Nowadays, increasing environmental temperature acts in combination with other factors and influences the reproductive success of insects. Factors linked to changes in land cover and biodiversity loss, such as the presence of chemical contaminants, higher rates of parasitism, or the decrease in the availability of resources, can have negative consequences on the survival and reproduction of insects (Folt et al., 1999; Siviter et al., 2021; Verheyen and Stoks, 2022). When two or more stressors occur simultaneously, their combined effect on organisms can be additive (the sum of their individual effects), antagonistic (smaller effect together than alone), or synergistic (greater effect in combination of stressors than the sum of their individual effects). The latter are of particular concern, due to their great threat to living organisms (Folt et al., 1999; Verheyen et al., 2019; Siviter et al., 2021; Verheyen and Stoks, 2022).
Insects are particularly vulnerable to temperature changes since they depend on environmental temperature to control their internal temperature, and because their small size makes them more susceptible (Atkinson, 1994; González-Tokman et al., 2020a; Mastore et al., 2023). Moreover, adaptation to a different temperature is unlikely to occur quickly, increasing the threat of fast changes (García-Robledo et al., 2016), particularly for species or populations already living at the edge of their critical thermal limits (Sunday et al., 2014).
Heat can potentiate the toxicity of polluting compounds found free in the environment (Noyes et al., 2009). For example, an increase in temperature combined with the insecticide chlorpyrifos increases mortality and decreases growth rate and thermal tolerance in larvae of the damselfly Ischnura elegans (Odonata; Verheyen et al., 2019). Similarly, Culex pipens mosquitoes exposed to high temperature combined with chlorpyrifos had lower survival, developmental time, and size at emergence, indicating that some pesticides are more toxic at higher temperature (Tran et al., 2018).
Temperature also plays an important role in the activation of the immune system, since a thermal shock can be decisive in the survival of parasitized insects, or of their offspring (Wojda, 2017). For example, Gryllus texensis and Tenebrio molitor can modify their behavior when they are parasitized, inducing anorexia or causing behavioral fever, to face the immune challenge, and deal with the allocation of energy to their different metabolic pathways (Adamo et al., 2010; Catalan et al., 2012). Therefore, small changes in environmental temperature can induce physiological, morphological and behavioral changes, in addition to enhancing the effect of other biological and chemical stress factors (Sunday et al., 2014; Wojda, 2017).
Cattle pastures are habitats where different stressors occur, including excessive use of herbicides and veterinary medications that are excreted in dung, besides naturally occurring pathogens and parasites (Adamo et al., 2010; Cruz et al., 2012; Villada-Bedoya et al., 2019; Villada-Bedoya et al., 2020; Servín-Pastor et al., 2021). These stressors threaten coprophagous fauna, mainly flies and beetles. Worryingly, these harmful effects could be enhanced by the increasing environmental temperature, potentially resulting in negative effects on beetle fitness (Verdú et al., 2020; Ambrožová et al., 2021). For example, for the dung fly species Scatophaga stercoraria, the increase in temperature combined with the presence of ivermectin, one of the most widely used antiparasitic drugs in cattle, synergistically decreases the survival of developing larvae (González-Tokman et al. al., 2022).
Dung beetles (Coleoptera: Scarabaeinae) play a fundamental role in the functioning of cattle pastures (Doube, 1990; Nichols et al., 2008; Cruz et al., 2012; Doube, 2018). When they bury cattle dung to feed and reproduce, they fertilize the soil, reduce the exposure of cattle to feces, and reduces the emission of methane from dung into the atmosphere (Cruz et al., 2012; Miranda-Flores et al., 2020; Verdú et al. al., 2020). However, the presence of different stressors at high temperatures can reduce the survival and reproductive success of these organisms, affecting their function in the ecosystem (González-Tokman et al., 2017a; Ishikawa and Iwasa, 2019; Servín-Pastor et al., 2021). For example, an increase in temperature modifies the production of brood balls (each containing dung and an egg; Mamantov and Sheldon, 2020) and parasitic pressure reduces the size of such balls (Servín-Pastor et al., 2021). Medicines as, ivermectin delays development (Martínez et al., 2017; Ishikawa and Iwasa, 2019), reduces larval survival (Cruz et al., 2012; González-Tokman et al 2017b; Ishikawa and Iwasa, 2019), and the number of brood balls produced during reproduction (Pérez-Cogollo et al., 2016; Ishikawa and Iwasa, 2019), besides having other sublethal negative effects on their physiology and behavior (Verdú et al., 2018). Regarding pesticides as herbicides have been less studied, but some studies have shown them to be less harmful (González-Tokman et al., 2017a).
Even though some experiments have evaluated the effect of isolated stressors on some aspects of insect life history, it is still unknown whether an increase in temperature enhances the negative effects of other stressors that are applied to grassland, such as veterinary medicines and agrochemicals, or natural factors such as parasites that exert pressure on the coprophagous fauna. Pesticides are the second major threat for dung beetles in cattle pastures, only after habitat loss (Alvarado et al., 2017), but their effect in combination with high temperature has not been evaluated. The same is true for the effect of parasitism, which has not been evaluated in dung beetles but could also become more dangerous at high temperatures. Furthermore, since males and females may differ in their sensitivity to different environmental stressors, stressful conditions may cause changes in sex ratio, with potential implications for reproductive behavior and mate availability, and ultimately with consequences on population sizes and dynamics (Kappeler et al., 2023). But the combined effect of heat with other stressors on beetles remains untested.
Here, we evaluated the hypothesis that heat enhances the negative effects of three stressors: ivermectin, an herbicide, and parasitism, on the reproductive success, larval development time, sex ratio and survival of adult dung beetles Euoniticellus intermedius. We expected particularly synergic effects between heat and ivermectin, which is known to be highly toxic for these insects.