The application of insecticides has made great contributions to the development of agriculture around the world. Among insecticides, most of the neonicotinoid insecticides are considered as classic neurotoxins since they can irreversibly target to nicotinic acetylcholine receptors and cause paralysis and eventual death in most arthropods rather than in vertebrates (Matsuda et al., 2005; Bradford et al., 2020). Moreover, diamide insecticides could damage muscle contraction via activating ryanodine receptors and releasing stored calcium from the sarcoendoplasmic reticulum, which is an important mechanism to control lepidopteran pests (Cordova et al., 2006). Sulfoximine insecticides (such as sulfoxaflor) could kill insects by damaging Ca²⁺ homeostasis in muscle cells (Guo et al., 2019).

After release into the environment, insecticides potentially cause toxicity to environmental organisms. As reported in previous studies, organochlorines, organophosphates, carbamates, pyrethroids, and neonicotinoid insecticides could cause damage on reproduction, feeding, and avoidance of predation in poultry populations (Walker, 2003). In addition, exposure to insecticides also poses a great threat to some soil organisms—for example, cycloxaprid, a novel neonicotinoid insecticide, could induce neurotoxicity in earthworms, such as neurological dysfunctions, stress responses, and damage on calcium binding (Qi et al., 2018).

Due to the properties of short lifespan, small size, short life cycle, and high sensitivity to pollutants (Leung et al., 2008; Haegerbaeumer et al., 2018; Queirós et al., 2019; Wang, 2020; Zhang et al., 2022), Caenorhabditis elegans has been used as an ideal model for examining the response to environmental toxicants or stresses (Wang et al., 2021a; Liu et al., 2021c; Sun et al., 2021). Recently, it was reported that some neonicotinoid insecticides could cause a dysfunction in the development, reproduction, and locomotion behaviors of C. elegans (Bradford et al., 2020). Moreover, C. elegans is an important animal model for the developmental study of nervous systems because of its simple nervous system structure composed of 302 neurons in adult hermaphrodite (Bargmann, 1998; Corsi et al., 2015). C. elegans is helpful for investigating different functions of the nervous system, such as learning and memory (Ardiel and Rankin, 2010; Bargmann et al., 1993). In addition, C. elegans is a powerful animal model for assessing neurotoxicity induced by different toxicants (Wang, 2019; Liu et al., 2021a)—for example, in C. elegans, the development of dopaminergic nervous systems and their functions (such as sensory perception behaviors) were found to be affected by exposure to nanoparticles (Qu and Wang, 2020). Considering the fact that ∼80% of C. elegans proteome has human homologous genes, specifically neurodevelopmental genes (Riddle et al., 1997), C. elegans is an important animal model for both biomedical and environmental toxicology. Due to the availability of some disease models (Moy et al., 2009; Griffin et al., 2017), C. elegans is also useful for pharmaceutical screening.

Sensory perception is an important function for organisms to sense and detect the existence and alteration of environmental stimuli. Nevertheless, the possible toxic mode of action on sensory perception behaviors induced by exposure to insecticides remains largely unclear in organisms. Thus, the aim of this study is to compare the possible damage of six insecticides (acetamiprid, nitenpyram, thiacloprid, thiamethoxam, dinotefuran, and sulfoxaflor) on the sensory perception behaviors of C. elegans as representative for other nematodes. Among them, acetamiprid, nitenpyram, thiacloprid, thiamethoxam, and dinotefuran are neonicotinoid insecticides. Sulfoxaflor belongs to sulfoximine insecticides. C. elegans has been widely used for the assessment of pesticide toxicity (Gomez-Eyles et al., 2009; Roh and Choi, 2011; Ruan et al., 2012; Du et al., 2015; Yu et al., 2020). The well-described backgrounds of development and functions of the nervous system of C. elegans provide a strong support for this research. The sensory perception behaviors of thermotaxis, avoidance to copper ion, chemotaxis to NaCl, and chemotaxis to diacetyl were examined in this study. Thermotaxis reflects a sensory perception to physical stimuli. The chemotaxis to NaCl and avoidance to copper reflect a gustatory perception to soluble chemicals. The chemotaxis to diacetyl indicates an olfactory perception to volatile chemicals. Our results demonstrated the potential of exposure to the examined insecticides in the range of micrograms per liter (μg/L) in causing damage on sensory perceptions to different degrees in nematodes. The obtained data highlights the toxicity of long-term exposure to the examined insecticides at low concentrations in inducing damage on sensory perceptions in environmental organisms.

Normally, the environmental concentrations of insecticides in surface water were in the range of microgram or nanogram per liter (μg/L or ng/L) (Faria et al., 2020). This is one of the important reasons for our focus on insecticide concentrations in the range of μg/L—to determine the effect of insecticide exposure on sensory perception in this study. Besides this, according to our study, exposure to the six insecticides examined at a concentration of 1–100 mg/L caused significant lethality, but exposure to the six insecticides (0.1 mg/L) examined did not induce significant lethality (Supplementary Figure S1). Thus, the observed damage on sensory perceptions in nematodes exposed to the six insecticides examined at a concentration of ≤100 μg/L might be largely not due to the lethal effect. This is another reason for our focus on insecticide concentrations in the range of μg/L—to determine the effect of insecticide exposure on sensory perception in nematodes.

Using C. elegans as an animal model, it has been shown that exposure to certain insecticides (such as neonicotinoid insecticides) could result in oxidative stress and damage on reproduction, locomotion behaviors, and growth (Rajini et al., 2008; Han et al., 2017; Kudelska et al., 2017; Zeng et al., 2017; Bradford et al., 2020). In this study, we found that prolonged exposure to insecticides in the range of μg/L potentially caused damage on multiple aspects of sensory perception in nematodes.

Temperature is an important environmental factor for the survival of organisms. Meanwhile, temperature is also an important environmental stimulus for environmental animals to sense. C. elegans has highly sensitive and sophisticated thermosensory mechanisms to detect environmental temperatures (Takeishi et al., 2020). As shown in Figure 2, exposure to neonicotinoid insecticides potentially noticeably affected thermotaxis behavior in nematodes. Sulfoxaflor had the weakest toxic effect on thermotaxis behavior among the six insecticides examined.

Avoidance of copper ion reflects a form of gustatory perception of worms, and this form of gustatory perception is controlled by ASH sensory neurons (Wang, 2019). All the six insecticides examined (10–100 μg/L) suppressed the avoidance of copper and showed the following toxicity order: dinotefuran > thiamethoxam > thiacloprid > nitenpyram > acetamiprid > sulfoxaflor (Figure 3). The heavy metal Cu is neurotoxic for worms (Wang and Xing, 2009; Zhang et al., 2010). These observations suggested that exposure to the examined insecticides in the range of μg/L may cause the difficulty for organisms to avoid harmful compounds or noxious stimuli in the environment. It was also reported that exposure to some insecticides might adversely influence the avoidance of predation in the poultry populations (Walker, 2003).

Chemotaxis towards NaCl is another form of gustatory perception of worms, and ASE sensory neurons controlled this form of gustatory perception (Bargmann and Horvitz, 1991). Similar to the observations on thermotaxis behavior and avoidance of copper ion, these six insecticides at concentrations of 10–100 μg/L caused a remarkable deficit in chemotaxis to NaCl and exhibited the following toxicity order: dinotefuran > thiamethoxam > thiacloprid > nitenpyram > acetamiprid > sulfoxaflor (Figure 4). In honeybees, exposure to thiamethoxam also caused a deficit in sensory perception of sugar (Aliouane et al., 2009).

Chemotaxis to diacetyl reflects the ability of olfactory perception in worm, and this sensory perception is mainly controlled by AWA sensory neurons (Metaxakis et al., 2018). After exposure at concentrations of 10–100 μg/L, the toxicity order for the six insecticides examined in affecting chemotaxis to diacetyl was dinotefuran > thiamethoxam > thiacloprid > nitenpyram > acetamiprid > sulfoxaflor (Figure 5). These observations demonstrated that, in worms exposed to the examined insecticides in the range of μg/L, both gustatory perception and olfactory perception would be noticeably damaged. Besides the four forms of sensory perceptions examined, there are also some other forms of sensory perceptions in C. elegans (Metaxakis et al., 2018). The possible effects of exposure to the examined insecticides on other forms of sensory perceptions still remain unclear in nematodes.

After exposure, 100 μg/L of the six insecticides examined obviously decreased the locomotion behavior as reflected by the endpoint of body bend (Supplementary Figure S2). In contrast, dinotefuran, thiamethoxam, thiacloprid, nitenpyram, acetamiprid, and sulfoxaflor at concentrations of ≤100 μg/L did not affect the locomotion behavior (Supplementary Figure S2). Therefore, the observed damage of the six insecticides examined at concentrations ≤100 μg/L on sensory perceptions was not due to the deficit in locomotion behavior.

In nematodes, exposure to insecticides (such as lindane) could induce oxidative stress (Yu et al., 2021). Ascorbate is a normally used antioxidant against oxidative stress in nematodes (Li et al., 2012). We further found that post-treatment with the antioxidant ascorbate could effectively suppress damage by the six insecticides examined in reducing themotaxis, in suppressing avoidance of copper ion, in decreasing chemotaxis toward NaCl, and in reducing chemotaxis toward diacetyl (Figures 6B–E). Meanwhile, post-treatment with the antioxidant ascorbate could also obviously suppress the ROS production induced by exposure to the six insecticides examined (Figure 6A). Therefore, the activation of oxidative stress can act as an important cellular contributor to the observed damage of the examined insecticides in affecting sensory perceptions. In nematodes, the ROS signals are mainly detected in the pharynx and the intestinal cells (Yang et al., 2021a). This implied that the examined insecticides might potentially bind to certain group(s) of biomolecules so as to induce oxidative damage in primary biological barriers, such as pharynx barrier, in nematodes. After that, neurotoxicity on sensory perceptions will be further induced after insecticide exposure—that is, the damage on the examined four forms of sensory perceptions might be largely due to the fact that both sulfoxaflor and neonicotinoid insecticides could cause the activation of oxidative stress. This is also helpful to explain the fact that the sensory perceptions were affected in the same way by the examined insecticides.

The possible adverse effects of insecticides at various aspects have also been determined with other organisms, such as Daphnia magna and Ceriodaphnia dubia (Sheets et al., 2016; Raby et al., 2018). Moreover, it was observed that exposure to neonicotinoid insecticides could result in developmental neurotoxicity (Sheets et al., 2016)—that is, at least at high concentrations, the observed damage on sensory perceptions to temperature, copper ion, NaCl, and diacetyl might also be possibly due to the developmental deficits in related neurons and/or neuronal circuit(s) governing these sensory perceptions in nematodes.

The potential damage on sensory perceptions by exposure to six insecticides (dinotefuran, thiamethoxam, thiacloprid, nitenpyram, acetamiprid, and sulfoxaflor) was compared in C. elegans. Exposure to these insecticides at concentrations of 10–100 μg/L caused severe deficits in sensory perceptions to temperature, copper ion, NaCl, and diacetyl. Activation of oxidative stress acted as an important cellular mechanism for the observed damage of the six insecticides examined in affecting sensory perceptions. Therefore, our results suggested the potential of long-term exposure to the examined insecticides in the range of μg/L in inducing damage on sensory perceptions in organisms.