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 right margin for further information).
An interesting presentation here 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.
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.
“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 iis 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 are important because they can be present at field margins, or can occur through residues from previous treatments. They may contaminate the environment during application, or contaminate puddles and water sources.
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 non‐target species abundance 2010 showed that in aquatic ecosystems, imidacloprid leads to an increase in WATER MITES (hydracarina – again, of the taxon ‘Acari’)
"The order Hydracarina showed an opposite effect, as abundances were higher at high imidacloprid concentrations."
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:
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?
A healthy bee SHOULD be able to deal with mites in normal circumstances, but after exposure to sub-lethal doses of pesticide?..........
Of course, our pesticide regulatory system does not consider wild bees and pollinators currently, nor does it regard impacts of sub-lethal pesticide doses, nor interrelationships between pesticides and predators and diseases of non-target invertebrates.
But then - as stated before, the standards for pesticide regulatory tests are largely set by the agro-chemicals industry anyway.
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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):