Varroa mite chews the fat

Illuminating research on Varroa destructor has recently been presented at the 2019 Apiculture NZ conference.

19 July 2019

Hannah Davidson spoke to EPA staff members Dr Elodie Urlacher and Marieke Soeter following their attendance at the 2019 Apiculture New Zealand Conference, which had Rotorua buzzing with beekeepers, scientists and sniffer dogs alike. Read to hear about keynote speaker Dr Sammy Ramsey’s discovery, maybe see a sneaky dog video.

Read Hannah Davidson's biography

Read Dr. Elodie Urlacher's biography

Read Marieke Soeter's biography

Busy bees (and dogs) at the Apiculture NZ Conference

On 27 to 29 July 2019, Dr. Elodie Urlacher and Marieke Soeter attended the annual Apiculture New Zealand conference in Rotorua. As experts in insect impact assessments, staying up to date and engaged with the latest bee news and stakeholders is important to the EPA.

The three day conference, primarily pitched towards beekeepers and industry stakeholders, featured a full programme (with just the right amount of bee-related puns) including practical seminars, photography, and presentations on emerging issues facing beekeepers in New Zealand and abroad.

A number of sessions were focussed on the highly infectious bee disease, American foulbrood (AFB), which is caused by a spore-forming bacteria that affects entire colonies. It’s a grim prognosis for a hive with AFB. Under New Zealand law, the bees and associated equipment must be destroyed by fire, due to the persistent nature of the spores. An exciting and innovative new technique for detecting AFB in hives was demonstrated at the conference, and it involved dogs!

The unique smell of AFB infections has allowed for the training of AFB sniffer dogs, which can inspect a lot of hives in a much shorter timeframe than possible using us humans.

Watch a demonstration of an AFB sniffer dogs – YouTube

Whilst diseases like AFB are concerning enough on their own, the leading cause of global honey bee health decline remains Varroa destructor, more commonly known as the varroa mite.

Varroa fake news, debunked!

Of the speakers on this omnipresent global issue, one in particular stood out - Dr Samuel (Sammy) Ramsey. Hailing from the University of Maryland in the USA, Dr. Ramsey presented his recent debunking of the long held misconception that the varroa mite is more of a vampire than a flesh-eating variety of monster, a conclusion backed by an neat array of different puzzle pieces, that fall into place to align with experimental observations about the effects of Varroa on bees.

Since the first paper on the topic in the 1960s, scientists have carried the assumption that Varroa feed on honey bee haemolymph (a more insect-y form of blood). This paper, however, was written in Russian, and assumptions gathered from translation of the abstract made their way into the first English-language paper, henceforth becoming unquestioned fact.

Dr. Ramsey and co-workers, however, had noted several inconsistencies in this theory (Ramsey et al., 2019).

Solving the puzzle – what do varroa mites eat?

They considered the anatomy of the varroa mite, which appeared to lack essential features for feeding on the watery haemolymph. Instead, they found that their digestive system is structured in a way consistent with organisms that feed on semisolid tissue via extraoral digestion. This is a nightmarish process in which an organism spits outs a digestive enzyme soup into the tissue of their victim, then slurps it back up using specialised mouthparts. Varroa possess these mouthparts, and a look to their family tree suggests that they are closely allied phylogenetically with other extraoral digesting mites.

Three SEM images of the Varroa feeding site (labelled a and c) and the Varroa mite’s mouthparts (e). Scale bars read 2mm, 100 µm and 100 µm respectively for (a), (c) and (e).

Figure 1. SEM images of the feeding site on an adult honey bee (a). A close-up of the wound (c) shows a match to the mouthparts of the varroa mite (e). From Figure 2 in Ramsey et al, 2019.

To support their growing suspicions that Varroa are not well-adapted to sucking blood, Dr. Ramsey and co-workers looked to the poo, or the guanine waste product excreted by the mites. They found it to be characteristic of guanine associated with organisms that eat lots of protein, and do not consume much water. They proposed the hypothesis that, actually, Varroa feed on fat body tissue, not haemolymph, and performed a variety of experiments that approached the hypothesis from different angles.

By examining infected bees, they discovered that the mites have a strong preference (95.2%) for attaching to the lower abdomen of the bee, or metasoma, an area where fat tissue is found in adult bees. Using imaging techniques such as scanning electron microscopy (SEM), they could look closely to see if the mites left wounds in this area, and indeed they did (Figure 1). They also found evidence of digestive enzyme flesh soup near the wound site – a tell-tale sign of extraoral digestion. These pieces of evidence not only support the hypothesis that Varroa feed on fat body tissue, but also discredit another widely believed theory that Varroa feed solely on baby bees, and only use the adults to hitch rides.

In a separate experiment, the researchers used two fluorescent dyes that would associate only with the fat body tissue (red) or the hemolymph (yellow) of the bee. Mites were allowed to have a feed, and then their guts were examined using another sophisticated microscopy technique – confocal laser scanning microscopy. As expected, they glowed bright red (Figure 2).

Three images of the digestive system of Varroa who have been fed biostained haemolymph. (a) and (b) are dark with faint fluorescent green outlines. (c) shows the digestive system glowing a fluorescent red.  Scale bars read 200 µm for each image.

Figure 2. Images of the digestive system of Varroa fed upon biostained haemolymph (a), a control mite fed neither (b),and biostained fat body tissue (c). From Figure 6 in Ramsey et al, 2019.

To further bolster their findings, one last experiment was performed that saw the first in vitro – that is, in a petri dish – replication of the life cycle of this parasitic mite. The Varroa were fed haemolymph, fat body tissue or a mix of the two and monitored for survival. The results consistently showed that the bees fed fat body tissue survived longer and reproduced, whilst those fed on haemolymph died.

Multifaceted evidence for flesh eating Varroa

All of these proverbial puzzle pieces fit nicely together to shed light upon the bigger picture. Honey bee fat body tissue plays a crucial role in processes such as protein storage and synthesis, immune function, and pesticide detoxification. It makes sense then that known phenomena inability to replace lost protein, impaired immune function, and reduced pesticide tolerance occur when this tissue is compromised, something that haemolymph consumption could not explain. A deeper understanding of these issues might lead to the development of more effective treatments for Varroa infections and inform how we think about pesticide exposure.

It’s no wonder that this elegant piece of research was published in the prestigious Proceedings of the National Academy of Sciences, with a front cover feature, quite simply titled, “Varroa destructor feeds primarily on honey bee fat body tissue and not haemolymph” (Ramsey et al., 2019).

Thank you to Elodie and Marieke for sharing their experiences from the conference. Elodie also recently delivered an evening talk at Zealandia for Bee Awareness month, on 17 September. Her talk, entitled Bright brains on wings – the world of honey bees, explored the world of honey bees and all the amazing things they get up to.

References

Ramsey, S. D., Ochoa, R., Bauchan, G., Gulbronson, C., Mowery, J. D., Cohen, A., et al. (2019). Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph. Proceedings of the National Academy of Sciences, 116(5), 1792-1801. doi:10.1073/pnas.1818371116

Hannah Davidson's biography

Hannah hails from Otago/Southland, where she completed a Master of Science in Chemistry at the University of Otago, focussed on the topic of improving alternative plastic processes. Hannah joined the EPA in 2019 as Science Research assistant, bringing a passion for science communication and its role in environmental protection.

Dr Elodie Urlacher's biography

Elodie did her PhD in France (Toulouse, 2007-2011) on the effects of stress on learning and memory in honey bees. She then came to NZ to do a post doctorate with Prof Alison Mercer at the University of Otago, which was founded by a French grant (Fyssen, 2011/2012) then a Marsden grant entitled: En garde! The development of a stress response in bees and its impact on learning and memory (2013-2016). Elodie joined the EPA in 2016 as an advisor in the Hazardous Substances team.

Marieke Soeter's biography

Marieke did a Bachelor in Biology and a Master in Biological Sciences at the University of Amsterdam focussing on Ecotoxicology. She collaborated with several institutes (IBED, Alterra, NIOO) with a key interest in the effects of biotic interactions on the distribution of pollutants. After gaining regulatory experience in  a research lab in the Netherlands as a Study Director – Ecotoxicology she moved to New Zealand to work for the EPA as an ecotoxicology advisor (2016). In 2019 Marieke was appointed as a senior advisor in Ecotoxicology.