Based on the science available to date, there is evidence to support the claim that bats are being negatively affected by neonicotinoid insecticides in several different ways, indirectly through reduction in insect abundance and directly through impairment.
Despite being generalists, bats are highly dependent on an abundant supply of insects in the right size class. Their foraging expenditures are such that they require high insect densities in order to provide a net energetic balance and their echolocation system limits the size range of insects available to them. Reduced prey availability will cause bats to stop foraging altogether and wait for better conditions.
There is current evidence for reduced insect densities associated with agricultural intensification. Although this intensification predated the neonicotinoid class of insecticides, there is increasing evidence that neonicotinoids are worsening the situation and hastening insect declines. This is because of their persistence, mobility and systemic nature, their very high invertebrate toxicity as well as their indiscriminate use in prophylactic treatments.
Most of the evidence to date for a neonicotinoid-accelerated decline is for terrestrial insects (e.g. pollinators, butterflies, predators and parasitoids). However the extent and level of aquatic contamination from neonicotinoid insecticides as well as the heavy use of surface water by many of our bat species suggest that a reduction in aquatic emerging insects is a key aspect of the bat vs. neonicotinoid insecticide question – indeed as has been shown to be the case with insectivorous bird species (Hallman et al. 2014).
The impact of neonicotinoid insecticides is very much extended both in time and space compared to the aquatic impacts from older products such as organophosphorous or pyrethroid insecticides. There is evidence that entire watersheds are being contaminated at damaging levels on a year-long basis.
In addition, the best available evidence strongly suggests that neonicotinoid insecticides are affecting bats directly. Despite the fact that these insecticides tend to be of lower toxicity than some of their predecessors (e.g. many organophosphorous insecticides previously registered), bats can be exposed to toxic levels, especially on the short and medium term (i.e. acute and summer-long exposures).
There is a real potential for bats to be acutely affected if they forage in or on the edges of treated fields or tree crops. Levels of residues expected, whether from foliar, air blast or seed treatment uses are high enough to put bats at risk of motor impairment and death. It is somewhat counter-intuitive that risk can be as high as or higher from seed treatments than from sprays applications. This is because of the high amount of dust generated in the seeding process. Because seed treatments are currently used prophylactically on a large proportion of the total crop area for our major field crops (e.g. corn, soy, cereals, oilseeds), there will be a risk to bats on most of our agricultural crop area.
Mineau, P. and C. Callaghan 2018. Neonicotinoid insecticides and bats: an assessment of the direct and indirect risks. Canadian Wildlife Federation. 87 pp.