Introduction
The new year got off to a snowy start here in the Highlands, and the first full moon of the year, a stunning wolf moon, slipped down slowly like a hot air balloon with a slow puncture below the treeline as daylight crept in today on 3 Jan. Today is also a special day because my lovely grandson Leopold turned two and went sledging for the first time. Leo will probably be a beekeeper and he already helps me collect the varroa boards to count the fallen mites. Speech and language are developing a pace and ‘varroa’ has been added to the ever-expanding vocabulary. ‘Gaga’s bees’ hold a special fascination for this small boy and he loves to watch them fly.
Wax Moths
I found a lesser wax moth larva (Achroia grisella) on the varroa floor of a hive this morning. This is not a great surprise to me as I have often found one or two there. In fact, a few years ago out of curiosity, I put a couple in a collecting pot with a magnifying lid and watched their development into mature moths. Because these moths have never done damage inside a hive or in stored equipment of mine, I’ve never given them more than a passing thought—until recently that is!
The greater wax moth, Galeria mellonella, is another story and if I found a tangle of silken threads on frames, or boat-shaped depressions in the woodwork where their offspring are raised, I would be worried and have to question my apiary hygiene standards and take some action.
If you want more information about these moths in terms of what they look like, what they do, and what you need to do if you have them, see this useful site: https://www.nationalbeeunit.com/diseases-and-pests/other-bee-diseases-and-viruses/wax-moth
Invertibrate Iridescent Virus
The American Bee Journal recently featured an article by Dr. Alison McAfee1 about invertebrate iridescent virus (IIV) and the suggestion that it may have played a part in last year’s very high overwintering losses in the U.S. That piqued my interest and I had to discover more about this virus which affect arthropods (invertebrates such as insects, spiders or crustaceans) and especially those living in damp, watery places worldwide. I was fascinated to see an infected woodlouse that had turned a violet colour so I investigated a little further and discovered that greater wax moths are common hosts too2. Could this be a problem for honey bee colonies where greater wax moths are prevalent? And, are lesser wax moths also hosts? I wanted to know.
Virus Profile
But firstly, what is this virus? Most honey bee viruses are RNA viruses that consist of single-stranded RNA inside a capsid or protein shell which protects the RNA genome. However, there are currently only two known DNA viruses made up of double-stranded DNA inside a lipid membrane, and IIV is one of them. Invertebrate iridescent virus-6 belongs to the Iridoviridae family which are large viruses and described as icosahedral because of their shape which is 20-sided with triangular faces. These particles are 120-180 nm in diameter and arranged in such a way that light gets refracted from the crystalline patterns and interferes with incident light which reveals the iridescent colours. These colours are indicative of an infection, and disease at the larval or pupal stage of the insect is lethal for them. They are highly infectious viruses if injected into the organism but not if ingested. So, if a honey bee larva that gets infected it will become iridescent and die. It is uncertain how the adult honey bee is affected, if at all.
Iridoviruses have been known about for some time but they have not been greatly studied in relation to honey bees. IIV-24 was thought to have been the cause of colony losses in India affecting Apis cerana 2. IIV-6 was isolated from Varroa destructor mites in collapsed U. S. colonies back in 1998. In 2010, the U.S. army biowarfare lab worked on a project to train honey bees to detect bombs. When some of the colonies collapsed, they were tested and the virus was found in varroa mites.
World expert and leader in the field of iridoviruses Dr. Trevor Williams hosts a really interesting educational website about these viruses and has published numerous papers, though not all of them relate to honey bees. I don’t have permission to reproduce photographs of the actual virus and infected larvae from his site, but you will find them here along with lots of other interesting information on the subject: http://www.trevorwilliams.info/Iridovirus.htm
I did email Dr. Williams though to ask if lesser wax moths also carry IIV-6 and he kindly replied at once with this very reassuring answer, “in my opinion wax moths of any type are unlikely to pose an IIV threat to honeybee colonies. The fact that the moth larvae can be infected with IIV type 6 by injection is mainly of academic interest. There is an IIV reported from bees (Apis cerana) in Kashmir (India/Pakistan) but I have never been able to infect Galleria mellonella larvae with that virus despite several attempts. So, in summary, I see no IIV risk associated with wax moth infestations, although they might be able to harbour other bee-infecting viruses (like dicistroviruses, iflaviruses etc.)”.
We Need More Research
It seems likely that more research will be done on these viruses, especially if overwinter colony losses in the U.S continue to be high. Although we haven’t heard of them causing problems here in the UK it is worth knowing about them and being mindful to keep varroa levels as low as possible, wax moths under control, and look out for Vairimorpha ceranae (nosema) which was also implicated in the heavy U.S. overwintering losses2. There is no treatment here in the UK for Vairimorpha spp. infections but ensuring colonies are well nourished might protect them and reduce risks. As with all life forms, adequate nutrition is key to colony health.
Why We Need Viruses
One last thing about viruses—much as we usually dread getting one, we actually need them in our world. Viruses put pressure on organisms to improve their defence systems which in turn speeds up genetic diversity and helps to improve our immune systems. Viruses keep animals in balance, and other life forms such as bacteria and algae are also kept in check. Ocean going viruses kill off bacteria that would otherwise be harmful to marine life. The dead bacteria get recycled and the nutrients are reused to support the ocean food chain. It might surprise you to know that viruses move genes (horizontal transfer) between organisms which, for example, allows plants to develop new and more useful traits which quickly shape their evolution. And these are just a few examples of the positive things that viruses do in our amazing world.
References
1 Alison McAfee, (2025) How Invertebrate Iridescent Virus Became Infamous, American Bee Journal, November 2025, page 1223.
2 Jerry Bromenshank, Colin Henderson and Dave Wick, (2025) The Uncontroversial Truth About Invertebrate Iridescent Virus, Nosema Caranae, And Nosema Apis, American Bee Journal, December 2025, page 1289.
Campbell and Reece, (2002) Biology Sixth Edition, Chapter 18.
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Happy new year Ann! Thank you for all of the research and knowledge you impart every year. Extension challenge for little Leo: “Varroa-resistance”😉
Underlying problem in North America is the export of over 1/2 million non varroa-resistant queens per annum from Hawaii, which lack the hygienic ability to cope with mites.
In the south-east of England, we are now seeing eggs and the first chewed-out pupal bits on the insert boards where the bees are interrupting early varroa breeding (pic to follow). Indicates brood rearing started in December.
Hello and happy new year to you too, Steve. I’ve been speaking to a couple of my Canadian friends who despair about all the imports from NZ and the U.S. that make selecting for varroa-resistance nearly impossible.
Thank you very much for those photos. What a surprise to learn that the colonies in the south are dealing with varroa so early in the season. The queens in my colonies are just starting to lay but I only know this from the clear wax flakes on the varroa boards.
I’ve identified the colonies that kept varroa levels low enough over the last year.
All the best for your new season, Ann.
Thank you for a great dive into iridescent viruses! If these viruses are implicated in the high losses seen in the US last year, can you explain a little more about how the virus affects the honey bee?, is it when the greater wax moths come in contact with the honeybee larva or is it the wax moth larva that can transfer the virus? With the varroa mite, transfer of viruses are when the mite “bites”, into the pupa. Is this a similar mechanism of transfer with these viruses?
Hello, Kevin. Thank you for commenting on the blog post. I don’t think that it’s known how the virus affects the adult honey bee apart from killing the larvae which become iridescent when infected, just that it was isolated from bees studied following last winter’s heavy U.S. colony losses. With hindsight, although Bromenshenk advises beekeepers to take action on wax moth infestations, I should probably not have given them such prominence because the experiments on them were done in a lab and not in the field, and, as Trevor Williams’ said in his email, he doesn’t think that wax moths will vector the virus in a harmful way for honey bees. He was involved with the paper I’m referencing at the end and you will see from the discussion around how wax moths would transmit IIV to bees that they can only really speculate.
“VOL. 45, NO. 2 SOUTHWESTERN ENTOMOLOGIST JUN. 2020
Replication of Invertebrate Iridescent Virus 6 (IIV-6) in European Honey Bees –
Potential Involvement in Colony Collapse Disorder?
Joseph L. McInnis1, Trevor Williams2, Ya-Chen Chuang3, and Douglas A. Gregg4
The study demonstrated that IIV-6 is very pathogenic to honey bee larvae.
The finding supports the demonstration of experimental infection of adult honey bees
with iridescent virus in a caged colony along with rapid colony loss following coinfection
with IIV-6 and Nosema (Bromenshenk et al. 2010). Honey bees are very
susceptible to the virus, which should be considered one of the pathogens potentially
contributing to colony collapse disorder. Further controlled studies are required to
establish if multiple agents are involved in colony collapse disorder. If IIV-6 persists
naturally in bee colonies, two additional hive inhabitants might be involved in the
persistence and transmission of the pathogen. Covertly infected wax moths might
pass non-lethal infection through the egg to developing larva as suggested by
transovarial transmission of IIV-6 in the citrus aphid (Hunter et al. 2001). If covert
infection is triggered into a patent lethal disease, the virus load in the wax moth larva
can exceed 10 logs (Constantino et al. 2001). Such large quantities of virus produced
in a single wax moth larva, could contaminate the hive with IIV particles and expose
healthy bees to abundant viral inoculum. Varroa mites could also serve as a vector
for IIV-6 from worker bees to honey bee larvae on which they prefer to feed. Feeding
by Varroa mites on larvae in a capped cell of the comb involves piercing the larval
tegument and might be a route for introduction of virus particles into the larval
hemolymph, which is the most efficient route for transmission of the viruses. Because
covert IIV infection has debilitating effects on development and adult survival of some
insects (Marina et al. 1999) and seemed to be lethal to honey bee larvae in this study,
the relationship between IIV infection and the health of honey bee colonies should be
studied further. At least, IIV-6 might lead to attrition of the worker bees in a hive.
Wax moths and Varroa mites might be more than a nuisance and might be vectors of
disease in the honey bee colony.”