Varroa Mite and Neonicotinoid Pesticides

[From Feb 2011]
Varroa mite is one of the biggest threats to honey bee colonies. In the UK, it was first discovered in 1992. Exposure to this pest causes viruses and diseases to be transmitted to honey bees, such as Deformed Wing Virus. But could it be that neonicotinoid pesticides play a role in Varroa too?

So far, it seems that some people who contend that neonicotinoid pesticides do not pose "unacceptable risk" to honey bees and other inverterbrates, also insist that colony losses in honey bees are largely due to diseases transmitted through Varroa.

Treatments against Varroa mite are increasingly found to be ineffective, the mites supposedly having developed a resistance to the chemical treatments available.

However, a key natural defence for honey bees against Varroa is for the bees to become “hygienic” – this means, the bees are able to groom and remove the mites from larvae and their bodies - something it seems, many of our bees are not doing – for some reason. In fact, there are currently efforts to breed “Hygienic bees” that are more likely to engage in this crucial grooming behaviour. But, given the mode of action of neonicotinoids, is it surprising if these insecticides have, in the first place, hindered the ability of bees to develop this grooming ability?

Actually, Bayer advertises the mechanism by which sub-lethal dose of imidacloprid, one of their neonicotinoids, kills colonies of social insects. The key is that disoriented social insects stop grooming and thus get infected with natural pathogens. Here is the quote from the Premise 200SC leaflet. Premise 200SC, is a Bayer product for Termites, which like bees, are social insects. The leaflet reads:

“The termites are susceptible to diseases or fungi found in soil. A principle part of their defence mechanism is their grooming habits, which allows the termites to get rid of the fungal spores before these spores germinate and cause disease or death. Premise 200SC interferes with this natural process by lowering defence to nature’s own weaponry.”

"What is Premise 200SC plus Nature?

Low doses of imidacloprid such as the edge of the Treated Zone, disoriented the termites and caused them to cease their natural grooming behaviour. Grooming is important for termites to protect them against pathogenic soil fungi. When termites stop grooming, the naturally occurring fungi in the soil attack and kill the termites. Imidacloprid makes fungi 10,000 times more dangerous to termites. Nature assists imidacloprid in giving unsurpassed control. This control is called Premise 200SC plus Nature."

Could it be, then, that neonicotinoids interfere with grooming behaviour in honey bees, making them more likely to succumb to Varroa mites? It seems to me this issue adds further weight in favour of a precautionary suspension, in line with the request from Invertebrates charity, Buglife.

Further evidence of the effects of neonicotinoids on grooming behaviour has also been seen in beetles, including having an impact on their larvae. This published study, titled Synergism of imidacloprid and entomopathogenic nematodes against white grubs: the mechanism; by Albrecht M. Koppenhöfer et al in 2000, states:

"The major factor responsible for synergistic interactions between [LOW DOSE!] imidacloprid and entomopathogenic nematodes appears to be the general disruption of normal nerve function due to imidacloprid resulting in drastically reduced activity of the grubs. This sluggishness facilitates host attachment of infective juvenile nematodes. Grooming and evasive behavior in response to nematode attack was also reduced in imidacloprid-treated grubs."

"Brushing (legs or mouth parts swept across body)……and chewing ….occurred significantly more often in grubs not treated with imidacloprid in the presence of nematodes and this response was reduced by 42--70% after imidacloprid treatment."

It has also been demonstrated that the interaction between the microsporidia Nosema and a neonicotinoid (imidacloprid), significantly weakened honeybees. This study by Alaux et al was published in Environmental Microbiology 2009: Interactions between Nosema microspores and a neonicotinoid weaken honeybees (Apis mellifera)



Grooming and Varroa

Andinof, G.K. & G.J. Huntf - A NEW ASSAY TO MEASURE MITE GROOMING BEHAVIOR - Grooming behavior is one of the known mechanisms of defense for honey bees against parasitic mites. Varroa destructor is often considered the biggest beekeeping problem within the U.S. and around the world. Mite-grooming behavior has been described as the ability of the adult bees to remove Varroa mites during grooming and has been associated with mites that have been chewed by the bees' mandibles, but the proportion of chewed mites is extremely tedious to measure.
We developed an easier assay to measure mite-grooming behavior that can be used for selection in breeding programs. Wood cages with screened tops and bottoms were used to hold a frame of bees collected from the brood nest. Bees were transferred to comb containing pollen and nectar but without brood. The mites removed during grooming were collected in sticky boards for three days at room temperature (22-25 °C) and then counted. The remaining mites on the adult bees were collected and counted using carbon dioxide (CO2) to anesthetize the bees and powdered sugar to remove the mites. The percentage of the mites removed was calculated.
A significant relationship (p = 0.0285) was found between the proportion of mites removed in the lab assay and the proportion of chewed mites on sticky boards from the source colonies. This relationship indicates that the colonies that removed the highest percentage of mites in the caged adult bees were also the colonies that had the highest percentage of chewed mites (Figure). These results suggest that the method used to measure mite-grooming behavior is effective. In addition, we also found a negative relationship (p = 0.0072) between the percentage of mites removed and mite infestation of adult bees, which indicates that the colonies with the highest percentage of mites removed in the cage assay, had the lowest population of mites on adult bees. These results suggest that the low population of mites present on the adult bees is due to grooming.