Bird decline, insect decline and neonicotinoids

The UK Government and retailers are under pressure to impose a ban on sale of pesticides linked to bee population decline following new research which groups call a 'growing body of evidence'. Environmental groups including the Soil Association and Buglife are making a renewed call for an end to the use of neonicotinoid pesticides, which are among the most commonly used pesticides worldwide, after a new study linked them to a decline in bee populations.

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 in the journal Toxicology (online).

The essence of the Druckrey–Küpfmüller equation dt^n = constant (where d = daily dose and t = exposure time-to-effect, with n>1) for chemical carcinogens is 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 that if both receptor binding and the effect are irreversible, exposure time would reinforce the effect. The Druckrey–Küpfmüller equation explains why toxicity may occur after prolonged exposure to very low toxicant levels. Recently, similar dose–response characteristics have been established for the toxicity of the neonicotinoid insecticides imidacloprid and thiacloprid to arthropods. This observation is highly relevant for environmental risk assessment. Traditional approaches that consider toxic effects at fixed exposure times are unable to allow extrapolation from measured endpoints to effects that may occur at other times of exposure. Time-to-effect approaches that provide information on the doses and exposure times needed to produce toxic effects on tested organisms are required for prediction of toxic effects for any combination of concentration and time in the environment.

The neonicotinoid insecticide imidacloprid is the first highly effective insecticide whose mode of action has been found to derive from almost complete and virtually irreversible blockage of postsynaptic nicotinergic acetylcholine receptors in the central nervous system of insects. The toxicity of imidacloprid to midges (Chironomus tentans) follows Haber’s rule, which is characterised by a linear relationship (on logarithmic coordinates) between exposure concentration and median time to effect, i.e. mortality. Similar dose : response relationships (in accordance with the Druckrey-Küpfmüller equation) have now also been demonstrated for the toxicity of imidacloprid to the freshwater ostracod Cypridopsis vidua and to Daphnia magna as well as for the toxicity of the neonicotinoid insecticide thiacloprid to Gammarus, Sympetrum and Simulium species, which indicates that exposure time may actually reinforce the toxicity of these substances to arthropods (recent article attached). 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.

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.

The European Plant Protection Products Registration Directive (91/414/EEC) requires that there is not an unacceptable impact on non-target organisms in the aquatic and terrestrial environment and that the annual average concentration of an active substance or relevant metabolite does not exceed 0.1 microgram per liter in any ground water. Leaching is a major process for the transport of pesticides to ground and surface water. Four factors govern the potential for groundwater contamination by pesticides passing through the soil: properties of the soil and of the pesticide, hydraulic loading (total amount of water applied to the soil) on the soil, and crop management practices. The most sensitive soil is an irrigated sandy soil with very low organic matter content. The least sensitive soil is a well-drained clayey soil with high organic matter content.

Pesticides may cause problems when they seep out of storage or are washed out of the soil into waterways and groundwater. The chemicals are then diffused through the environment and may affect marine and bird life.

Monitoring data reveal significant changes in the number of meadow birds in the Netherlands. The overall yearly decline has nearly quadrupled in recent years, i.e. from 1.2% in 1990-2000 to 4.6% since 2000. All meadow birds in the monitoring scheme (Skylark Alauda arvensis, Northern Shoveler Anas clypeata, Yellow Wagtail Motacilla flava, Oystercatcher Haematopus ostralegus, Black-tailed Godwit Limosa limosa, Northern Lapwing Vanellus vanellus, Common Redshank Tringa totanus, Meadow Pipit Anthus pratensis en Tufted Duck Aythya fuligula) are declining since 2000. Songbirds like Skylark, Meadow Pipit and Yellow Wagtail declined up to 30% from year to year in some regions which may lead to a population drop of 75% in only five years. Particularly alarming is the steep decline of meadow birds in the western peat land of the Netherlands (with an overall annual decline of 13% since 2000). The bird decline observed since the nineties correlates well with the increasing use of imidacloprid, which was first introduced in agriculture in 1994 and is now by far the most widely used insecticide. Imidacloprid is may leach from soil and is a major surface water pollutant in the Netherlands, particularly in the western part of the country.