Neonicotinoids And Varroa Mite

  • Some research suggests a link between neonicotinoids and increased abundance (even outbreaks)  of certain mites belonging, like Varroa mite, to the arachnid taxon ‘Acari’ (i.e. mites and ticks).

  • Honey bee colony losses have been blamed on Varroa mite by some parties – especially the agrochemicals industry, who seek to deflect attention away from neonicotinoids as a primary cause of bee deaths.  But is there actually a relationship between neonicotinoids and increased Varroa mite abundance?
  • Of course, Varroa are specific to honey bees, but other species of bees and insects, are susceptible to mite predators. 

    Could wild species be more vulnerable to mites because of neonicotinoids?

A couple of years ago, I wrote a piece about bees and Varroa, specifically raising the question of whether or not neonicotinoids hinder the insects ability to groom away, and deal with Varroa mites.  (see the block below for further information). 

The importance of grooming has also been highlighted as a defence against Varroa mite in both Apis cerana (Peng et al., 1987) and Apis mellifera (Ruttner and Haenel, 1992).  

Indeed, it is well known that insects and some other invertebrates engage in grooming behaviours as a way of maintaining cleanliness, cleaning away fungi, dirt, and mites.  Even a casual search on YouTube reveals bees and other insects engaging in such behaviours.  You can see some examples here (clicking these links opens a new window):

One interesting presentation proposes how the general weakening of a honey bee colony due to pesticide exposure, ultimately favours Varroa mite, interfering with brood care, reduced jelly in brood cells, and ultimately enabling Varroa mite to reproduce more prolifically.

Do Neonicotinoids 'Assist' Mites In Some Way?

A paper by Zen and Wang 2008, investigated the impact of the neonicotinoid imidacloprid, on a mite:

Chun-Xiang Zeng and Jin-Jun Wang 2008:  Influence of exposure to imidacloprid on survivorship, reproduction and vitellin content of the carmine spider mite, Tetranychus cinnabarinus.

It states:

“The results showed that the field-relevant dose of imidacloprid did not significantly affect the hatch rate of eggs or pre-imaginal survivorship of the mite, while sublethal doses of imidacloprid, previously determined for Myzus persicae, led to a significant increase in the hatch rate of eggs and pre-imaginal survivorship of the mite compared to the untreated control.”

Note, although the pesticide was intended to control the ‘Green Peach Aphid’ Myzus persicae, it highlights that at sub-lethal doses, the effect of imidacloprid was to increase the hatch rate and survival of another pest – a mite.

The issue of 'sub-lethal doses' is important. 

The fact is that:

  • sub-lethal doses (i.e. tiny doses that do not cause immediate death, but can impair function, ultimately leading to death) are barely assessed in regulatory tests for pesticides - and this issue has been picked up on by EFSA.
  • Sub-lethal doses can be present at field margins, or can occur through residues from previous treatments in the soil. 
  • Sub-lethal doses may contaminate the environment during application, or contaminate puddles and water sources. 

As you'll see below, Bayer themselves acknowledge the importance of sub-lethal effects in the killing of 'pest species', so it's ironic that such effects are not adequately assessed for product registrations.

Neonicotinoids may harm the natural insect predators of mites....

Another paper found that outbreaks of a mite were linked to a neonicotinoid, imidacloprid, in part by debilitating predators of mites:

Conclusion: “Imidacloprid increases spider mite fecundity through a plant-mediated mechanism. Laboratory experiments provide evidence that imidacloprid debilitates insect predators of spider mites suggesting that relaxation of top-down regulation combined with enhanced reproduction promoted a non-target herbivore to pest status”.

Tessa Van Dyk’s study: Effects of neonicotinoid pesticide pollution of Dutch surface water on nontarget species abundance 2010 showed that in aquatic ecosystems, imidacloprid leads to an increase in WATER MITES (hydracarina – again, of the taxon ‘Acari’)

Page 27:

"The order Hydracarina showed an opposite effect, as abundances were higher at high imidacloprid concentrations."

Of course, the above information relates to mites generally, and ways in which neonicotinoids almost appear to 'favour' the mite taxon in some way.  The research papers above do not specifically refer to Varroa mite. 

Nevertheless, it would take a major leap of faith for me to assume that neonicotinoids could in some way, increase populations of just "certain mites", yet conveniently have no effect on Varroa mite - what do you think?

At the very least, I suggest investigation is needed.

Impact of neonicotinoids on natural defences

Bees have numerous defenses against mites, fungi and predators.  It’s no surprise if neonicotinoids are actively hindering those natural defences.

So far, I think questions need to be asked:

  1. do neonicotinoids hamper the bee's first line of defence - i.e. grooming away mites (- see Varroa Mite And Honey Bee Defences  to watch a video and read an interesting Bayer Cropscience leaflet!)

  2. do neonicotinoids compromise the bee's immune system, meaning they are more likely to die from diseases spread by Varroa?  This issus has already been raised by Alaux et al in his Nosema paper

    This issue has been raised in relation to bees and pesticides generally, as well as other wildlife.

  3. as detailed in the link above, do neonicotinoids hamper the colony such, that the ideal conditions are created for Varroa to thrive?

And there is a further question:

.........if neonicotinoids can and do help to cause a proliferation of mites, how is this affecting not only honey bees, but also bumblebees, solitary bees, and other non-target invertebrates, all of which have their mite predators to contend with?

The regulatory system for insecticides is inadequate to robustly assess these issues.

  Pssst ... spread the word!

leafcutter bee on sweet pea plant sweet peas for bees