Concept: Varroa destructor
Hop (Humulus lupulus L.) beta acids (HBA) were tested for miticidal effects on varroa destructor Anderson and Trueman, a parasitic mite of the honey bee (Apis mellifera L.). When varroa were placed on bees that had topical applications of 1 % HBA, there was 100 % mite mortality. Bee mortality was unaffected. Cardboard strips saturated with HBA and placed in colonies resulted in mite drop that was significantly greater than in untreated hives. HBA was detected on about 60 % of the bees in colonies during the first 48 h after application. Mite drop in colonies lasted for about 7 days with the highest drop occurring in the first 2-3 days after treatment. There was a reduction in the percentages of bees with HBA and in the amounts on their bodies after 7 days. Bee and queen mortality in the colonies were not affected by HBA treatments. When cardboard strips saturated with HBA were put in packages of bees, more than 90 % of the mites were killed without an increase in bee mortality. HBA might have potential to control varroa when establishing colonies from packages or during broodless periods.
The health of the honeybee and, indirectly, global crop production are threatened by several biotic and abiotic factors, which play a poorly defined role in the induction of widespread colony losses. Recent descriptive studies suggest that colony losses are often related to the interaction between pathogens and other stress factors, including parasites. Through an integrated analysis of the population and molecular changes associated with the collapse of honeybee colonies infested by the parasitic mite Varroa destructor, we show that this parasite can de-stabilise the within-host dynamics of Deformed wing virus (DWV), transforming a cryptic and vertically transmitted virus into a rapidly replicating killer, which attains lethal levels late in the season. The de-stabilisation of DWV infection is associated with an immunosuppression syndrome, characterized by a strong down-regulation of the transcription factor NF-κB. The centrality of NF-κB in host responses to a range of environmental challenges suggests that this transcription factor can act as a common currency underlying colony collapse that may be triggered by different causes. Our results offer an integrated account for the multifactorial origin of honeybee losses and a new framework for assessing, and possibly mitigating, the impact of environmental challenges on honeybee health.
Varroa mites and viruses are the currently the high-profile suspects in collapsing bee colonies. Therefore, seasonal variation in varroa load and viruses (Acute-Kashmir-Israeli complex (AKI) and Deformed Wing Virus (DWV)) were monitored in a year-long study. We investigated the viral titres in honey bees and varroa mites from 23 colonies (15 apiaries) under three treatment conditions: Organic acids (11 colonies), pyrethroid (9 colonies) and untreated (3 colonies). Approximately 200 bees were sampled every month from April 2011 to October 2011, and April 2012. The 200 bees were split to 10 subsamples of 20 bees and analysed separately, which allows us to determine the prevalence of virus-infected bees. The treatment efficacy was often low for both treatments. In colonies where varroa treatment reduced the mite load, colonies overwintered successfully, allowing the mites and viruses to be carried over with the bees into the next season. In general, AKI and DWV titres did not show any notable response to the treatment and steadily increased over the season from April to October. In the untreated control group, titres increased most dramatically. Viral copies were correlated to number of varroa mites. Most colonies that collapsed over the winter had significantly higher AKI and DWV titres in October compared to survivors. Only treated colonies survived the winter. We discuss our results in relation to the varroa-virus model developed by Stephen Martin.
The Varroa mite, Varroa destructor, is an acarine ecto-parasite on Apis mellifera. It is the worst pest of Apis mellifera, yet its reproductive biology on the host is not well understood. In particular, the significance of the phoretic stage, when mites feed on adult bees for a few days, is not clear. In addition, it is not clear whether the preference of mites for nurses observed in the laboratory also happens inside real colonies. We show that Varroa mites prefer nurses over both newly emerged bees and forgers in a colony setting. We then determined the mechanism behind this preference. We show that this preference maximizes Varroa fitness, although due to the fact that each mite must find a second host (a pupa) to reproduce, the fitness benefit to the mites is not immediate but delayed. Our results suggest that the Varroa mite is a highly adapted parasite for honey bees.
Here we present results of a three-year study to determine the fate of imidacloprid residues in hive matrices and to assess chronic sublethal effects on whole honey bee colonies fed supplemental pollen diet containing imidacloprid at 5, 20 and 100 μg/kg over multiple brood cycles. Various endpoints of colony performance and foraging behavior were measured during and after exposure, including winter survival. Imidacloprid residues became diluted or non-detectable within colonies due to the processing of beebread and honey and the rapid metabolism of the chemical. Imidacloprid exposure doses up to 100 μg/kg had no significant effects on foraging activity or other colony performance indicators during and shortly after exposure. Diseases and pest species did not affect colony health but infestations of Varroa mites were significantly higher in exposed colonies. Honey stores indicated that exposed colonies may have avoided the contaminated food. Imidacloprid dose effects was delayed later in the summer, when colonies exposed to 20 and 100 μg/kg experienced higher rates of queen failure and broodless periods, which led to weaker colonies going into the winter. Pooled over two years, winter survival of colonies averaged 85.7, 72.4, 61.2 and 59.2% in the control, 5, 20 and 100 μg/kg treatment groups, respectively. Analysis of colony survival data showed a significant dose effect, and all contrast tests comparing survival between control and treatment groups were significant, except for colonies exposed to 5 μg/kg. Given the weight of evidence, chronic exposure to imidacloprid at the higher range of field doses (20 to 100 μg/kg) in pollen of certain treated crops could cause negative impacts on honey bee colony health and reduced overwintering success, but the most likely encountered high range of field doses relevant for seed-treated crops (5 μg/kg) had negligible effects on colony health and are unlikely a sole cause of colony declines.
Honey bees are increasingly important in the pollination of crops and wild plants. Recent reports of the weakening and periodical high losses of managed honey bee colonies have alarmed beekeeper, farmers and scientists. Infestations with the ectoparasitic mite Varroa destructor in combination with its associated viruses have been identified as a crucial driver of these health problems. Although yearly treatments are required to prevent collapses of honey bee colonies, the number of effective acaricides is small and no new active compounds have been registered in the past 25 years. RNAi-based methods were proposed recently as a promising new tool. However, the application of these methods according to published protocols has led to a surprising discovery. Here, we show that the lithium chloride that was used to precipitate RNA and other lithium compounds is highly effective at killing Varroa mites when fed to host bees at low millimolar concentrations. Experiments with caged bees and brood-free artificial swarms consisting of a queen and several thousand bees clearly demonstrate the potential of lithium as miticidal agent with good tolerability in worker bees providing a promising basis for the development of an effective and easy-to-apply control method for mite treatment.
Varroa destructor, the introduced parasite of European honey bees associated with massive colony deaths, spreads readily through populations of honey bee colonies, both managed colonies living crowded together in apiaries and wild colonies living widely dispersed in natural settings. Mites are hypothesized to spread between most managed colonies via phoretically riding forager bees when they engage in robbing colonies or they drift between hives. However, widely spaced wild colonies show Varroa infestation despite limited opportunities for robbing and little or no drifting of bees between colonies. Both wild and managed colonies may also exchange mites via another mechanism that has received remarkably little attention or study: floral transmission. The present study tested the ability of mites to infest foragers at feeders or flowers. We show that Varroa destructor mites are highly capable of phoretically infesting foraging honey bees, detail the mechanisms and maneuvers by which they do so, and describe mite behaviors post-infestation.
Several factors threaten the health of honeybees; among them the parasitic mite Varroa destructor and the Deformed Wing Virus play a major role. Recently, the dangerous interplay between the mite and the virus was studied in detail and the transition, triggered by mite feeding, from a benign covert infection to a devastating viral outbreak, characterized by an intense viral replication, associated with some characteristic symptoms, was described. In order to gain insight into the events preceding that crucial transition we carried out standardized lab experiments aiming at studying the effects of parasitization in asymptomatic bees to establish a relationship between such effects and bee mortality. It appears that parasitization alters the capacity of the honeybee to regulate water exchange; this, in turn, has severe effects on bee survival. These results are discussed in light of possible novel strategies aiming at mitigating the impact of the parasite on honeybee health.
Honeybee disappearance is one of the major environmental and economic challenges this century has to face. The ecto-parasitic mite Varroa destructor represents one of the main causes of the worldwide beehive losses. Although halting mite transmission among beehives is of primary importance to save honeybee colonies from further decline, the natural route used by mites to abandon a collapsing colony has not been extensively investigated so far. Here, we explored whether, with increasing mite abundance within the colony, mites change their behaviour to maximize the chances of leaving a highly infested colony. We show that, at low mite abundance, mites remain within the colony and promote their reproduction by riding nurses that they distinguish from foragers by different chemical cuticular signatures. When mite abundance increases, the chemical profile of nurses and foragers tends to overlap, promoting mite departure from exploited colonies by riding pollen foragers.
When a female varroa mite, Varroa destructor (Anderson & Trueman), invades a honey bee brood cell, the physiology rapidly changes from feeding phoretic to reproductive. Changes in foraging and malvolio transcript levels in the brain have been associated with modulated intra-specific food searching behaviors in insects and other invertebrates. Transcription profiles for both genes were examined during and immediately following brood cell invasion to assess their role as potential control elements. Vdfor and Vdmvl transcripts were found in all organs of varroa mites with the highest Vdfor transcript levels in ovary-lyrate organs and the highest Vdmvl in Malpighian tubules. Changes in transcript levels of Vdfor and Vdmvl in synganglia were not associated with the cell invasion process, remaining comparable between early reproductive mites (collected from the pre-capping brood cells) and phoretic mites. However, Vdfor and Vdmvl transcript levels were lowered by 37 and 53%, respectively, in synganglia from reproductive mites compared to early reproductive mites, but not significantly different to levels in synganglia from phoretic mites. On the other hand, in whole body preparations the Vdfor and Vdmvl had significantly higher levels of transcript in reproductive mites compared to phoretic and early reproductive, mainly due to the presence of both transcripts accumulating in the eggs carried by the ovipositing mite. Varroa mites are a critical component for honey bee population decline and finding varroa mite genes associated with brood cell invasion, reproduction, ion balance and other physiological processes will facilitate development of novel control avenues for this honey bee parasite.