Insecticides

Der Toxikologe Dr. Henk Tennekes weist in einer aktuellen Studie nach, dass die Langzeitrisiken der Insektizide Imidacloprid und Thiacloprid weitaus größer sind, als bislang angenommen. In der Studie "The significance of the Druckrey-Küpfmüller equation for risk assessment - The toxicity of neonicotinoid insecticides to arthropods is reinforced by exposure", erklärt Tennekes das gängige Verfahren zur Bestimmung der Auswirkungen von bestimmten Dosen eines Wirkstoffs auf einen ausgewählten Organismus. Er belegt, dass die Expositionsdauer einen wesentlichen Einfluss darauf hat, wie hoch eine Dosis sein muss, um einen schädigenden Effekt auf den Testorganismus auszuüben. Setzt man beispielsweise Honigbienen über einen längeren Zeitraum einer niedrigen Konzentration von Imidacloprid aus, so kommt es zu letalen Effekten obgleich die Gesamtbelastung 60-6000mal unterhalb der Dosis liegt, die den gleichen Effekt in Testverfahren zur Ermittlung der akuten Toxizität hat.

Das Risiko von Pestiziden wie Imidacloprid und Thiacloprid wird demnach erheblich unterschätzt. Dies gilt besonders für Wasserlebewesen, Bodenorganismen und Bienen. Die bislang gültigen Grenzwerte wurden weitgehend aus Kurzzeit-Tests abgeleitet. Würde man Langzeit-Versuche durchführen, könnten schon bei wesentlich geringeren Konzentrationen verheerende Schäden auftreten.

Tennekes kommt in seiner Studie zu dem Schluss, dass die Belastung mit niedrigen Neonicotinoid-Dosen einen negativen Einfluss auf das Sammelverhalten und das Lernvermögen von Honigbienen haben können und somit in Folge subletale Imidacloprid-Konzentrationen sich schädigend auf die Bienenkolonie auswirken und dadurch den Zusammenbruch einer Kolonie verursachen können.

Le texte ci-dessous est une traduction de : "The Significance of the Druckrey-Küpfmüller Equation for Risk Assessment - The Toxicity of Neonicotinoid Insecticides to Arthropods is Reinforced by Exposure Time" à paraître in "Toxicology". Le texte complet en français est joint en PDF.

Dans cet article théorique, puisant aux sources de la toxicologie et des mathématiques, Henk TENNEKES s'intéresse à un domaine, ô combien crucial en toxicologie, celui des relations "doses - effets".

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."

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 Dutch population of the Great Reed Warbler Acrocephalus arundinaceus has declined by 90% from 1960 to 1990 (from 5,000 breeding pairs in 1950-1960 to 400-550 in 1989-1991), and continued to decline by 6% per annum since 1990. The territories of a large Great Reed Warbler colony in the shallow Reeuwijk lakes (at 52◦2’ N and 4◦45’E in the Western part of the Netherlands, near Gouda), which originated from peat-digging in the 16th and 17th century, decreased from 90-100 in 1975, 40 in 1993, 20 in 1997, 14 in 2000, 8 in 2004 to just 6 in 2005. The Reeuwijk polder is mainly used for agriculture and surplus water during wet periods is discharged via the Reeuwijk lakes to the Breevaart canal, which may expose insects in the Reeuwijk lakes to surface water contaminated with pesticides. Surface water analyses in the vicinity of the Reeuwijk lakes have revealed excessively high concentrations of imidacloprid and carbamates (carbendazim and propoxur) that are bound to be toxic to insects. There is supporting evidence. Monitoring data for the characteristic dragonfly Aeshna viridis (a food source for the Great Reed Warbler) in the Reeuwijk lakes since 1998 indicate that this population is declining. The populations of other dragonflies characteristic of peat bogs in the Netherlands, i.e. Leucorrhinia pectoralis (large white faced darter) and Sympecma paedisca are also in decline since the 1960s.

The evidence provided by the Dutch Water Boards on imidacloprid contamination of surface water (attached) indicates that, in any case in the western part of the Netherlands, high concentrations of imidacloprid are diffused through the environment, which may kill or debilitate insects and possibly other arthropods. Attached is a map of Holland showing the agricultural areas where the use of imidacloprid is permitted.

The use of imidacloprid in Dutch agriculture rose from 668 kg on 5,335 hectares in 1995 to 6, 377 kg on 40,007 hectares in 2004 . Since 2004 major contamination of Dutch surface water with imidacloprid has been detected by the Water Boards, particularly in the western part of the country. The highest concentration was measured in Noordwijkerhout in December 2005: 320,000 ng/L. The maximum permissible limit for imidacloprid in Dutch surface water is 67 ng/L. Imidacloprid is stable to breakdown by water at neutral pH and degrades with a half-life of 355 days in more basic solutions.

Soil acts as a major sink for bulk of the pesticides used in agriculture and public health programs. Leaching is a major transportation process responsible for ground water contamination, which is a major concern worldwide as ground water is a source of drinking and irrigation water in many countries. An Indian study demonstrates high mobility of imidacloprid in soil and high potential for leaching. Thiamethoxam also has a potential to leach down under heavy rainfall conditions.