Global Wildlife AIDS
Associated With
Neonicotinoid Insecticides

This feature is a continuation of an article by Dr Rosemary Mason and Palle Uhd Jepsen. To go to the beginning of the feature,

Go to beginning of feature by: Rosemary Mason and Palle Uhd Jepsen.

Amphibians in the US

In 1991, the first of a group of novel insecticides was introduced; the systemic neonicotinoids. They started to replace many of the older insecticides. Their sales escalated (see below), so now in 2011 they occupy a dominant position in the global pesticide market.

Amphibians, particularly tadpoles, are considered to be an environmental indicator because of their unique sensitivity to pollutants.

In the US in 2001, there were reports of wild frogs with grotesque limb deformities in at least 43 US States and five Canadian provinces in areas of pesticide run-off. Kiesecker (2002) found that atrazine (herbicide) and malathion (pesticide) made frogs more susceptible to a parasite, a burrowing trematode worm, which affected tadpoles [12].

Even very low levels of exposure (“at concentrations considered safe for drinking water by the US Environmental Protection Agency”) could produce “dramatic effects on the immune response of the animals”.

Field studies showed “considerably higher rates of limb deformities where there was pesticide exposure”. At that time, the systemic neonicotinoid insecticides were (and still are) ‘beneath the radar’, since they do not feature in the 2009 US Geological Survey (USGS) National Water-Quality Assessment Program (NAWQA) Report: Pesticide Trends in Corn Belt Streams and Rivers (1996-2006) [13].

The USGS authors of the Report said: “The declines in pesticide concentrations closely followed the declines in their annual applications, indicating that reduced pesticide use is an effective and reliable strategy for reducing pesticides contamination in streams.”

One of the first national studies on the presence of pesticides in ground-water had been published in 2008 [14]. Laura Bexfield who conducted the data analysis said:

“The results of this study are encouraging for the future state of the nation’s ground-water quality with respect to pesticides”...

“Despite sustained use of many popular pesticides and the introduction of new ones, results did not indicate increasing detection rates or concentrations in shallow drinking water resources over the 10 years studied”.

The authors of both reports expressed satisfaction with the results because they were (mistakenly) under the impression that pesticide use was decreasing and that the reduction in pesticides in ground-water was commendable.

However, the chemicals that NAWQA were measuring were only those that they knew about. Imidacloprid, thiamethoxam and clothianidin do not appear in the USGS list of monitored pesticides [15]. The US Environmental Protection Agency was not measuring neonicotinoid levels either.

In 2001, in response to claims in a pesticide fact sheet by Cox, Bayer experts from different scientific fields had issued a ‘position paper’ on imidacloprid:

 “The use of imidacloprid in agriculture does not entail unacceptable harmful effects for the environment as the substance will disappear under all circumstances from the compartments soil, water and air” [16].

However, contrary to Bayer’s claims, in Europe Tennekes [1,2] and Van Dijk [17] showed that measurements of imidacloprid in surface water by Dutch Water Boards in intensively-farmed areas of the Netherlands had been steadily increasing (in some areas to more than five times the MTR) and that the levels were inversely correlated with declines in Diptera flying insect species.


(The Maximum Tolerable Risk (MTR) norm is an ecotoxicological standard for general environmental quality and the minimum quality level that is desirable for all surface water in the Netherlands. The MTR-value for a substance is the environmental concentration of that substance, at which the species in an ecosystem are safe from effects caused by the substance.)

In 1999, two other pathogens were described in amphibians in the journal Emerging Infectious Diseases; the chytrid fungus and the ranavirus [18].

In 2000, the significance of these new infections on biodiversity and human health was starting to be discussed [19]. Soon, two species of once common frogs that had inhabited the thousands of lakes and ponds in California’s Sierra Nevada were being wiped out by chytridiomycosis, a disease caused by the pathogen Batrachochytrium dendrobatidis (Bd).

Vredenburg et al. (2010) [20] described the progress of the infection in a study area that comprised three lake basins separated by 20-50 km. At the beginning of the study they found no evidence of chytridiomycosis in the frog populations in these three basins.

However, the three were immediately adjacent to basins where frogs had been recently infected and had died from the disease. Bd. was first detected in the smallest basin in June 2004 and in the two larger basins in August 2004 and July 2005 respectively.

It took only one year to spread to virtually all the frog populations in the small basin and 3-5 years in the other two. The scientists followed the pattern and rate of spread in one of the basins. They concluded that Bd. was a novel pathogen spreading through naïve host populations.

However, it did not appear to obey the epidemiological theory that a pathogen should fade out when the host population is driven below some threshold density. (In fact Bd. repeatedly led to extinction of local frog populations over the next 5 years). 

For the decade after they were first reported, these two pathogens, chytrid fungus and ranavirus had between them caused mass deaths across the US in a wide variety of amphibian populations. By 2007 they had been detected in six continents [20].

In 2010, it was reported that there was still “no cure yet for the chytrid fungus which is devastating frog populations”[21].

In 2011, a joint study from four centres (Copenhagen, Spain, Portugal and the US) showed that amphibian population declines far exceed those of other vertebrate groups, with 30% of all species listed as threatened by the International Union for Nature Conservation. They thought the probable causes of these declines included climate change, land-use change and spread of the pathogenic fungal disease chytridiomycosis [22].

However, the WWF Living Planet Report 2010 has shown that biodiversity is declining faster in freshwater, than in any other biome, including coral reefs and tropical forests [23].

We found maps of imidacloprid and thiomethoxam use for 2002 on the NAWQA website (with a cautionary notice to say that it was compiled by the Croplife Protection Research Institute and was an estimated average use over a period of five years from 1999 to 2004).

Imidacloprid was applied to 10 different crops. The top three, sorghum, potatoes and tobacco represented respectively about 26%, 16% and 12% of national pesticide use. Thiomethoxam was also applied to ten different crops. 

The top three, cotton, sorghum and potatoes represented respectively about 57%, 23% and 10% of the national pesticide use. The densest rate of application of imidacloprid and thiomethoxam were in strips running parallel to California’s Sierra Nevada.

At about the same time, Davidson et al. (2002) were studying Californian amphibian declines in a similar area [24].

They used a geographic information system to test and compare patterns of amphibian declines with four hypotheses; climate change, habitat destruction, ultraviolet radiation and pesticide drift.

In four species there was a strong positive association between declines and the amount of upwind agricultural land use. They suggested that windborne pesticides may be an important factor in declines.

Joseph Mendelson, an amphibian scientist, wrote in 2011:

 “The reality of amphibian declines and extinctions has shifted the ecological baseline in so many ecosystems, that an entire generation of biologists is conducting their research in a framework that has been very recently remodelled… [25]” 

He has now decided to call himself a Forensic Taxonomist and his “colleague Dr Tim Halliday (Open University) has taken to calling himself ‘an extinction biologist’.”

Amphibians in Europe and the UK

The Zoological Society of London (ZSL) first discovered the ranavirus in 1995 and the chytrid fungus emerged in 1998 [18].

In 2006, in localised areas of the UK, the ranavirus caused infected frogs either to bleed to death or to develop skin ulceration [26].

By 2007, a similar condition was found in toads and laboratory experiments showed that transmission could occur by inoculation from an infected frog to a toad [27].

Scientists from Natural England suggested that it could have been present for years, but something had changed to turn a commensal organism into a pathogen.

By July 2008, the ZSL and Froglife reported that the ranavirus and chytrid fungus were starting to affect amphibian populations over a wider area and appealed to the public to report outbreaks to the ZSL [28].

Dr Andrew Cunningham said:

“Amphibians are being devastated by disease on a global scale, but we have an extremely limited picture of what is going on in our own back yard.”

At a ZSL meeting in 2008 it was predicted that more than half of Europe’s amphibians faced extinctions by 2050 [29]. By October 2010, the devastation that had earlier ravaged US populations had hit the UK as well.

In Animal Conservation, researchers reported that the rapidly spreading ranavirus “is killing common frogs in the UK in areas where it has never been seen before.” [30] Population declines of 81% had occurred over a period of 12 years.

Continues.... bats and bumblebees in the US, UK and Europe