Lethal and sublethal toxicity of neonicotinoid and butenolide insecticides to the mayfly, Hexagenia spp.

Highlights

• Toxicity of neonicotinoid and butenolide insecticides was assessed in Hexagenia.

• Differences in toxicity occurred among compounds and endpoints (>2700-fold).

• Behaviour and mobility endpoints were more sensitive than lethality.

• Acute acetamiprid and thiacloprid exposures caused long-term impacts.

• Effects of imidacloprid were observed at environmentally relevant concentrations.

Abstract

Neonicotinoid insecticides are environmentally persistent and highly water-soluble, and thus are prone to leaching into surface waters where they may negatively affect non-target aquatic insects. Most of the research to date has focused on imidacloprid, and few data are available regarding the effects of other neonicotinoids or their proposed replacements (butenolide insecticides). The objective of this study was to assess the toxicity of six neonicotinoids (imidacloprid, thiamethoxam, acetamiprid, clothianidin, thiacloprid, and dinotefuran) and one butenolide (flupyradifurone) to Hexagenia spp. (mayfly larvae). Acute (96-h), water-only tests were conducted, and survival and behaviour (number of surviving mayflies inhabiting artificial burrows) were assessed. Acute sublethal tests were also conducted with imidacloprid, acetamiprid, and thiacloprid, and in addition to survival and behaviour, mobility (ability to burrow into sediment) and recovery (survival and growth following 21 d in clean sediment) were measured. Sublethal effects occurred at much lower concentrations than survival: 96-h LC50s ranged from 780 μg/L (acetamiprid) to >10,000 μg/L (dinotefuran), whereas 96-h EC50s ranged from 4.0 μg/L (acetamiprid) to 630 μg/L (thiamethoxam). Flupyradifurone was intermediate in toxicity, with a 96-h LC50 of 2000 μg/L and a 96-h EC50 of 81 μg/L. Behaviour and mobility were impaired significantly and to a similar degree in sublethal exposures to 10 μg/L imidacloprid, acetamiprid, and thiacloprid, and survival and growth following the recovery period were significantly lower in mayflies exposed to 10 μg/L acetamiprid and thiacloprid, respectively. A suite of effects on mayfly swimming behaviour/ability and respiration were also observed, but not quantified, following exposures to imidacloprid, acetamiprid, and thiacloprid at 1 μg/L and higher. Imidacloprid concentrations measured in North American surface waters have been found to meet or exceed those causing toxicity to Hexagenia, indicating that environmental concentrations may adversely affect Hexagenia and similarly sensitive non-target aquatic species.

Introduction

Neonicotinoids are the most widely used class of insecticides in the world, and they have been marketed primarily as a safer alternative to organophosphates, carbamates, and pyrethroids since the 1990s. The first commercially available compound, imidacloprid, was registered for use by the United States Environmental Protection Agency (US EPA) in 1994, and by Canada’s Pest Management Regulatory Agency (PMRA) in 1995 (PMRA, 2001; US EPA, 1994). Several additional neonicotinoids have been registered for use in North America since that time, including thiamethoxam, clothianidin, thiacloprid, acetamiprid, and dinotefuran (e.g., Health Canada 2017; US EPA, 2017). Imidacloprid, thiamethoxam, and clothianidin are the most widely used neonicotinoids. For example, in Canada there are currently 96, 23, and 16 registered end-use products containing these compounds, respectively, while acetamiprid and thiacloprid are used to a lesser extent (7 and 2 registered products, respectively; Health Canada 2017). Dinotefuran is not registered in Canada, although it has been registered for use in the United States since 2004 (US EPA, 2004).

Neonicotinoids are neurotoxic, and they disrupt the central nervous system by acting as agonists of the nicotinic acetylcholine receptor (nAChR; Goulson 2013). Low concentrations of neonicotinoids stimulate the nervous system, whereas higher concentrations cause receptor blockage, paralysis, and death (Goulson 2013). Neonicotinoids are preferentially toxic to insects while displaying a low toxicity towards vertebrates, as they bind more strongly to insect nAChRs than those of vertebrates (Anderson et al. 2015; Goulson 2013); this selective toxicity is one of the advantages leading to the rapid and ubiquitous use of these compounds. In addition, neonicotinoids provide effective pest control at low concentrations, their high water-solubility allows them to act systemically (i.e., they are taken up by the entire plant), and they can be used in a variety of applications (e.g., seed coating, foliar and soil sprays, drench, and tree injection; Anderson et al. 2015).

However, the characteristics that make neonicotinoids such an effective group of insecticides have also contributed to a growing number of environmental concerns regarding these compounds. First, unlike many of the pesticides they were intended to replace, neonicotinoids are often used prophylactically, and their widespread use as seed coatings for preventative treatment against target pests (Simon-Delso et al. 2015) marks a significant move away from integrated pest management (Goulson 2013). Second, neonicotinoids are water-soluble and non-volatile, and thus they readily move into surface water and ground water, entering the aquatic environment by spray drift, runoff, and leaching (Anderson et al. 2015; Goulson 2013). Third, neonicotinoids can be environmentally persistent. Imidacloprid, thiamethoxam, and clothianidin readily undergo direct photolysis in surface waters (half-lives of 0.20–3.3 d); however, acetamiprid and thiacloprid are relatively stable (half-lives of 8.8–68 d; Lu et al. 2015). In addition, the photolysis of thiamethoxam became negligible at water depths >8 cm due to light attenuation by natural organic matter, and the persistence of other neonicotinoids may be similarly affected (Lu et al. 2015). The pervasive use, high water solubility, and environmental persistence of neonicotinoids have led to their widespread occurrence in North American surface waters (Hladik et al. 2014; Main et al. 2015; Starner and Goh 2012; Struger et al. 2017), and several monitoring studies have measured imidacloprid concentrations exceeding the interim Canadian Water Quality Guideline for the Protection of Aquatic Life (CWQG) (230 ng/L, CCME, 2007; Main et al. 2015; Starner and Goh 2012; Struger et al. 2017). Fourth, because of their selective toxicity toward insect pests and their effectiveness at low concentrations, deleterious effects have been observed in non-target terrestrial and aquatic insects (as reviewed in Anderson et al. 2015; Goulson 2013; Morrissey et al. 2015).

The global presence of neonicotinoids in surface waters and toxicity to non-target aquatic biota have sparked a flurry of research on environmental fate, degradation, and transformation; occurrence and distribution in surface waters; and acute and chronic toxicity to various aquatic organisms (e.g., insects, oligochaetes, crustaceans, molluscs, fish) (Anderson et al. 2015; Goulson 2013; Morrissey et al. 2015; Struger et al. 2017). However, the bulk of this research has focused on imidacloprid, comprising 66% of the 214 toxicity tests reviewed by Morrissey et al. (2015). Consequently, the development of most regulations and guidelines are also focused on imidacloprid; for example, Canada’s only federal freshwater guideline for neonicotinoids is for imidacloprid (CCME, 2007). Few studies are available for the other neonicotinoids registered for use; therefore, the relevance of extrapolating the information on imidacloprid to other neonicotinoids is unknown. The few comparative studies that do exist have reported variability in toxicity among different neonicotinoids to benthic invertebrates (Beketov and Liess 2008b; Cavallaro et al. 2017; Maloney et al. 2017; Van den Brink et al. 2016), indicating that more research is needed to assess the risk of, and develop guidelines for, neonicotinoids other than imidacloprid. Furthermore, flupyradifurone is a butenolide insecticide that is being marketed as an alternative to neonicotinoids: it has a mechanism of action similar to neonicotinoids and has recently been registered for use in Canada and the United States (PMRA, 2015; US EPA, 2015), yet very few data are available on its environmental concentrations and toxicity.

Aquatic insect larvae are especially sensitive to neonicotinoids in both acute and chronic exposures, in particular, those belonging to the orders Ephemeroptera, Trichoptera, and Diptera (Morrissey et al. 2015). A range of effects has been reported, including lethality, impairment of mobility, delayed emergence, reduced growth, feeding inhibition, and altered swimming behaviour, and effects on sublethal endpoints have been found to occur earlier and at lower concentrations than lethality (Goulson 2013; Morrissey et al. 2015). In addition, short-term pulse exposures of imidacloprid and thiacloprid have been demonstrated to cause long-lasting and delayed impacts from four days up to seven months following the initial exposure (Alexander et al. 2008; Beketov and Liess 2008a; Beketov et al. 2008).

The objective of this research was to investigate the toxicity of seven insecticides, six neonicotinoids (imidacloprid, thiamethoxam, acetamiprid, clothianidin, thiacloprid, and dinotefuran) and one butenolide (flupyradifurone), to the burrowing mayfly Hexagenia spp. (order Ephemeroptera). The genus Hexagenia is widespread in North America, in particular the Great Lakes region and the US Midwest (Fremling and Mauck 1980; Harwood et al. 2014). Hexagenia nymphs are ecologically important, both as a food source for other organisms and for their role in processing detritus (Fremling and Mauck 1980). They spend most of their life cycle as benthic organisms at the silty bottoms of lakes, rivers, and ponds; the aquatic portion of their life cycle is typically one year, and the adult terrestrial stage is very short, only one to two days (Fremling and Mauck 1980; Harwood et al. 2014). Thus, populations of Hexagenia are highly likely to be exposed to neonicotinoids, due to both the overlap in distribution with that of neonicotinoid use (Hladik et al. 2014; Struger et al. 2017) and the duration of the aquatic portion of their life cycle.

Acute (96-h), water-only tests were conducted with Hexagenia, and lethal and sublethal (behaviour) endpoints were assessed. Hexagenia have been observed to leave their burrows when injured, stressed, or just prior to death (Fremling and Mauck 1980; Harwood et al. 2014; Henry et al. 1986), which would leave them vulnerable to predation in the natural environment; therefore, this behaviour was monitored at the end of the test. Sublethal acute tests were also conducted with three neonicotinoids (imidacloprid, acetamiprid, and thiacloprid). In addition to monitoring the endpoints described above, effects on mobility were quantified by recording the time for mayflies to burrow, as burrowing indicates the ability to move in a coordinated fashion, and healthy Hexagenia generally burrow into sediment immediately (Harwood et al. 2014). Survival and growth of Hexagenia from the sublethal acute tests were assessed after a 21-d recovery period to determine if short-term exposures to neonicotinoids resulted in long-term or delayed effects. This research will contribute to developing water quality guidelines to protect aquatic life for neonicotinoid and butenolide insecticides in Canada and abroad, providing extensive comparative ecotoxicological effects of the six most commonly used neonicotinoids and a novel butenolide to Hexagenia, an ecologically relevant species.