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Neonicotinoids and their effects on aquatic macroinvertebates

To be completely honest, I do not know what took me so long to write about neonicotinoids or "neonics", as they are commonly called other than I knew it was going to take a lot of time and research for me to understand the topic.


Certainly we all know a bit about nicotine, a rather addictive drug used by tobacco users worldwide. Nicotine is an alkaloid, a group of chemicals that includes quinine, which gives flavor to your gin and tonic; the caffeine in your morning coffee, and cocaine if you are into a much bigger "pick me up". Alkaloids are naturally occurring organic chemicals produced by organisms to defend themselves from predators, herbivores, or competitors. We use alkaloids in medical applications (e.g. quinine, morphine, and many compounds), in psychoactive drugs like psilocybin, and, of course, in agricultural / pesticide applications, the point of this post.

Almost all of us have heard about the issues facing bees and other pollinators and maybe have seen their declines linked to neonicotinioids in the environment (Lundin et al. 2015, Van der Sluijs et al. 2013). They have also been implicated in the decline of aquatic macroinvertebrates (Alsafran et al. 2022, Barmentlo et al. 2021, Morrissey et al. 2015). Neonics attack the nervous system of insects and are rather indiscriminate in their effects. That is they do not target specific taxa but rather are general insect killers. I plan to dig deeper into neonicotinoids and their effects on the environment, most notably trout streams.


What are Neonicotinoids and Why are they Used?


Neonicotinoids literally means "new, nicotine-like" substances. They are a group of insecticides produced to mimic the effects of nicotine, an alkaloid compound produced by members of the nightshade (Solanaceae) family which includes tobacco, tomatoes, potatoes, eggplant, and chili peppers, among many other species. As we so often do, we have found uses for these compounds - they are in our morning coffee, cigarettes and cigars, hot sauce, tonic water and malarial medications, pain killers and anesthesia, recreational drugs, and of course our insecticides.

Neonics are used in commercial insecticides because they work and they are easily - and relatively cheaply - manufactured. They are relatively new, first produced in a laboratory in 1970, were first sold as insecticides in the mid-80's, and by the late 90's, their use became much more common and widespread. Jeschke and Nauen (2008) call the discovery of neonicotinoids "a milestone in insecticide research" in Pest Management Science, a publication of the the Society of Chemical Industry. While this is certainly an industry-friendly view, neonics do have some significant advantages. They are fairly invertebrate-specific which means that they should have less effect on birds, mammals, fishes, and other vertebrates. They are easily applied, systematic (occur throughout the entire plant), and are the best defense against sucking insects like aphids, weevils, leafhoppers, and other common insect pests. They are water-soluble which means that they are readily taken up and absorbed by plants. However, because they are water soluble, they also are transported in the soil, groundwater, and runoff and are easily transported to streams and lakes where they have negative impacts on "non-target species" - aquatic invertebrates and fishes - which will be our main concern here.

Neonics attack the nervous system of insects - much like their effect on us. But the dose makes the poison and we are MUCH larger than your typical insect. They, like a number of drugs and medicines, mimic neurotransmitters which results in continuous, uncontrolled nerve stimulation. This overstimulation results in the paralysis and eventual death of insects. A large part of the calories taken in by any heterotroph, humans included, are used to maintain the resting potential across the membranes of neurons. Neonics disrupt this by causing the continual firing of neurons, a rather energy expensive task in animals.


Neonicotinoids in the Environment


Before we move to the aquatic environment, there are a host of other documented environmental issues with neonicotinoids with bees and colony collapse disorder (CCD; van der Sluijis et al. 2013, Lundin et al. 2015) receiving the most attention. However, research suggests that there is not a single, "smoking gun" explanation for CCD, rather a host of factors (parasites, disease, stress due to colony transportation, loss of habitat, environmental chemicals, and others) may lead to CCD and different factors may be responsible in different locations. While more needs to be known, some countries and states have eliminated or at least greatly reduced the use of neonicotinoids.

The Wisconsin Neonic Forum was hosted in the fall of 2024 and these videos contain a great amount of information about neonics and their effects.

The rest of the series:


Neonics are widely used because they are systemic, that is they move into all parts of the plant, and remain in the environment for a long time. They are also water soluble, a characteristic that allows them to relatively easily move from where they were applied to groundwater, streams, and other waterbodies. They have high specificity to insects and jhave low toxicity to mammals, a characteristic that makes them seem "safe". Unlike many other agricultural chemicals, they do not affect humans, at least not at the concentrations they are typically found in nature.


Neonics are most commonly used as seed treatments. In fact, it is often difficult for farmers to buy seeds that are not treated despite the fact there is little evidence that they are cost effective. From Mourtzinis et al. (2019):

Despite widespread use, this practice appears to have little benefit for most of soybean producers; across the entire region, a partial economic analysis further showed inconsistent evidence of a break-even cost of FST or FST + nSt. These results demonstrate that the current widespread prophylactic use of NST in the key soybean-producing areas of the US should be re-evaluated by producers and regulators alike.

Despite a lack of evidence of their cost effectiveness and much evidence of their ecological impacts, neonicotinoids are commonly available and used in most US states. A few states (California, Colorado, Maine, Maryland, Minnesota, Nevada, New Jersey, New York, Rhode Island, and Vermont; link) have banned or restricted the use of neonics. The European Union has been well ahead of the states in limiting the use of neonics (link). In Wisconsin, more than 500 neonics are registered for use (Senger et al. 2018) and neonics are common in groundwater and surface waters in the state (Senger et al. 2018, Bradford et al. 2018). Wisconsin currently has no restrictions on neonic use.

Image from US EPA and USGS. Neonics have grown in their use in the past two decades.
Image from US EPA and USGS. Neonics have grown in their use in the past two decades.

The environmental effects of neonics are widespread and not limited to their effects on insects. Insects play many important ecological roles such as pollination of plants, as predators and herbivores, and numerous other roles. In aquatic systems, insects are an important part of secondary production - they move plant matter to fishes (stream productivity post). Aquatic insects are important in the River Continuum Concept (RCC post) because they play the functional roles of grazers, shredders, and collectors - activities that move primary productivity into predator food webs. Neonics have indirect effects on fishes by altering stream food webs (Sánchez-Bayo et al. 2016, Yamamuro et al. 2019) and have direct effects on fishes (Gibbons et al. 2015).


Effects on Aquatic Invertebrates and Food Webs


Neonicotinoids are indiscriminant insect killers - they are rather specific to insects - not that they do not have some effects on vertebrates like fishes. However, their largest effect on fishes are through altering food webs through their effects on secondary production. Trout, particularly before reaching 12 to 14 inches, are highly insectivorous and the forage of larger trout are heavily dependent upon invertebrate food sources.

A rock from a Driftless Area stream - lots of productivity.
A rock from a Driftless Area stream - lots of productivity.

Some aquatic macroinvertebrates are more affected by neonics than are others. Mayflies (order Ephemeroptera) are among the most susceptible to neonics. Caddis (Trichoptera) and midges (family Chironomidae) are also quite effected where as crustaceans are generally less effected. Are neonics responsible for the decline in our hatches? They probably - or almost certainly - are part of the puzzle but they are probably one of the many factors that have had an effect. I'll dig deeper in a future post.


Effects in Wisconsin


The concentrations of neonics in Wisconsin waters are greatest in areas with sandy soils so the Central Sands and the Wisconsin River valley are most affected. A recent study found that 67% of 317 samples from 91 central Wisconins wells were positive for TMX (thiamethoxam, a neonicotinoid) and 78% of well tested positive at least once during the study (Bradford et al. 2018). The other high detection area is the sand plains within the lower Wisconsin River valley. Not surprisingly, neonics are more mobile in sandy soils and less mobile in clay soils.

Map of neonicotinoid detections in groundwater wells from the Wisconsin Department of Agriculture, Trade, and Consumer Protection (DATCP). (link)
Map of neonicotinoid detections in groundwater wells from the Wisconsin Department of Agriculture, Trade, and Consumer Protection (DATCP). (link)

I plan to write more about the effects of neonics but have kept it short as this post has grown long enough. Want to learn more about neonics? I suggest watching the videos from the 2024 forum (linked above) and visiting the links below.


Links


Literature Cited / References











Sánchez-Bayo, F., Goka, K. and Hayasaka, D., 2016. Contamination of the aquatic environment with neonicotinoids and its implication for ecosystems. Frontiers in Environmental Science, 4, p.71.






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