Feature: Protecting the pollinators part 2 - bees and disease
5 April 2011. By Chrissie Giles
For years, a pathogen called deformed wing virus existed in honeybees in the UK. It caused no visible symptoms and appeared to have little effect on the health of the infected bees. This all changed when the Varroa destructor mite invaded the UK in 1992. The levels of deformed wing virus in bees exploded, leading to catastrophic consequences for hives all over the country.
The Varroa mite that transmits deformed wing virus can have a devastating effect on honeybees, but it and this virus are by no means the only threat to these insects. A number of other viruses and microorganisms can infect honeybees, wiping out colonies. Agricultural chemicals - including the pesticides used to kill off Varroa - are also suspected of causing harm.
Combined with a change in habitats in the UK, exposure to chemicals and disease are thought to be behind a fall in the number of different species of insect pollinators over recent years.
The nine projects funded through the Insect Pollinators Initiative are setting out to understand these threats better. Four of these, detailed below, are concerned with aspects of bee health and disease. Find out about the other five projects in first of this pair of features.
How do diseases affect the honeybee, and could they spread to other bee species?
"We've picked what we think are the most important disease organisms for the honeybee," says Dr Robert Paxton, from Queen’s University Belfast and the University of Halle, Germany. His team is studying deformed wing virus, carried by the Varroa destructor mite, and a fungus-like microorganism called Nosema ceranae.
"Before the Varroa mite came to the UK, deformed wing virus was found in maybe 1 in 10 000 colonies," says Robert. This changed after its discovery in the UK in 1992, when the amount of virus carried by bees increased dramatically.
Varroa is a rusty-coloured mite, which feeds on the haemolymph (circulatory fluid - the equivalent of blood) of adult and pupal bees. This increases the amount of deformed wing virus carried by bees and can lead to symptoms including - as the name suggests - misshapen wings that prevent bees from flying. The grounded bees are taken by predators and the colonies suffer as their numbers drop.
Robert says half or more of colonies in the UK have clinical symptoms of deformed wing virus, severe infection with which can lead to the collapse of colonies.
Nosema, meanwhile, has spread from East Asia in the last 10-15 years to the Western honeybee. Robert suspects that the interaction between this and deformed wing virus may act as a "double whammy", greatly increasing the ill-effects on honeybees. Not only honeybees are at risk: these diseases also affect bumblebees, and there are fears that they will be transmitted to other pollinators too.
Dr Mark Brown and his team at Royal Holloway, University of London, will be investigating how bumblebees come to be infected with deformed wing virus and N. ceranae, and the impact that these emerging diseases have on individuals and colonies of the important native bumblebee species.
In their project, researchers are studying how the diseases affect the bees physically, and whether they have any impact on insects' flight behaviour, orientation and learning - so-called sub-lethal effects, which do not kill the bees but affect how they function.
"We're working with Juliet Osborne's team at Rothamsted Research that has very refined methods for tracking how individual bees fly," says Robert. "It will be really nice to understand the impact of these disease organisms on individuals."
Professor Vincent Jansen, also at Royal Holloway, will use the data collected to model the spread of the disease organisms in the pollinator community, to try to understand the threat to both honeybees and bumblebees.
In addition to all these efforts, the team will also attempt to find ways to treat these infections. Robert and his colleagues will be testing the use of RNAi (RNA interference) methods in controlling deformed wing virus, an RNA virus. The technique involves blocking the multiplication of the virus and has been effective on other RNA viruses in honeybees, though it is not completely understood how.
They are also looking in some detail at the bacterial species inside the honeybee. Researchers have only recently discovered that insect 'guts' hold a huge variety of lactic acid bacteria and related species, the kind you find in probiotic yoghurts. They will investigate whether the two diseases have an impact on these bacteria, and whether the bacteria can help to overcome the disease symptoms, particularly those caused by Nosema, which bees contract by swallowing spores.
How do honeybees, honeybee viruses and Varroa destructor interact?
"Varroa destructor was not in Britain 30 years ago," says Dr Eugene Ryabov from the University of Warwick. "When it arrived, all beekeepers' behaviour had to change. It's essentially the reason that there are almost no honeybees in the wild in Britain today, and, without intervention, European bees will not survive."
A virologist by training, Eugene is focusing on the interplay between the mite, the viruses it carries and the bees. The ultimate aim is to find whether there are specific bee genes that affect how they respond to the viruses and their susceptibility to Varroa infection, so that bees can be bred to be more resistant to these threats.
Eugene wants to find out more about how the bees fight viruses. He is particularly interested in deformed wing virus and the closely related Varroa destructor virus-1. It is believed that deformed wing virus has long been present in UK bees, but was relatively harmless. As mentioned already though, the introduction of the Varroa mite changed all that. "We saw an increase in viral replication in some bees, and a strong correlation between wing deformities and an increase in viral levels," he says.
But why would this happen? Eugene thinks that Varroa infection blocks any bee immune response, leaving it vulnerable to viruses. This is something he and his colleagues will be exploring in their project funded by the Insect Pollinators Initiative.
The team will also look in more detail at the bees' responses to the viruses, testing the idea that there may be genetic variation between bees in how they react to infection. "We do anticipate variation in antiviral response within a hive," Eugene says. "A queen mates with 10-20 drones, so, even sampling in one hive, you will sample up to 20 different sets of half-siblings."
They are also following up on their recent findings that, in the presence of Varroa, deformed wing virus and Varroa destructor virus-1 can combine to produce new strains, which may be more harmful than the original viruses. Working with beekeepers in the Inner Hebrides and their Varroa-free bees, they will attempt to re-create what happens when British honeybees are infested with the mite.
By sequencing viruses isolated from the mites and bees, they hope to understand the diversity of the strains of virus present.
"There has been research into honeybee genetics," says Eugene, "But there's little so far on the insects' response to disease.” The team's large-scale genotyping work is new, and they will also profile the genes that are active in Varroa-infested and Varroa-free bee pupae to find out more about the genes and the cellular pathways that respond to the viruses.
How can models be used to explore disease movement in pollinators?
When Dr Giles Budge began his role as research coordinator of the National Bee Unit, based at the Food and Environment Research Agency, York, he began to realise just how much data on honey bees the Agency holds. "It's cost a lot to get these data collected, but we generally use them for summaries of regional incidence," he says.
Realising that data-mining experts could potentially use models to interpret these data and answer biological questions of interest to beekeepers and farmers, he set about assembling a team of researchers. "Suddenly seeing how our understanding can benefit from data stored for 10-15 years is a wonderful thing."
"We're trying our best to use data that already exist - we're not planning to reinvent the wheel," he says. He and his colleagues are starting with a disease they know a lot about: European foulbrood, which is caused by a bacterium that infects bee larvae. For this work, he has recruited a team that includes modellers from a range of disciplines, a microbiologist and a mathematician.
"We have information on when and where the disease has been found over the last 20 years or so," says Giles. "But we also know where it wasn't found - that's the most difficult thing when it comes to modelling disease." He says this dataset is like a "starter for ten", to get people thinking about which modelling processes work and which don't.
"If we can understand how a disease is spreading and how current control methods influence this, we can try and decrease the impact of disease on the honey bee population," he says. As many of the diseases associated with honeybees can also affect wasps, bumblebees and other pollinators, these findings could be extrapolated to other insects.
The researchers will be collecting some new information along the way. For example, they will be looking at how genetic variation in bees and pathogens affects the expression or movement of disease. They will also compare colonies that are either coping or not with disease to see whether it is putting a selection pressure on the bees and pathogens, causing them to evolve faster.
And the models developed will be applicable to other diseases, according to Giles. "It doesn't matter too much what is causing the disease - you need to put in basic biological factors such as how it might spread, how it survives, whether it's transported by adults or not. Then you can rerun some of the modelling tools developed for honey bee disease, and apply them across species."
The Food and Environment Research Agency is currently involved in screening for 14 different honeybee diseases across England and Wales, by sampling 4600 apiaries over two years. "Suddenly we've got information on the distribution of 14 other diseases," Giles says. "We should be able to use the modelling tools we will develop to investigate the spread of other honeybee diseases too."
How do pesticides and other chemicals affect bees' behaviour?
It is not all about disease: pesticides and other agricultural chemicals used to maximise crop yields may be affecting the health of bees. Honeybees are also treated with pesticides - miticides - to try to prevent infestation with the Varroa mite, something that could also be detrimental to their wellbeing.
Dr Chris Connolly is a neurobiologist at the University of Dundee. Though usually found investigating the human brain, he is now applying his expertise to bees, having been inspired by a book about their decline.
"I was thinking about pesticides and realised that, although people have looked at the concentrations that kill pest and non-pest species, so-called sub-lethal doses may affect how bees behave. Moreover, there have been no studies to investigate whether the sub-lethal effects of multiple pesticides might create the 'perfect storm' through synergistic interactions," Chris says.
"I decided that this is more or less the kind of thing we've been doing on mammalian brain cells, so if we could apply this to the bee brain, we could find out if these chemicals have sub-lethal effects at the level of individual cells, neural networks [circuits of nerve cells], whole animals or entire colonies." There are fears that miticide exposure may affect the bees' abilities to move, communicate and find food.
The Varroa-killing miticides are a reformulation of the pesticides used in the field and one of the prime targets for synergistic toxicity. "If the bees encounter this and then another pesticide, then the double hit might be the problem," Chris says.
His team is designing assays to study the effects of different combinations of pesticide on brain cells. "Our next step will be to look at neural networks and to study the effects on the insects' abilities to learn at the neuronal level," he says.
To assess the impact of pesticides, research in the lab of Dr Geraldine Wright at Newcastle University will investigate how such chemicals affect learning and memory in both honeybees and bumblebees. Bees can be trained in a classical Pavlovian way: give them an odour followed by a sugar reward, which they take in with their proboscis, and they eventually learn to put out their proboscis out in response to the odour alone.
"After the bees have been exposed to chemicals, we can ask: are they slower at learning? Do they forget what they've learned?" says Chris.
This part of the project will involve the radiotagging of some 6000 bees, overseen by Dr Nigel Raine at Royal Holloway, University of London. A scanner will monitor bees as they enter and leave the hive. And the bees will be weighed, allowing researchers to work out not only each individual bee's contribution to the hive throughout its life but the performance of the whole colony throughout a season.
One of the challenges faced by this project and several others funded through the Insect Pollinators Initiative is a lack of stable cell lines for bee research. Working with Professor Neil Millar at University College London, Chris hopes to create the first-ever honeybee neuronal cell line vital for pesticide screening in the future.
Image: False-coloured scanning electron micrograph of a honeybee. Credit: David McCarthy and Annie Cavanagh/Wellcome Images.
- Robert Paxton is working with Dr Mark Brown at Royal Holloway, University of London and Dr Juliet Osborne at Rothamsted Research.
- Eugene Ryabov is working with Prof. David Evans, Dr Dave Chandler and Dr Jim Bull at the School of Life Sciences, University of Warwick, and is cooperating with Prof. Nigel Burroughs at the Warwick Systems Biology Centre.
- Giles Budge is working with Dr Ed Feil at the University of Bath, Prof. Stephen Rushton at Newcastle University and Prof. Matt Keeling at the University of Warwick.
- Chris Connolly is working with Dr Jenni Harvey at the University of Dundee, Dr Nigel Raine at Royal Holloway, University of London, Dr Geraldine Wright at Newcastle University and Prof. Neil Millar at UCL.
The Insect Pollinators Initiative is supported by the Biotechnology and Biological Sciences Research Council, the Department for Environment, Food and Rural Affairs, the Natural Environment Research Council, the Scottish Government and the Wellcome Trust, under the auspices of the Living With Environmental Change partnership.
