Abstract
Feeding a growing global population in the context of a changing climate will bring challenges for food producers and environmental managers seeking to mitigate the impact of intensifying agricultural practices. The control of insect pest populations through insecticide application is an important practice for improving crop yields, yet contamination of the environment with insecticides combined with impacts from climate change are together subjecting ecosystems to novel stressors and stressor combinations. Contamination of surface waters with neonicotinoids, the most widely used insecticides in the world, has become a chronic global problem. Despite a growing body of research investigating the impacts of neonicotinoids in aquatic ecosystems, prior to my research there were no published toxicity data for their effects on aquatic insects in New Zealand. Moreover, international data on chronic, neonicotinoid-mixture, interactive multiple-stressor and community-level effects in stream ecosystems were also lacking. My overall PhD research objective was to investigate the ecological impacts of neonicotinoid insecticides on New Zealand’s freshwater macroinvertebrate communities in a multiple-stressor and climate-change context.
I developed and optimized laboratory procedures for testing chronic toxicities of neonicotinoids and observing their lethal and sublethal effects on stream insect larvae in controlled laboratory experiments. Through a series of 4-week experiments, the chronic toxicities of three commonly used neonicotinoids (imidacloprid, clothianidin and thiamethoxam) to nymphs of the ubiquitous New Zealand mayfly Deleatidium spp. were determined (Chapter 2) and possible interactions between the three insecticides were investigated (Chapter 3). In a subsequent 6-week multiple-stressor laboratory experiment, also with Deleatidium nymphs, I focused on possible interactions of low-level imidacloprid exposure with simulated heatwaves and a period of food limitation (Chapter 4). I then conducted a stream-side mesocosm experiment in 128 flow-through stream channels to investigate the individual and combined effects of realistic, pulsed exposures of imidacloprid and raised water temperature on stream macroinvertebrate communities characteristic of fast-flowing and slow-flowing microhabitats (Chapter 5). This experiment is the first empirical evaluation of stream macroinvertebrate community dynamics in response to the world’s most widely used agricultural insecticide, increased water temperature and reduced flow velocity (simulating streams subject to reduced flows due to water abstraction and climate change). Six hundred and forty invertebrate drift and insect emergence samples each were collected throughout the experiment (on five occasions during and after the insecticide pulses), and 128 benthic invertebrate samples were collected after 24 days of heating and insecticide manipulations. In combination, the long-term laboratory experiments using larvae of a sensitive aquatic insect taxon and the field experiment in stream mesocosms allowed assessing neonicotinoid effects on stream macroinvertebrates at the individual, population and community level in a multiple-stressor and climate-change context.
The 28-day concentration-response experiments (Chapter 2) and the neonicotinoid mixture experiment (Chapter 3) revealed imidacloprid as the most toxic of the three neonicotinoids to Deleatidium nymphs and also showed its potential for synergistic interactions with the other, comparably less toxic neonicotinoids clothianidin and thiamethoxam. Imidacloprid was therefore chosen for the subsequent multiple-stressor experiments (Chapters 4 and 5). Both of these demonstrated the strong effects of raised water temperatures on stream invertebrates and especially on Deleatidium larvae. In the 42-day laboratory experiment, sublethal and lethal effects of exposure to 0.4 µg/L imidacloprid took 2436 days to manifest and were clearest in the absence of heatwaves and starvation because these stressors alone already strongly reduced Deleatidium survival. In the mesocosm experiment, all three manipulated factors strongly affected invertebrate drift community composition, with the first pulse of imidacloprid and increased temperature having a greater impact on communities in fast-flowing channels. Heating and imidacloprid exposure both generally resulted in increased emigration by drift. Increased temperature was the most pervasive stressor for the benthic invertebrate community, negatively affecting 80% of response variables. This was in part due to a natural 10-day heatwave which occurred during the manipulative period, raising temperatures in ambient mesocosms to 29.8°C and in heated channels to 32.9°C. Water temperature in the river reached 31.2°C during a second heatwave seven weeks after the end of the experiment. The snail Potamopyrgus antipodarum showed the only positive response to raised water temperatures, also responded positively to slow flow and was unaffected by imidacloprid highlighting the general tolerance of this invasive species to stressors. Monitoring drift and emergence patterns periodically throughout the experiment also provided insights into how invertebrate community composition changed in response to the natural heatwave. Taken together, the findings of my thesis demonstrate the importance of efforts to mitigate climate change and reduce contamination of surface waters with imidacloprid in order to protect our vulnerable freshwater ecosystems.