Abstract
Human-introduced stressors such as pollution, restructuring of natural habitats, or global warming affect all other organisms on earth as well. This dissertation investigates basic physiological, developmental and life history trait interactions between natural and artificial stressors in insects. My scientific approach assesses how a guild of closely related insect species responds to (1) basic nutritional parameters in their food and (2) environmental stressors such as pesticides, insecticides and nutrient depletion that they encounter daily in their natural grassland environment. I performed three laboratory experiments to understand how beneficial European sepsid dung flies (Diptera: Sepsidae) cope with various stressors, natural or artificial, applied in combination or alone, to predict how these stressors affect their population dynamics and future persistence should application by humans in agricultural landscapes remain unchanged. Chapter 1 aimed to describe how nutrient-enriched (dung enriched with yeast) and nutrient- depleted (autoclaved dung) substrates affect the growth and development of four coexisting European sepsid dung fly species (Sepsis cynipsea, S. punctum, S. thoracica, S. fulgens). Sepsid flies depend on cattle dung across all life stages for feeding and reproduction, and thus are extremely affected by dung texture, nutrient availability of and any extraneous substances in the dung. Our results confirmed that autoclaved, low-quality cow dung negatively affects most life history traits of most species assessed. In contrast, enriching dung by adding yeast did not enhance growth, development and reproduction of sepsid dung flies. This study shows that nutritional drivers of growth, which determine the ultimate fitness components (such as body size) and performance in nature, act in unpredictable ways and are still not completely understood for dung flies in particular and other species more generally. Therefore, further research is needed to unravel which nutrients affect a species’ life history precisely by which mechanisms, and how these developmental mechanisms interact with other extraneous stressors to potentially hamper performance in nature. Chapter 2 focused on the lethal and sublethal effects of one of the most widely used herbicides, glyphosate (Round Up©), on four dung-decomposing fly species (Sepsis cynipsea, S. punctum, S. thoracica, S. fulgens). While glyphosate putatively does not affect invertebrates, previous studies have shown that it may affect the fertility, fecundity and navigation of several invertebrates after all. Sepsid dung flies regularly encounter glyphosate in their daily environment on and around pastures, and our results showed delayed onset of egg laying as well as reduced offspring production in almost all species tested. These results suggest that exposure to realistic levels of common herbicides in agricultural landscapes can sub-lethally affect important life history traits of non-target insects, which may lead to drastic or subtle effects that ultimately can affect population dynamics and the stability of entire ecosystems. Chapter 3 aimed to investigate how a common veterinary antibiotic (Ivermectin) and a widely used bio-insecticide (Spinosad) applied alone or in combination affect five species of closely related dung-colonizing fly species in a close-to-natural scenario. Dung flies have previously shown high sensitivity to substances such as parasiticides excreted with cattle dung that their larvae feed on, with often strong effects on fly adults. However, studies combining substance effects over the flies’ entire life cycle are still scarce. My results are in line with previous studies showing that the combined exposure of two toxic substances across these dung flies’ juvenile and adult life stages negatively affect their life history, suggesting that more realistic future research should regularly test combinations of various stressors, as this is closer to today’s cultural scenarios. In summary, by employing a range of standard ecotoxicological but also elaborated experimental laboratory methods, I was able to study how various natural and human- introduced stressors, applied alone or in combination, affect the life history of one widespread and common, hence highly relevant insect study system of today’s agricultural landscape in Europe and beyond. From these results one could infer how such substances may cause lethal and, notably, sublethal effects that can alter these flies’ population dynamics to contribute to the recent insect decline observed.
Gourgoulianni, Natalia