecticides have time-dependent effects on arthropods, the risk of foraging worker bees feeding on tiny levels of residues becomes an issue that cannot and should not be ignored. In the example shown here, 50% of worker bees would die within 7-12 days if feeding on a field where 11% of plants have residues of imidacloprid in the specified range (Table 5). By contrast, standard hazard quotients (HQ) for dietary NOEL of 20 microgram per Litre are misleading because they suggest that imidacloprid poses no danger to honey bees. Given that honey bee workers can live up to a few months in winter time the NEC (No Effect Concentration) for imidacloprid is estimated as close to zero, which means that any residue concentration found in pollen will have a lethal effect provided there is sufficient time of exposure.

A new study appears to have confirmed suspicions that the neonicotinoid group of pesticides is in part responsible for the dramatic decline in UK honeybee numbers, the Telegraph reports. Insect research charity Buglife and the Soil Association "brought together a number of peer-reviewed pieces of research" which demonstrate that neonicotinoids "damage the health and life cycle of bees over the long term by affecting the nervous system". Matt Shardlow, Buglife chief exec, said: “Other countries have already introduced bans to prevent neonicotinoids from harming bees. This is the most comprehensive review of the scientific evidence yet and it has revealed the disturbing amount of damage these poisons can cause." Peter Melchett, director of the Soil Association, added: “The UK is notorious for taking the most relaxed approach to pesticide safety in the EU. Buglife’s report shows that this puts at risk pollination services vital for UK agriculture."

Despite widespread concern about declines in pollination services, little is known about the patterns of change in most pollinator assemblages. By studying bee and hoverfly assemblages in Britain and the Netherlands, we found evidence of declines (pre-versus post-1980) in local bee diversity in both countries; however, divergent trends were observed in hoverflies. Depending on the assemblage and location, pollinator declines were most frequent in habitat and flower specialists, in univoltine species, and/or in nonmigrants. In conjunction with this evidence, outcrossing plant species that are reliant on the declining pollinators have themselves declined relative to other plant species. Taken together, these findings strongly suggest a causal connection between local extinctions of functionally linked plant and pollinator species.

The population of the Kentish Plover Charadrius alexandrinus in the Netherlands has declined from 700-900 pairs in 1973-1977 to 180-210 pairs in 2008, three-quarters of which were breeding in the Dutch Delta area. Reproductive success of the Kentish Plover in the Delta area in 2000-2005 was on average only 0.39 chicks per pair per year, which is far too low to sustain the population. The most recently recorded (2007-2008) decline of the Kentish Plover population in the Delta area (estimated at 144 pairs in 2008) can be attributed to a large extent to changes in the breeding populations in the Grevelingen lake area, which showed a decline from 91 pairs in 2007 to 59 pairs in 2008. In its most important breeding area (Slikken van Flakkee) there was a decline from 62 pairs in 2007 to 32 pairs in 2008, which coincides with recent decline of the Red-listed bumblebee Bombus muscorum in this area (several dozens were sighted in 2005, but only 1 in 2009) and possibly other wild bee species as well. In addition, there is evidence of insecticide (imidacloprid and carbendazim) contamination of surface water in 2007 on the island of Goeree-Overflakkee, i.e. in the vicinity of the Slikken van Flakkee, at levels that are bound to be toxic to insects.

The Dwingelderveld National Park, located in the province of Drenthe in the north-eastern part of the Netherlands, is a heath and woodland reserve of 3,700 hectares, with 1500 ha of wet heath land, which makes it the largest wet heath land in western Europe. Sjouke van Essen surveyed the ground beetles (Coleoptera : Carabidae ) on 38 locations in the Dwingelderveld reserve in 1991 and 2008, in exactly the same manner, and observed a massive decline in the number of caught ground beetles, from 45,000 (94 species) in 1991 to 15,000 (79 species) in 2008. Within the same period there was a dramatic decline of the insectivorous Whinchat Saxicola rubetra (from 35 pairs in 1989 to 6-14 pairs in 1998-2003) and Northern Wheatear Oenanthe oenanthe (from 33 pairs in 1993 to just 3 in 2003) in the Dwingelderveld reserve.