Neonicotinoids (neonics) are the most commonly used pesticide globally. They are widespread partly because they are highly toxic to insects without being very toxic to mammals, and can be applied in a variety of ways. They are most commonly applied as a seed coat- when the plant germinates it draws the pesticide into all it’s cells, from leaves to pollen- but can also be applied as a spray.
Using a seed coat pesticide marks a shift away from an integrated pest management approach that uses natural enemies and only uses chemicals when certain thresholds of damage are surpassed, toward application of pesticides before they are needed, in fact, before the crop even germinates.
In a new review in Ecological Applications, Dr. Dave Goulson, outlines the environmental risks of this new class of pesticide that is sweeping agricultural areas throughout the world. He asks whether the evidence shows that neonicotinoids are safe, or whether they have dangerous consequences we should consider.
First, how much economic edge do neonics give farmers over other types of pesticides? Though this is relatively unstudied, Goulson suggests that the little evidence available points to little benefit. In trials of treated and untreated soybeans, there were no differences in crop yields between fields treated with the main neonic on the market, imidacloprid, and those that were not. In winter wheat, a seed coating cost more than the added benefits accrued from lack of pest damage. One study, again on wheat, found that there were benefits of a foliar spray, but none from a seed coating.
Pesticide coated seeds are sown directly into the ground, and evidence is increasingly showing that most of the pesticide doesn’t make it into the plant, but instead accumulates in the soil. Only about 16-20% of the active ingredient is absorbed by the crop. Some (~2%) flies off as dust while seed are being sown. The rest remains in the soil, building over time, or goes into waterways as part of agricultural runoff. The chemical can remain in the soil for between 200-1000 days depending on the soil conditions. Fields in France were randomly sampled for traces of neonicotinoids, and the majority of samples from conventional farms had traces of the chemical, even those that had not been treated for more than 2 years. This could lead to uptake of neonics by non-target plants, including crops and hedgerows.
Neonics have been detected in all kinds of waterways, including rivers, streams, storm-water ponds and tidal creeks. The substance has even been found in groundwater. Neonics may be under-detected because they are not currently part of water monitoring programs. The levels and frequencies of contamination may be higher than currently thought. Again, plants along waterways or irrigated with contaminated water may absorb the pesticide, even when application was not intended.
Having consistent levels of the chemical in the environment can lead to increases in resistance, which could be very damaging if it occurs in pest insects. Resistance has already been recording as occurring in as little as 3 years from introduction of the pesticide- which is heightened in species that have short generation times.
Given that neonics are present in may environments after they are sprayed, the next question Goulson asks is whether this has lingering effects on populations of invertebrates- particularly beneficial insects, such as pollinators. Neonics are commonly used as a seed coat on crops that provide floral resources to bees, particularly oilseed rape and sunflower. They are also used as foliar sprays on fruit crops, such as raspberries. The highest concentrations of the pesticide are found when they are applied through irrigation water. Bees collecting pollen and nectar can be exposed multiple times, which can either lead to death, or other non-lethal complications such as reduction in foraging and homing ability (being able to return to their nest). Bumble bees were found to produce fewer offspring when they foraged on neonic treated crops. Most studies to date have focused on adult bees, but larval bees that consume pollen could also be exposed to low concentrations of the active ingredient, and therefore exhibit similar problems as those seen with mature bees.
According to Goulson, it is apparent that the increasing rate of neonicotinoid application in crop fields is having a negative effect on biodiversity conservation in agricultural landscapes. This means a reduction in expected ecosystem services provided by beneficial insects such as pollinators and natural enemies. Concentrations in the soil may harm essential organisms that paly a role in soil health and fertility. He suggests re-evaluating the current high use of neonics in farming, and considering appropriate reductions that better balance all aspects that go into farming and stewardship.
Goulson, Dave. “Review: An overview of the environmental risks posed by neonicotinoid insecticides.” Journal of Applied Ecology 50.4 (2013): 977-987.