Neonicotinoid Thiamethoxam Impairs Honey Bee Flight

This is a brief review of a scientific paper published in 2017:

Tosi, Simone & Burgio, Giovanni & Nieh, James. (2017).

"A common neonicotinoid pesticide, thiamethoxam, impairs honey bee flight ability." 

Scientific Reports. 7. 1201. 10.1038/s41598-017-01361-8.

Quote From The Abstract

"Pesticides can pose environmental risks, and a common neonicotinoid pesticide, thiamethoxam, decreases homing success in honey bees.

Neonicotinoids can alter bee navigation, but we present the first evidence that neonicotinoid exposure alone can impair the physical ability of bees to fly.

We tested the effects of acute or chronic exposure to thiamethoxam on the flight ability of foragers in flight mills.

Within 1 hr of consuming a single sublethal dose (1.34 ng/bee), foragers showed excitation and significantly increased flight duration (+78%) and distance (+72%).

Chronic exposure significantly decreased flight duration (54%), distance (56%), and average velocity (7%) after either one or two days of continuous exposure that resulted in bees ingesting field-relevant thiamethoxam doses of 1.96–2.90 ng/bee/day.

These results provide the first demonstration that acute or chronic exposure to a neonicotinoid alone can significantly alter bee flight.

Such exposure may impair foraging and homing, which are vital to normal colony function and ecosystem services."

Introduction

As discussed elsewhere on this website, neonicotinoid pesticides are very widely used across the global, and on a very broad range of crops. 

Above: Honey bee drinking guttation on leaf stem

They are known to persist in the environment after their use. 

For the purposes of reviewing the above study, it is also important to remember that after use, neonicotinoids can be found:

  • in the nectar and pollen of plants visited by honey bees, and 
  • in the guttation (the drops of sap that gather on the leaves and stems of some plants – including those favoured by bees).

    Thus, bees visiting and feeding on such plants will be routinely exposed to neonicotinoid pesticides. 

    The authors of the above paper also note that other studies have shown: 

  • that chronic exposure to neonicotinoids can occur because bees can survive a single small dose but can then return to forage at the same contaminated food source, and 
  • that bees seem to prefer sucrose solutions containing neonicotinoids over pure sucrose – meaning that they return repeatedly to the contaminated food source.
  • that neonicotinoid pesticides (e.g. clothianidin, imidacloprid and thiacloprid) can impair bee navigation, foraging, homing and locomotion.  

Varroa-Free Bees Tested

Some of the studies examining the effect of neonicotinoid pesticides have been carried out using bee colonies known to have measurable levels of Varroa and it appears that the effect of the neonicotinoids used in these studies may be a synergistic one between neonicotinoid and Varroa, which leads to impaired flight distance in the bees. 

It has also been previously shown that ‘second generation’ neonicotinoid (thiamethoxam = TMX) reduces ‘forager return’ rates to the nest (i.e. in bee colonies that consume TMX, there is an increase in the number of bees that fail to return to the nest from foraging trips), suggesting that TMX impairs navigation or flight ability or both. 

The bee colonies used in this study did not have any measurable levels of Varroa, so the study demonstrates a direct effect of the study compound (thiamethoxam - TMX), rather than merely a synergistic effect between neonicotinoid and Varroa.

Therefore, this study demonstrates that TMX alone impairs the physical ability of bees to fly.

Key Study Design Features

  • The study was performed on 19 colonies – all confirmed not to have measurable levels of Varroa. 

  • The scientists used tethered bees in flight mills (a standard way of examining bee flying capabilities) to examine flight distance, flight duration, and flight velocity in bees.   
  • TMX in the treated group did not increase mortality (death) compared to the control group – it was a sub-lethal dose of TMX used.  This means that all the bees involved in the study remained alive for the duration of the study.  (Bees in the control group were given glucose solution without the TMX.)
  • The scientists examined both the acute (short-term) and chronic (long-term) effects. They examined four parameters in each part:
    - duration of flight;
    - distance flown;
    - mean velocity;
    - maximum velocity.  
  • To measure the acute effects of TMX exposure, each bee flew twice – once before exposure to TMX and again after exposure to TMX (glucose solution with 1.34ng TMX).  To measure the effects of chronic exposure to TMX, each bee flew just once after one or two day exposure.

Findings

Acute Effects.   

  • The ‘immediate’ effect of TMX exposure was an increase in parameters measured: flight time; flight distance and mean flight velocity – i.e. bees exposed to TMX flew longer, further, and quicker than non-exposed bees.  The effect of TMX was noted to occur within 1 hour of exposure.
  • Although the acute effect of TXM exposure seems to be positive (greater duration and distance of flight), it is actually negative, because it leads to exhaustion and energy depletion in individual bees.  
  • Additionally it has also been shown that similar doses of neonicotinoids also create ‘flight disorientation’ – bees fly more erratically and for longer distances, thus reducing the possibility of them successfully returning to the nest.

Chronic Effects.   

  • The scientists used four different concentrations of TMX – 1.95ng, 2.90ng, 3.71ng, 4.53ng.  Even the largest of these doses is significantly lower than the ‘worst case scenario’ volume that bees might consume in the wild according to an EFSA estimate.  
  • The scientists showed that chronic exposure to TMX reduce flight time, flight distance, and flight velocity.

Conclusion

The authors of this study state that a sub-lethal dose or chronic neonicotinoid exposure is sufficient to significantly alter honey bee flight ability.  This alteration may impair foraging and homing in affected bees – thereby creating a negative impact on colony function.


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