How will insecticides affect streams under climate change?

How climate change will alter the effects of toxicants is a key concern in the 21st Century. Rising water temperature can increase the toxicity of some contaminants to stream-dwelling animals in laboratory conditions, as has been shown for the most widely used insecticides in the world – the neonicotinoids. But until this study, whether this translates to more realistic environmental scenarios remained to be tested. In this blogpost, Sam Macaulay talks about his mesocosm experiment studying the effects of rising water temperatures and neonicotinoids on stream invertebrate communities.

Why do this research?

As average global temperatures rise due to climate change, it is becoming increasingly important to understand how this might affect the toxicity of contaminants in the environment. Temperature is known to alter the toxicity of many contaminants, and I had previously shown that this occurred in laboratory experiments using stream mayflies exposed to a neonicotinoid insecticide across a range of temperatures. In order to see if this would happen in the real world, I set out designing a large-scale stream mesocosm experiment to test the combined effects of raised water temperature and pulses of the world’s most widely used insecticide on stream invertebrate communities.

What did I do?

In an experimental system comprising 128 small, circular stream channels that receive water and organisms from a nearby river (Figure 1), I manipulated three variables. Firstly, half of the streams had reduced flow velocity which is a common effect of agriculture – due to abstracting water for irrigation – and climate change, due to increased severity of droughts. In these fast-flowing and slow-flowing streams, I then subjected half to raised water temperature by 3 °C, simulating future climate warming under a high emissions scenario (for New Zealand, where the experiment took place). Lastly, I added the neonicotinoid insecticide imidacloprid, at four environmentally realistic concentrations from 0 to 4.6 micrograms/L (Figure 2).

Figure 1: Circular tream mesocosms used in the experiment (cc: James Orr)
Figure 2: Schematic of the experiment (by Sam Macaulay)

Although pesticides are applied onto land, many easily dissolve in water and get washed into surface waters including streams and rivers where they can kill aquatic insects which are important for the health of aquatic ecosystems and the surrounding terrestrial ecosystems that they support. The highest concentrations of pesticides and other contaminants enter waterways during rainfall as runoff pulses. To simulate such exposure scenarios, I added the insecticide in my experiment in three, 48-hour pulses over a 24-day period. During these pulses, I collected the stream invertebrates that drifted out of the channels and emerged as adult insects. These are active responses that stream organisms can use to evade stream conditions they don’t like – like pesticide runoff or warmer temperatures!

What happened?

One thing you can’t control when doing an experiment in the field, is the weather. Rain, hail, or shine – the experiment must go on! During my mesocosm experiment, we had all three. But the latter occurred a bit more than we expected. During the 10 days from the second to third insecticide pulse, an intense heatwave struck. The result was that we serendipitously captured the effects of a natural heatwave under future climate warming scenarios – and the results were drastic. 

Warming was, by far, the most pervasive stressor in the experiment overall, negatively affecting 80% of responses in the benthic invertebrate community (the bugs living in the stream-bed). The combination of simulated climate warming and the natural heatwave strongly reduced populations of heat-sensitive insects such as mayflies and caddisflies , thereby causing a shift to communities dominated by more tolerant invertebrates such as stream-dwelling worms, crustaceans and snails. 

Due to these strong temperature effects, my hypothesis that warming would worsen the effects of the insecticide was not supported. Instead, the invertebrate communities in unheated streams were most negatively affected by the insecticide pulses because these contained higher numbers of pollution-sensitive species. The same type of result occurred when looking at the combined effects of reduced flow velocity and imidacloprid (the insecticide we used): invertebrates in fast-flowing streams were more negatively affected by imidacloprid than those in the slow-flowing streams. These results have important implications for management and conservation of streams.

Why does it matter?

The key finding that invertebrate communities from fast-flowing and unheated streams – which often contain more pollution-sensitive species – were most strongly affected by insecticide runoff suggests that these healthy streams should receive protection from water abstraction (which reduces current velocity) as well as pesticide contamination to maintain their important biological diversity. This biodiversity is important not just for the health of the stream ecosystem, but also for the surrounding ecosystems. The emergence of aquatic larval insects into flying adults provides a key link to terrestrial ecosystems, as these adult insects become an important food source for many land-dwelling animals including birds, bats, reptiles and spiders.

Our study suggests that increasingly severe and frequent heatwaves occurring at higher average global temperatures will significantly affect freshwater invertebrate communities, especially those in streams with high proportions of sensitive insects. In addition, these ecosystems will also be degraded by imidacloprid where its prophylactic use is widespread and contamination of the environment unmonitored. Although crop protection methods are crucial for reducing yield losses from pests, sustainable pest-control procedures must be implemented to mitigate the environmental harm and reductions in ecosystem services from widespread use of systemic pesticides.

The world is acutely aware of the impacts of climate change and there are global efforts being made to reduce their severity. However, despite a global call from scientists to restrict use of neonicotinoids in 2018 – especially their prophylactic coating of seeds – most countries outside of the EU and Canada have taken no steps to mitigate their use. The biodiversity, ecological integrity and long-term sustainability of many vulnerable ecosystems will depend on continued global action to combat climate change and to alleviate the environmental impacts of harmful synthetic chemicals such as neonicotinoid insecticides.

The attached link is a short video shot at the mesocosm system field site where the experiment took place – Video

This study was authored by Sam Macaulay, Kimberly Hageman, Jeremy Piggott, Noël Juvigny- Khenafou and Christoph D. Matthaei and was published in Global Change Biology in August 2021.