A comparison of the reproductive ability of <Emphasis Type="Italic">Varroa destructor</Emphasis> (Mesostigmata:Varroidae) in worker and drone brood of Africanized honey bees (<Emphasis Type="Italic">Apis mellifera</Emphasis>)

Colony infestation by the parasitic mite, Varroa destructor is one of the most serious problems for beekeeping worldwide. In order to reproduce varroa females, enter worker or drone brood shortly before the cell is sealed. To test the hypothesis that, due to the preference of mites to invade drone brood to reproduce, a high proportion of the mite reproduction should occur in drone cells, a comparative study of mite reproductive rate in worker and drone brood of Africanized honey bees (AHB) was done for 370 mites. After determining the number, developmental stage and sex of the offspring in worker cells, the foundress female mite was immediately transferred into an uninfested drone cell. Mite fertility in single infested worker and drone brood cells was 76.5 and 79.3%, respectively. There was no difference between the groups (X ² = 0.78, P = 0.37). However, one of the most significant differences in mite reproduction was the higher percentage of mites producing viable offspring (cells that contain one live adult male and at least one adult female mite) in drone cells (38.1%) compared to worker cells (13.8%) (X ² = 55.4, P < 0.01). Furthermore, a high level of immature offspring occurred in worker cells and not in drone cells (X ² = 69, P < 0.01). Although no differences were found in the percentage of non-reproducing mites, more than 74% (n = 85) of the mites that did not reproduce in worker brood, produced offspring when they were transferred to drone brood.     

Reproduction of Varroa destructor in worker brood of Africanized honey bees (Apis mellifera)

Reproduction and population growth of Varroa destructor was studied in ten naturally infested, Africanized honeybee (AHB) (Apis mellifera) colonies in Yucatan, Mexico. Between February 1997 and January 1998 monthly records of the amount of pollen, honey, sealed worker and drone brood were recorded. In addition, mite infestation levels of adult bees and worker brood and the fecundity of the mites reproducing in worker cells were determined. The mean number of sealed worker brood cells (10,070 ± 1,790) remained fairly constant over the experimental period in each colony. However, the presence and amount of sealed drone brood was very variable. One colony had drone brood for 10 months and another for only 1 month. Both the mean infestation level of worker brood (18.1 ± 8.4%) and adult bees (3.5 ± 1.3%) remained fairly constant over the study period and did not increase rapidly as is normally observed in European honey bees. In fact, the estimated mean number of mites fell from 3,500 in February 1997 to 2,380 in January 1998. In May 2000 the mean mite population in the study colonies was still only 1,821 mites. The fertility level of mites in this study was much higher (83–96%) than in AHB in Brazil(25–57%), and similar to that found in EHB (76–94%). Mite fertility remained high throughout the entire study and was not influenced by the amount of pollen, honey or worker brood in the colonies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effect of acaricide resistance on reproductive ability of the honey bee mite Varroa destructor

The reproduction of pyrethroid-resistant Varroa destructor mite, a brood parasite of honey bees, was observed in Weslaco, Texas, and the results compared with known susceptible mite populations from other studies. Seven Apis mellifera colonies that had mite populations resistant to the acaricide Apistan^™ were used. Pyrethroid-resistance was confirmed when only 17% rather than 90% of mites confined in dishes containing Apistan^™ died after 12 h of exposure. The average number of eggs laid by resistant mites invading worker and drone cells was 4.4 and 5.4 respectively. This is similar to the number of eggs laid by susceptible mites in worker (4.4–4.8) or drone (4.7–5.5) cells. Also the average number of fertilised V. destructor female mites produced by resistant mites in worker (1.0) and drone (2.1) cells were similar to the number produced by susceptible mites in worker (0.9) and drone (1.9–2.2) cells. In addition, no major differences between the resistant and susceptible mite populations were observed in either worker or drone cells when six different reproductive categories and offspring mortality rates were compared. Therefore, it appears that there is little or no reproductive fitness cost associated with pyrethroid resistance in V. destructor in Texas. This revised version was published online in July 2006 with corrections to the Cover Date.     

Brood cell size of <Emphasis Type="Italic">Apis</Emphasis> <Emphasis Type="Italic">mellifera</Emphasis> modifies the reproductive behavior of <Emphasis Type="Italic">Varroa</Emphasis> <Emphasis Type="Italic">destructor</Emphasis>

We undertook a field study to determine whether comb cell size affects the reproductive behavior of Varroa destructor under natural conditions. We examined the effect of brood cell width on the reproductive behavior of V. destructor in honey bee colonies, under natural conditions. Drone and worker brood combs were sampled from 11 colonies of Apis mellifera. A Pearson correlation test and a Tukey test were used to determine whether mite reproduction rate varied with brood cell width. Generalized additive model analysis showed that infestation rate increased positively and linearly with the width of worker and drone cells. The reproduction rate for viable mother mites was 0.96 viable female descendants per original invading female. No significant correlation was observed between brood cell width and number of offspring of V. destructor. Infertile mother mites were more frequent in narrower brood cells.     

Behavior of varroa mites in worker brood cells of Africanized honey bees

The ectoparasitic mite Varroa destructor is currently the most important pest of the honey bee, Apis mellifera. Because mite reproduction occurs within the sealed cell, the direct observation of varroa activity inside the cell is difficult. A video observation method using transparent polystyrol cells containing infested brood was used to analyze the behavior of varroa mites in worker brood of Africanized honey bees. We recorded how mites feed on the larva and pupa, construct a fecal accumulation site and how the bee larva carried out some longitudinal movements around the cell. The feeding activity of the foundress mite varies during the course of the cycle. On the prepupa mites were found to feed often (0.3 ± 0.2 bouts h⁻¹) for a period of 8.7 ± 8.4 min h⁻¹ and there was no preference for a specific segment as feeding site. On the opposite, during the pupal stage mites fed less often (0.1 ± 0.1 bouts h⁻¹) for a period of 6.2 ± 4.0 min h⁻¹ and almost always at a particular site (92.4%). On pupa, 83.7% of the feeding was on the 2nd abdominal segment (n = 92), and only few perforations were found on the thorax. Varroa shows a preference for defecation in the posterior part of the cell (cell apex), close to the bee′s anal zone. We found a high correlation between the position of the feeding site on the pupa and the position of the fecal accumulation on the cell wall. Most infested cells have only one fecal accumulation site and it was the favorite resting site for the mite, where it spent 24.3 ± 3.9 min h⁻¹. Longitudinal displacements were observed in 28.0% (n = 25) of the analyzed bee larvae. Turning movements around the cell, from the bottom to the top, were carried out by these larvae, mainly during the second day (47.7 ± 22.5 min h⁻¹), just before pupation, with a total time of 874.9 ± 262.2 min day⁻¹ (n = 7 individuals). These results in worker brood of Africanized bees demonstrate adaptations of varroa mites to parasitizing the developing bee inside the capped brood cells.     

Ontogenesis of the mite Varroa jacobsoni Oud. in drone brood of the honeybee Apis mellifera L. under natural conditions

A study carried out during the summer of 1994, in southern England, investigated the developmental times and mortality ofVarroa jacobsoni inApis mellifera drone cells. The position and time of capping of 2671 naturally infested drone cells were recorded. Six hours after the cell was capped, 90% of the mites were free from the brood food to start feeding on the developing drone. The developmental time of the mite's first three female offspring (133±3 h) and the male offspring (150 h) and the intervals between egg laying (20–32 h) were similar to those found in worker cells. However, the mortality of the offspring was much lower in drone cells than worker cells. The mode numbers of eggs laid were six and five in drone and worker cells, respectively. All offspring had ample time to develop fully in drone cells with the sixth offspring reaching maturity approximately 1 day before the drone bee emerged. Normal mites (those which lay five or six viable eggs) produced on average four female adult offspring but since only around approximately 55% of the mite population produced viable offspring the mean number of viable adult female offspring per total number of mother mites was 2 to 2.2 in drone cells.     

Reproductive biology of <Emphasis Type="Italic">Varroa destructor</Emphasis> in Africanized honey bees (<Emphasis Type="Italic">Apis mellifera</Emphasis>)

Since its first contact with Apis mellifera, the population dynamics of the parasitic mite Varroa destructor varies from one region to another. In many regions of the world, apiculture has come to depend on the use of acaricides, because of the extensive damage caused by varroa to bee colonies. At present, the mite is considered to contribute to the recent decline of honey bee colonies in North America and Europe. Because in tropical climates worker brood rearing and varroa reproduction occurs all year round, it could be expected that here the impact of the parasite will be even more devastating. Yet, this has not been the case in tropical areas of South America. In Brazil, varroa was introduced more than 30 years ago and got established at low levels of infestation, without causing apparent damage to apiculture with Africanized honey bees (AHB). The tolerance of AHB to varroa is apparently attributable, at least in part, to resistance in the bees. The low fertility of this parasite in Africanized worker brood and the grooming and hygienic behavior of the bees are referred as important factors in keeping mite infestation low in the colonies. It has also been suggested that the type of mite influences the level of tolerance in a honey bee population. The Korea haplotype is predominant in unbalanced host-parasite systems, as exist in Europe, whereas in stable systems, as in Brazil, the Japan haplotype used to predominate. However, the patterns of varroa genetic variation have changed in Brazil. All recently sampled mites were of the Korea haplotype, regardless whether the mites had reproduced or not. The fertile mites on AHB in Brazil significantly increased from 56% in the 1980s to 86% in recent years. Nevertheless, despite the increased fertility, no increase in mite infestation rates in the colonies has been detected so far. A comprehensive literature review of varroa reproduction data, focusing on fertility and production of viable female mites, was conducted to provide insight into the Africanized bee host-parasite relationship.     

Ontogenesis of the mite Varroa jacobsoni Oud. in worker brood of the honeybee Apis mellifera L. under natural conditions

A study was carried out during May 1993, in southern England, on eight chemically untreated Apis mellifera L. colonics heavily infested with Varroa jacobsoni (15–40% of worker sealed brood). The position and time of capping of 3.228 worker sealed brood were recorded. At two hour intervals, starting from when each cell was capped, groups of worker cells were uncapped and their contents recorded. It was found that each V. jacobsoni female could deposit five or sometimes six larvae in a worker cell, of which four (1 male and 3 females) may reach maturity before the bee emerged from its cell. However, mortality of the offspring resulted in only 1.45 female offspring reaching maturity, for each normally reproducing mother mite, before the bee emerged. The mean development time of the first three female offspring was 134 hours (±=3 h.n=3), shorter than that of the male (154 hours). The first larva was deposited approximately 60 hours after the cell was capped, and developed into a male. Subsequent larvae were deposited at intervals varying from 26–32 hours, and all developed into females.     

A Comparative Study of Varroa Jacobsoni Reproduction in Worker Cells of Honey Bees (Apis Mellifera) in England and Africanized Bees in Yucatan, Mexico

The aim of this study was to investigate an underlying mechanism of the apparent tolerance of Africanized honey bees (AHB) to Varroa jacobsoni mites in Mexico. This was achieved by conducting the first detailed study into the mites' reproductive biology in AHB worker cells. The data was then compared directly with a similar study previously carried out on European honey bees (EHB) in the UK. A total of 1071 singly infested AHB worker cells were analyzed and compared with the data from 908 singly infested EHB worker cells. There was no significant difference between the number of mother mites dying in the cells (AHB = 2.0%, EHB = 1.8%); the mean number of eggs laid per mite (AHB = 4.86, EHB = 4.93); the number of mites producing no offspring (AHB = 12%, EHB = 9%); and developmental times of the offspring in worker cells of AHB and EHB. However, there was a major difference between the percentage of mother mites producing viable adult female offspring (AHB = 40%, EHB = 75%). This was caused by the increased rate of mite offspring mortality suffered by the first (male) and second (female) offspring in AHB worker cells. Therefore, only an average of 0.7 viable adult female offspring are produced per mite in AHB, compared to 1.0 in EHB.     

Reproductive capacity of varroa destructor in four different honey bee subspecies

Varroa tolerance as a consequence of host immunity may contribute substantially to reduce worldwide colony declines. Therefore, special breeding programs were established and varroa surviving populations investigated to understand mechanisms behind this adaptation. The aim of this study was to investigate the reproductive capacity in the three most common subspecies of the European honey bee (Carnica, Mellifera, Ligustica) and the F2 generation of a varroa surviving population, to identify if managed host populations possibly have adapted over time already. Both, singly infested drone and worker brood were assessed to determine fertility and fecundity of varroa foundresses in their respective group. We found neither parameter to be significantly different within the four subspecies, demonstrating that no adaptations have occurred in terms of the reproductive success of Varroa destructor. In all groups mother mites reproduce equally successful and are potentially able to cause detrimental damage to their host when not being treated sufficiently. The data further suggests that a population once varroa tolerant does not necessarily inherit this trait to following generations after the F1, which could be of particular interest when selecting populations for resistance breeding. Reasons and consequences are discussed.     

Reproduction of parasitic mites Varroa destructor in original and new honeybee hosts

The ectoparasitic mite, Varroa destructor, shifted host from the eastern honeybee, Apis cerana, to the western honeybee, Apis mellifera. Whereas the original host survives infestations by this parasite, they are lethal to colonies of its new host. Here, we investigated a population of A. cerana naturally infested by the V. destructor Korea haplotype that gave rise to the globally invasive mite lineage. Our aim was to better characterize traits that allow for the survival of the original host to infestations by this particular mite haplotype. A known major trait of resistance is the lack of mite reproduction on worker brood in A. cerana. We show that this trait is neither due to a lack of host attractiveness nor of reproduction initiation by the parasite. However, successful mite reproduction was prevented by abnormal host development. Adult A. cerana workers recognized this state and removed hosts and parasites, which greatly affected the fitness of the parasite. These results confirm and complete previous observations of brood susceptibility to infestation in other honeybee host populations, provide new insights into the coevolution between hosts and parasites in this system, and may contribute to mitigating the large‐scale colony losses of A. mellifera due to V. destructor.     

Honey bee colonies from different races show variation in defenses against the varroa mite in a ‘common garden’

Honey bee [Apis mellifera L. (Hymenoptera: Apidae)] genetic diversity may be the key to responding to novel health challenges faced by this important pollinator. In this study, we first compared colonies of four honey bee races, A. m. anatoliaca, A. m. carnica, A. m. caucasica, and A. m. syriaca from Turkey, with respect to honey storage, bee population size, and defenses against varroa. The mite Varroa destructor Anderson & Trueman (Acari: Varroidae) is an important pest of honey bee colonies. There are genetic correlates with two main defenses of bees against this parasite: hygienic behavior, or removing infested brood, and grooming, which involves shaking and swiping off mites and biting them. In the second part of this study, we examined the relationship of these two types of defenses, hygiene and grooming, and their correlation with infestation rates in 32 genetically diverse colonies in a ‘common garden’ apiary. Mite biting was found to be negatively correlated with mite infestation levels.     

Natural selection of Varroa jacobsoni explains the different reproductive strategies in colonies of Apis cerana and Apis mellifera

In colonies of European Apis mellifera, Varroa jacobsoni reproduces both in drone and in worker cells. In colonies of its original Asian host, Apis cerana, the mites invade both drone and worker brood cells, but reproduce only in drone cells. Absence of reproduction in worker cells is probably crucial for the tolerance of A. cerana towards V. jacobsoni because it implies that the mite population can only grow during periods in which drones are reared. To test if non-reproduction of V. jacobsoni in worker brood cells of A. cerana is due to a trait of the mites or of the honey-bee species, mites from bees in A. mellifera colonies were artificially introduced into A. cerana worker brood cells and vice versa. Approximately 80% of the mites from A. mellifera colonies reproduced in naturally infested worker cells as well as when introduced into worker cells of A. mellifera and A. cerana. Conversely, only 10% of the mites from A. cerana colonies reproduced, both in naturally infested worker cells of A. cerana and when introduced into worker cells of A. mellifera. Hence, absence of reproduction in worker cells is due to a trait of the mites. Additional experiments showed that A. cerana bees removed 84% of the worker brood that was artificially infested with mites from A. mellifera colonies. Brood removal started 2 days after artificial infestation, which suggests that the bees responded to behaviour of the mites. Since removal behaviour of the bees will have a large impact on fitness of the mites, it probably plays an important role in selection for differential reproductive strategies. Our findings have large implications for selection programmes to breed less-susceptible bee strains. If differences in non-reproduction are mite specific, we should not only look for non-reproduction as such, but for colonies in which non-reproduction in worker cells is selected. Hence, in selection programmes fitness of mites that reproduce in both drone and worker cells should be compared to fitness of mites that reproduce only in drone cells. © Rapid Science Ltd. 1998     

Non-reproducing Varroa jacobsoni Oud. in honey bee worker cells—status of mites or effect of brood cells?

The parasitic mite Varroa jacobsoni Oud. reproduces in sealed honey bee brood cells. Within worker cells a considerable fraction of the mites do not produce offspring. It is investigated whether variation in the ratio of cells without reproduction is caused by properties of the worker brood, or by the state of the mites entering cells. Pieces of brood comb were taken from colonies of 12 different bee lines and were placed simultaneously into highly infested colonies. Non-reproduction was independent of the origin of the brood pieces, indicating a minor role of a variation due to different brood origin. Between colonies used for infestation, however, it differed considerably. A comparison of the proportion of cells without reproduction when infested by one Varroa mite or when infested by two or three Varroa mites showed, that non-reproduction was mainly related to the state of the mites entering cells, and only to a minor degree to an influence of the brood cells. A high ratio of worker cells without reproduction was consistently reported in bee lines which survive the disease without treatment, and a high level of non-reproduction is thus regarded to be a key factor in breeding bees for high Varroa tolerance. The current results indicate, that differences in this trait are only to a minor degree related to differences between bee lines in the ability of the bee brood to induce oviposition. These differences seem rather to depend on other, unknown colony factors influencing the reproductive state of Varroa when they enter cells for reproduction.     

Uncapping activity of Apis mellifera L. (Hymenoptera: Apidae) towards worker brood cells infested with the mite Varroa destructor Anderson & Treuman (Mesostigmata: Varroidae)

Varroosis, a disease caused by the mite Varroa destructor Anderson and Treuman has killed hundreds of thousands of Apis mellifera L. colonies in various parts of the world. Nevertheless, the damage caused by this mite varies with the type of bee and climate conditions. Varroa causes little damage to Africanized bee colonies in Brazil, as the infestation rates are relatively stable and low. We evaluated the hygienic behavior (uncapping and removal of brood) of highly hygienic Africanized bees using combs with worker brood cells infested (naturally) and no infested with V. destructor. The daily uncapping rate, measured in eight colonies during six days, was 3.5 fold higher in the combs infested with varroa compared to no infested combs. The results show that the Africanized bees are able to recognise and remove brood cells naturally infested with V. destructor what is an important mechanism for tolerance against varroa.     

Differential periods ofVarroa mite invasion into worker and drone cells of honey bees

Invasion ofVarroa mites into honeybee brood cells was studied in an observation hive, using combs with cell openings at one side only. The cell bottoms had been replaced by a transparent sheet, through which mites were clearly visible after invasion into a cell. Mites invaded worker cells from 15–20 h preceding cell capping, whereas they invaded drone cells from 40–50 h preceding capping. The larger number of mites generally found in drone cells, when compared to worker cells, may be partly due to the longer period of mite invasion into drone brood.     

The effects of temperature and dose of formic acid on treatment efficacy against Varroa destructor (Acari: Varroidae), a parasite of Apis mellifera (Hymenoptera: Apidae)

In order to decrease the variability of formic acid treatments against the honey bee parasite the varroa mite, Varroa destructor, it is necessary to determine the dose-time combination that best controls mites without harming bees. The concentration × time (CT) product is a valuable tool for studying fumigants and how they might perform under various environmental conditions. This laboratory study is an assessment of the efficacy of formic acid against the varroa mite under a range of formic acid concentrations and temperatures. The objectives are 1) to determine the effect of temperature and dose of formic acid on worker honey bee and varroa mite survival, 2) to determine the CT₅₀ products for both honey bees and varroa mites and 3) to determine the best temperature and dose to optimize selectivity of formic acid treatment for control of varroa mites. Worker honey bees and varroa mites were fumigated at 0, 0.01, 0.02, 0.04, 0.08, and 0.16 mg/L at 5, 15, 25, and 35 °C for 12 d. Mite and bee mortality were assessed at regular intervals. Both mite and bee survival were affected by formic acid dose. Doses of 0.08 and 0.16 mg/L were effective at killing mites at all temperatures tested above 5 °C. There was a significant interaction between temperature, dose, and species for the CT₅₀ product. The difference between the CT₅₀ product of bees and mites was significant at only a few temperature-dose combinations. CT product values showed that at most temperatures the greatest fumigation efficiency occurred at lower doses of formic acid. However, the best fumigation efficiency and selectivity combination for treatments occurred at a dose of 0.16 mg/L when the temperature was 35 °C. This revised version was published online in July 2006 with corrections to the Cover Date.     

Repellency of the oily extract of neem seeds (<Emphasis Type="Italic">Azadirachta indica</Emphasis>) against <Emphasis Type="Italic">Varroa destructor</Emphasis> (Acari: Varroidae)

A crude oil extract of neem seed (Azadirachta indica, Sapindales: Meliaceae) was evaluated for repellency on Varroa destructor Anderson and Trueman. Burgerjon’s tower was used to spray worker bee pupae with 0.0, 0.3, 0.7, 1.3, 2.6, 5.3, 10.6 and 21.1% neem extract concentrations. Sprayed pupae were attached to observation arenas and incubated at 32 ± 2°C and 70 ± 10% RH. The ability of V. destructor to locate and feed on treated and untreated pupae was monitored from 30 min to 72 h after spray. Higher and more stable repellency was achieved with 2.6, 5.3, 10.6 and 21.1% neem extract. At the highest concentration, 98% of V. destructor were prevented to settle on bee pupae, resulting in 100% V. destructor mortality at 72 h.     

Behavioural responses of Varroa destructor (Acari: Varroidae) to extracts of larvae, cocoons and brood food of worker and drone honey bees, Apis mellifera (Hymenoptera: Apidae)

Abstract. Varroa destructor is a parasitic mite of the honey bee species Apis cerana Fabr . and A. mellifera L. Mature females reproduce on the immature stages of their hosts, producing more viable female offspring on drone hosts than on worker hosts. Thus, immature drones are more likely to be infested with mites than immature workers. To investigate the hypothesis that differences in host chemistries underlie the biased distribution of mites between worker and drone brood, the arrestment responses of mites to solvent extracts of a number of stimuli normally encountered by a mite during its life cycle were measured. Mites were arrested by cuticular extracts of worker and drone larvae obtained at 0, 24 and 48 h prior to the time when cell capping is completed. Mites were also arrested by extracts of worker and drone, brood food and cocoons, and by a blend of synthetic fatty acid esters previously shown to be active in the host acquisition process. In a wind tunnel bioassay, mites were attracted to odours from living fifth-instar worker and drone larvae, but not to volatiles from cocoons, brood food or a blend of fatty acid esters. The sex of the host was not an important factor affecting the behavioural responses of the mites in any assay. We conclude that host kairomones play a role in the host acquisition process, but we found no evidence to support the hypothesis that mites use these substances to differentiate between worker and drone brood.     

The removal response ofApis mellifera L. colonies to brood in wax and plastic cells after artificial and natural infestation withVarroa jacobsoni Oud. and to freeze-killed brood

LikeApis cerana colonies,A. mellifera colonies also show removal response toVarroa-infested brood cells. Infested worker brood cells of artificially and naturally infested combs were detected by the worker bees to various degrees in all types of comb-material used.The bees uncap brood cells and remove larvae or pupae infested with one or two mites. The removal response of worker bees was stronger towards brood cells containing two mites than cells with one mite.The specific signals which cause the removal of brood cells infested withVarroa mites are unknown. Removal response toVarroa-infested brood cells in plastic comb-material (Jenter-and ANP-comb) was significantly higher than to brood in wax combs. Up to now we do not know to what extent this tolerance mechanism is influenced by genetic and environmental factors.Our experiments comparing the removal of freeze-killed brood with the removal of brood infested withVarroa mites demonstrate positive correlations. Considering the time-consuming method of the artificial infestation with living mites, the hygienic behaviour-including the removal of brood cells infested with mites-of large series of colonies can be tested using freeze-killed brood.