The Varroa mite – a deadly and dangerous bee parasite Honey bees – Small insects, big impact Big tasks for little insects: the work done by Contents honey bees is of great importance to humans. The Varroa mite – The honey bee is essential for pollination of a wide number of crop plants, making them an important part of food production. A large a deadly and dangerous bee parasite proportion of our food is produced, to some extent, with the help of bee and other insect Introduction pollination. That is why protecting and improving the health of bees is so important. 3 Honey bees – Small insects, big impact 5 Varroosis: the infestation of bee colonies 6 The expansion of the Varroa mite Although the number of honey bee colonies 7 Mite migration has grown across the world in the past 50 Biology years, poor bee health has reached alarming levels in some regions of the world. One of 8 The biology of the parasite the main causes, and one of the honey bee’s 8 Body structure worst enemies, is a tiny mite called Varroa 9 Gender differences 10 Sensory perception of the Varroa mite destructor. There are currently only a few 11 The reproduction process of Varroa destructor ways to protect bees from the mite. Using the 12 Egg laying in the brood cell few effective control products and measures Mating behavior 13 Developmental stages of the Varroa mite available, combined with good beekeeping 14 After the bee exits the cell management practices, it is possible to 15 Transmission of honey bee viruses reduce the impact of Varroa and keep the 16 Honey bee viruses impact of this parasite in check. Infection 19 Infection with the Varroa mite Combating Þ 21 Combating the Varroa mite Diagnostics 1 24 Measures to combat Chemical methods for use during the breeding season 26 Chemical methods for use outside the breeding season 27 Biotechnical methods If an article or picture is marked with this sign, you can download Outlook this material with the following link: www.beecare.bayer.com/varroa 28 What’s next in bee research? 29 Varroa gate technology We hope this material may be useful 30 Further research activities for your Varroa training courses. The Varroa mite | Introduction 3 Varroosis: the infestation of bee colonies Þ 1 Download the picture at: www.beecare.bayer.com/varroa Right: Varroa mites at various development stages in a bee colony brood comb Left: Varroa mite attached to adult bee showing symptoms of deformed wing virus (DWV) It is small and yet highly dangerous: the Varroa destructor mite is the most destructive enemy of the Western honey bee (Apis mellifera). The parasite has now spread to almost all parts of the world – except for Australia. Mite infestation has hit Europe and North America particularly hard, and the parasite is a serious threat to bee health. Without human intervention, a bee colony infested with mites will typically die off in these regions within three years. In addition to the threat posed by the Varroa mite itself, there is also the danger of secondary infection from various mite-vectored diseases, which have also become more widespread and additionally weaken the bee colonies. The parasitic Varroa mites – much like ticks – transmit diseases that often prove fatal to adult honey bees and their brood. Combating the mite is a difficult task for researchers. This is because – despite a number of promising ideas – they have not yet managed to develop simple and long-lasting treatments for fighting the bee parasite, nor have they yet managed to breed a Varroa-resistant strain of the Western honey bee. The Varroa mite | Introduction 5 The expansion of the Varroa mite Mite migration The Varroa mite is originally native to Asia, where it was first discovered on the island of Java in Indonesia over 100 years ago. The Dutch zoologist Anthonie Cornelis Oudemans gave it the name Varroa jacobsoni. The mite initially preyed on the Asian honey bee (Apis cerana). But over thousands of years the bee successfully adapted its behavior to the parasite. The bees fend off the mites through their intensive cleaning habits in the hive, thus minimizing harm to the colony. When European settlers brought the Western honey bee (Apis mellifera) to Asia, it also fell prey to the Varroa mite. Through these infested colonies the parasite was then introduced to Europe, where since the 1970s it has continued to spread. Recent genetic investigations have revealed that Varroa jacobsoni comprises 18 different genetic variants No information No reports with two main groups: Varroa jacobsoni and Varroa destructor. Varroa destructor, the newly identified type, inflicts a great deal of harm in Europe, North America and elsewhere because the Western Spread of the Varroa mite in the selected countries with the year of detection in the corresponding decade. honey bee lacks sufficient defense mechanisms. Clearly, the equilibrium between Varroa destructor and the Western honey bee has not yet been established. 2000s 1980s 1970s 1960s 1950s First detection The mite is now found in many areas of the world: it is common not only Source: adapted from Webster TC, Delaplane KS 2001, Mites of the Honey Bee, (ECPA) in China and Russia but also in Central Europe and North and South America. Even New Zealand and Hawaii reported cases of infestation in the first decade of the 21st century. Australia is the only part of the // The Varroa mite originated in Asia, but has since spread towards world where the mite has not yet spread, mainly as a result of intensive the west and now threatens the Western honey bee almost all biosafety protocols at the borders. over the planet. // Only Australia has managed to keep the parasite at bay to date. 6 The Varroa mite | Introduction The Varroa mite | Introduction 7 The biology of the parasite Body structure Gender differences Varroa destructor literally means “destructive mite.” And Male and female mites differ significantly from one another. Male Varroa although the parasite’s name more or less says it all, this mites are more rounded and yellowish-white. And, measuring in at only tiny arachnid is not much larger than a millimeter and lacks 0.7 to 0.9 mm, they are also markedly smaller than females, which hearing and sight. The body of the mite has four pairs of are approximately 1.1 mm long and 1.6 mm wide. Females are also more legs and piercing and sucking mouthparts. It uses the heavily sclerotized, giving them a harder cuticle, and are brownish in numerous sensory hairs all over its body as receptors to color. sense its environment. The Varroa mite’s flattened shape and the suckers on its feet enable it to optimally grip the In addition, the mouthparts of females are much more pronounced bee’s body. It uses its mouthparts to pierce the bee’s than those of males. This means that only females can penetrate the exoskeleton and feed on its hemolymph, a circulatory exoskeleton of honey bees and their brood; males, on the other hand, fluid similar to blood. cannot reach the hemolymph without damaging themselves. The male mite is confined to the brood cells and feeds on the bee brood at a feeding spot created by the female mite. For this reason, only females can survive outside the brood cells. The function of male mites is limited // Body structure: to mating with the females. idiosoma (main body Þ region) and gnathosoma (mouth and feeding parts). 1 Download the picture at: www.beecare.bayer.com/varroa // The mite has four pairs of legs and piercing and Female morphology sucking mouthparts. • 1.1 mm long and 1.6 mm wide • flattened body // The hairs on the top and • heavily sclerotized = mechanical bottom of the mite’s protection body enable it to cling to • brownish in color the honey bee. • have suckers (apoteles) on the last section of each leg (tarsus) // The parasite feeds on • t wo teeth are situated at the end of their the hemolymph of both jaws (chelicerae) adult bees and their brood. Male morphology • smaller than the female • 0.7 mm long and 0.9 mm wide • yellowish-white in color • rounded body Female Varroa mite top right • can survive only in the capped brood cells and male bottom left, below 8 The Varroa mite | Biology The Varroa mite | Biology 9 The reproduction process of Varroa destructor Sensory perception of the Varroa mite The Varroa mite orients itself without any sense of sight or hearing – it can only differentiate between light and dark. However, the receptors on its sensory hairs are highly developed: the mite is able to detect differences Varroa mite enters Varroa mite in temperature, moisture and chemical stimuli. The parasite uses its cell with larva inside reproduction atatsrhheecnseenutpe stVsneooma nnrsryaasreo ileonvlae esr– sgm e tsta oeooint neffs ls vteoioosincbu rsaerycaetn hest diid ytotosso niw tn esefi en.man dvI dts i erw umointsisntaaeh mlbwsl tl ecaehitnaysesvt .atfsi htrRreyooen enu opsstnn eeald esarit ag rtsocshs fhife t rmse o hbm nutaeotcese hll lehlao gaialcvsnssaeo. dt i aenTd tn shiatsedhescc e tcomet sabv it netrhueo rrspe’oosedidu c h t gktchihg heeuah lipaltrls y Queen bee lays eggWsofrekeerdi nbge le arva ...................Larsvtaal cglgeor. soeWwst orhc ketlelo r fiwbintePahleu ppwahatiaxs.oen ................... Yofurnoga md btuelhte e Vhcaaertlrlc ohawi ets mh i t es in the beehive. ... .. .. ... ... ... . . . . . . . . . . Electron microscope picture of a Varroa mite Close-up detail showing Varroa mite front Development of Varroa mites in a honeycomb cell legs, with mouthparts in between The parasite preys on both adult honey bees and their brood. Varroa females can also survive outside the brood cells by attaching themselves to adult bees. However, the parasite only reproduces in the sealed brood cells of the honey bee. Shortly before the // THE FRONT LEGS OF VARROA DESTRUCTOR CONTAIN brood cells are capped, the Varroa female mites enter and crawl to the bottom of these AN OLFACTORY SENSE ORGAN THAT ENDOWS THE cells – they protect themselves from the bees that tend to the brood by hiding under the larvae. Here they first immerse themselves in the liquid brood food. Once this is PARASITE WITH THE ABILITY TO SMELL AND TASTE. depleted, the Varroa mite feeds directly on the bee larvae. The parasite has strongly IT USES ITS FRONT LEGS LIKE ANTENNAE TO SENSE adapted to its host in terms of habitat and food. ITS ENVIRONMENT. // 10 The Varroa mite | Biology The Varroa mite | Biology 11 9 10 Egg laying in the brood cell 8 11 DAY DAY AFTER CELL AFTER CELL DAY CAPPED CAPPED DAY The pheromones emitted by the bee larvae and the capping of 7 AFTER CELL AFTER CELL 12 CAPPED CAPPED the honeycomb cells with wax by worker bees activate Varroa egg DAY DDAAAAYY development – so-called oogenesis – approximately six hours after AFTER CELL AAFFTTEERR CCEELLLL CAPPED CCAAPPPPEEDD infestation. After finalization of oogenesis – approximately 60 hours after 6 the brood cell is sealed – the female mite starts to lay eggs. The first egg is not fertilized and always develops into a male. The mite lays at daily DAFATY ER CELL Matingg CAPPED intervals the remaining four to five fertilized eggs, which become female mite offspring. The nymphs are white and mature into adult female mites 5 m 4itteh egg in about six days, passing through various development stages during DAY AFTER CELL this process. Males need about seven days to mature. The juvenile mites CAPPED are unable to feed themselves, so the mother mite pierces a hole in the 3rd bee pupa to create a communal feeding site for her offspring. During the 4 mite egg 17181920 development period the mother mite has to expend considerable energy DAY 1516 21 ttohe m maiitnetsa.i n the communal feeding site, as it is the sole source of food for CAFATPEPRE3 DCELL A u pfe tmo a5l efe mrtiiltiez elady esg gs m2nitde egg 1mstite egg 1113214110DINA99 YBS88R BO7EOED 6L CAER5LVLA4321 monFeeffimtwsep abarliernno dgVo ahdinre rvcora eadl lse DAY AFTER CELL Mating behavior CAPPED To complete the mating process, the mites remain for twelve days with 2 the developing worker brood – and even as long as 14 days in the cells DAFATYE R CELL A female mite enters the brood cell shortly before capping; approximately CAPPED 3 days later the first male egg is laid followed by up to five female eggs. of the drone brood. The drone brood is typically infested five to ten times Depending on the post-capping period, one or two mature daughter mites more often than the worker brood. Mating in the broodcell proceeds will leave the brood cell together with the mother mite and the hatching bee. 1 efficiently and purposefully: the males wait for the mature females near DAY AFTER CELL the common excrement area. Mating with each of the female mites needs CAPPED to occur before the bee hatches, because males and unmated females Development of the Varroa mite die after the bee emerges from the cell. The mating behavior shows that the Varroa mites have optimally adapted their feeding and reproduction to Graphic showing Varroa mite the hive environment of the honey bee. This enables parasite infestation development stages Unfertilized mite egg Fertilized mite egg to double every three to four weeks during the breeding season. The first development stage of the Varroa mite is as a six-legged larva which develops Male protonymph Female protonymph inside its closed egg. After hatching, the second development stage emerges – the eight-legged protonymph.This protonymph develops into the third phase - the Male deutonymph Female deutonymph deutonymph, which then becomes the adult mite. Following the final nymph phase, the // THE DRONE BROOD IS INFESTED FIVE TO TEN TIMES Young male mite Young female mite developing mite becomes immobile and MORE OFTEN THAN THE WORKER BROOD. // becomes a chrysalis. Young nymphs are white in color and after several moltings Adult male mite Adult female mite during the development process of the mite the last immobile stage – the deutochrysalis – turns brown, taking on the color of the female Adult male mite stays Immature female mite adult Varroa mite it will finally become. in the cell and dies stays in the cell and dies 12 The Varroa mite | Biology The Varroa mite | Biology 13 Transmission After the bee exits the cell of honey bee viruses A mite’s life span is two to three months in summer, and six to eight months in winter. But the parasite is strongly dependent on its host – it can only survive up to seven days without bees and their brood. Unlike its South-East Asian counterpart, the Western honey bee lacks Dead mites fall to the base of the hive. sufficient defense mechanisms to fend off the non-native parasites. Infested honey bees are weakened as a result of the mites feeding on Brood their hemolymph, which puts a strain on the bees’ immune system. This y sit adversely affects their performance and shortens their life span. When en the parasite feeds on the larva, it also transmits dangerous viruses d n Varroa mites directly into the bees’ hemolymph. The viruses can spread and harm the o ati bees during their vulnerable development stage. Varroa increases the ul extent of the infection, because in the hemolymph, many viruses become p Po deadly. Since there are no effective medicines to treat honey bee viruses, control of the Varroa mite to reduce the spread of viruses is essential. Bees One such virus that is very widespread is the deformed wing virus (DWV), which can occur both in the brood and in adult bees. Often an infection does not produce any visible symptoms, but if the parasite transmits the virus to bee pupae, the young bees will develop deformed Winter bees wings. These bees are unable to fly – and have a shortened life span Winter bee brood compared to healthy bees. Spring Summer Autumn Winter The Varroa mite also transmits other viruses such as the acute bee paralysis virus (ABPV), which can infect adult bees and larvae alike. Graphic: Varroa population It is primarily found in fat body cells and in the bees’ salivary glands, but does not produce any typical signs of disease. The mite transmits ABPV directly into the bees’ hemolymph. From there it spreads to the vital organs: once in the brain, the virus induces behavioral disturbances and Debris is the term used to describe the remains impairs orientation and development – all of which can have lethal effects that drop to the bottom. The honey bees’ different movements and activities cause the remains, such on the bees. An infection with ABPV is particularly critical in the case of as wax, cell caps, food and dead mites, to fall into winter bees – it severely affects their ability to survive until spring. an insert on the floor of the hive. The infestation level can be monitored by counting the number of dead mites (debris diagnosis). Bee larva with parasitizing Varroa mites // THE VARROA POPULATION CAN DOUBLE EVERY FOUR WEEKS DURING THE BREEDING SEASON. IT CAN GROW FROM 50 MITES UP TO AROUND 3,200 MITES FROM THE BEGINNING OF FEBRUARY TO THE END OF AUGUST – WHICH ENABLES IT TO WIPE OUT EVEN A STRONG BEE COLONY OVER THE WINTER. // 14 The Varroa mite | Biology The Varroa mite | Biology 15 Honey bee viruses Ü1 Download the table at: www.beecare.bayer.com/varroa Virus Abbreviation Symptoms of the disease Virus Abbreviation Symptoms of the disease Acute bee paralysis virus ABPV The virus thrives in the bees’ fat bodies and Slow bee paralysis virus SBPV SBPV infection typically produces no salivary glands, but the infection usually symptoms in bees. But the Varroa mite does not produce typical symptoms. transfers the virus directly into the bee’s Through the hemolymph, however, ABPV hemolymph, where the infection can be can reach the bee’s brain. In this case the deadly. virus causes problematic behavioral displays, such as bees flying to the wrong hive. They Deformed wing virus DWV The widespread DWV infects bees in all become disoriented and disturbed in their development stages. At first, the infection development and die within a short time. does not produce any symptoms. If the Varroa mite transfers the virus to a pupa, it Chronic bee CBPV Symptoms include black bees lacking hair, will develop deformed wings. The adult bee paralysis virus trembling and inability to fly. Affected bees is unable to fly and thus unfit for survival. are often seen around the hive entrance. Infestation may sometimes cause diarrhea. Sacbrood virus SBV SBV typically infects brood that is fed with Excrement spreads CBPV throughout the infected jelly. Diseased larvae fill with liquid entire hive. and their body loses structure within a tough, sac-like outer shell (exoskeleton). Israeli acute IAPV 2004 IAPV was first described in Israel, Eventually, the brood dries out and dies. A paralysis virus where infected bees were seen with shivering dark scab forms on the dead bodies. Adult wings, progressed to paralysis, and then bees typically do not show visible symptoms died outside the hive. IAPV is spread by bee of SBV. But they develop faster, collect less excrement in the hive. The Varroa mite can food and die sooner. also carry the virus and kill both pupae and adult bees very fast after transmission into Kashmir bee virus KBV Infection with KBV is deadly to adult bees in the hemolymph. a very short time. The Varroa mite increases its spread within the beehive. Wasps and bumble bees can also get infected by KBV. Symptoms include black bees lacking hair THE VARROA MITE HARMS HONEY BEES IN VARIOUS WAYS: and a higher number of dead or dying bees // IT WEAKENS THE BEE’S IMMUNE SYSTEM, CAUSING DISEASE PROGRESSION inside or outside the hive. KBV leads to TO BE MORE ACUTE. weakening of the colony. // IT TRANSMITS VIRUSES THAT SPREAD QUICKLY WITHIN AND BETWEEN BEE Cloudy wing virus CWV CWV spreads through the air in the entire COLONIES. beehive. Varroa mites strengthen the // IT TRANSMITS VIRUSES DIRECTLY INTO THE BEES’ HEMOLYMPH – PREVIOUSLY infestation of the brood. The bee’s wings HARMLESS VIRUSES CAN THUS BECOME LETHAL. become translucent. 16 The Varroa mite | Biology The Varroa mite | Biology 17 Infection with the Varroa mite Varroa destructor depends on honey bees to move from hive to hive. This is possible because when searching for food, bees regularly come into contact with bees from other colonies (drifting), even those located several kilometers away. As a result, even colonies that were treated for Varroa mites are at risk of being reinfested by bees from untreated colonies. A high density of honey bee colonies in some areas further facilitates the spread of the parasite. The spread of Varroa mites through so-called mite reinfestation was long underestimated, but is now Spread of the considered a significant contributor to rapid increases in Varroa numbers, Varroa mites particularly in the late summer and fall periods. Swarming is a colony’s natural method of reproduction. When the brood Shipping Live Bees nest becomes too crowded, roughly half of the bees will swarm out to establish a new colony with the old queen – taking the Varroa mites with them to their new home. The parasites mate inside the new brood cells, Split and the Varroa mite’s reproduction cycle starts all over again. The mites may also spread when beekeepers build up new honey bee colonies, for example through a process known as splitting, which involves Nucleus Colony removing brood combs from established honey bee colonies to start a new honey bee colony (nucleus colony). This method allows the nurse bees which stay with the brood to raise a new queen from fresh larvae material and set up a new honey bee colony. A split can also be carried out with Drifting adult honey bees: beekeepers initiate an artificial swarm to form a new colony from worker bees and an established queen to start a new colony on Honey Bee a broodless basis. After establishing on brood free honey combs the queen starts egg laying and the development of a new honey bee colony. Colony Artificial Swarm In a swarm prevention split, beekeepers relocate the existing hive including the old queen to a new location. A new hive will be placed instead, filled with brood material, pollen and nectar from the relocated hive. The foraging bees from the “old hive” will return to the old location Swarm Prevention Split and start immediately to raise a new queen from the larvae material as a foundation for a new honey bee colony. Splitting can be an efficient way to reduce the Varroa population. In all described splitting methods the mites hitch a ride on the bees from the old hive to the new one, although the natural degree of mite infestation is lower in the split colonies than Robbing in the parent colony. An additional Varroa treatment when using splits, artificial swarms or removing the queen to break the brood cycle helps to further reduce the parasites in the new colony. 18 The Varroa mite | Infection The Varroa mite | Infection 19