In order to understand the possible adverse effects of neonicotinoids on bees we must first look at how these substances actually work. Even though the neonicotinoids have a type of neurotoxic effect, as has been shown in many studies, the insects do not die from this. There is namely, another more fundamental neonicotinoid mechanism.
It is generally accepted that neonicotinoids have a neurotoxic effect. The effect of imidacloprid and similar substances (mode of action) is, in particular, attributed to the irreversible binding to nAChR (acetylcholine-N-receptors). In studies into the neurological effects of imidacloprid and similar substances, and in particular the binding to nAChR receptors, very high doses were used. It transpires that there are multiple nAChR receptors to which the neonicotinoids cling. The effect and the results of the 7 or so most common neonicotinoids is, in this respect, the same.
From the tests with rats and cases of imidacloprid poisoning, among others – in suicide attempts – a picture emerges that the effect as irreversibly bound neurotoxicant can more or less be excluded in practice. Most organisms that were exposed to high doses of imidacloprid could recover quickly (within a few days) and recover well. This has also been observed in bees. The neurological effects are the consequence of interactions with dopamine. The neonicotinoids interfere, therefore, with another part of the neurological system. That this error is made is related to the fact that other insecticides do indeed have an effect on the acetylcholine receptors and scientists assume that this also applies to the neonicotinoids.
Systemic effect in plants
Neonicotinoids act as transporters of certain metals. This enables these metals (minerals) to be transported by the plant. They then emerge via the guttation fluid on the leaves and stems of the plant, as well as in the nectar and pollen. The result is a different micro flora in these plant parts that protects the plant against diseases and parasites. When the bees ingest the guttation fluid, nectar or pollen, they also ingest these minerals in large quantities. In turn, this also gives the bees a different microbial composition. The result is a range of effects, such as impaired digestion, weakening, diseases and ultimately a deadly infection.
Neurological symptoms in bees
The neurological effects observed in bees may also be attributed to the influence of viruses, in particular to DWV (Deformed Wing Virus) and the ABPV complex (Acute Bee Paralysis Virus). It is unclear whether the strong presence of these types of viruses is due to the effect of neonicotinoids.
The neonicotinoids themselves, as has been frequently observed, do not cause the neurological symptoms or death of the bees. The metals are the guiding factor here.