How do neonicotinoids affect bees?

How do neonicotinoids affect bees? As I’m sure you know, changes in the the honey bee population has been a subject of much debate for the last decade. The chart below from Bee Informed shows that around 23% of bee colonies were lost in the US during the winter of 2015/2016, significantly higher than expected losses during hibernation. But bees are also being lost in spring and summer, through abrupt colony collapse disorder (CCD) or a gradual population decline, and the total loss in the US for 2016 was 44%.  

Frustrated with media coverage on the subject, which is rarely objective, I thought I’d spend some time researching and summarizing. It’s a complex issue of course, so the most I can hope to cover in one or two posts is the most commonly asked question: Do neonics have a significant effect on the bee population?

So briefly, some background:

How do neonicotinoids work?

How do neonicotinoids affect bees? Neonicotinoids (commonly known as neonics) are insecticides that act by binding to central nervous system receptors in insects, causing paralysis and death. The market is dominated by three insecticides: imidacloprid and clothianidin from Bayer and thiamethoxam from Syngenta. Seed coatings were developed in the 1990s to deliver the product more directly to the plant – for example, imidacloprid-coated seeds sold by Bayer under the trade name Gaucho. The insecticides are currently banned in Europe but widely used in some countries – almost all of the corn and most of the soybeans and cotton grown in the US in 2013 started as neonicotinoid-coated seeds. This article, published in Science, questions whether the coatings provide any benefit since sunflower and corn yields were unchanged after neonicotinoid-coated seeds were banned in France.

How do neonicotinoids affect bees?

There are concerns over the impact of neonicotinoids on aquatic and bird life, but I’ll just focus on honey bees here. Neonics are systemic insecticides – present in all parts of plant – so bees ingest them when they feed on pollen and nectar. There’s no doubt that they are ingested by bees and also toxic to them – the debate centers around the question of how much the bees ingest and whether this affects their health. Here’s a quick pictorial guide to some key numbers:

I’m showing conservative numbers here – the quantification in pollen and nectar and the lethal dose (LD₅₀) all come from a paper published by Bayer scientists in 2001 (Ref 1). The estimate on how much imidacloprid a bee ingests comes from a 2005 paper by French scientists (Ref 2). Other research groups have found higher concentrations of imidacloprid in nectar and pollen – for example, this group studied squash that was treated with the insecticide by spraying on soil or via drip irrigation after transplant. On average, they found 10 ppb imidacloprid in nectar and 14 ppb in pollen, and the highest concentration encountered in pollen was 28 ppb.

Are neonicotinoids toxic to bees?

We know they are toxic. There’s no debate there – they are insecticides after all. So, the key question is: Are neonicotinoids toxic to bees at levels encountered in the field?

Even taking the conservative numbers from Bayer (1.9 ppb in nectar) the estimated weekly intake (for a bee that’s foraging contaminated nectar) is in the range of a single lethal dose, but spread out over a week. To put it in a colorful way, if someone offered you a glass of poison, saying it’ll probably kill you if you drink it in one go but you need to drink a bit each day and finish it in a week, would you want to do it? Even if a bee is not killed directly, neonics have been shown to affect their ability to navigate, forage and reproduce. And since bees colonies operate as collectives, even small changes in the dynamic can cause the entire colony to collapse. I’ll cover these aspects in Part 3.

In the next part I’ll take a look at a key paper that’s often cited as evidence that neonics are not harmful to bees. It’s an unsettling story that suggests interference in the scientific process by the companies that make neonics via corruption in the US Dept. of Agriculture, a retired professor, and a journalist with an industry-sponsored agenda.

(The original title of this post was Daily Footprint, #15 – Honey: So what exactly is happening to the bees?)