Parasite preferences for large host body size can drive overdispersion in a fly-mite association

Body size generally correlates intraspecifically with insect fitness but can also correlate with parasite abundance (number of parasites). Host preferences by parasites, and variation in host immunity, could contribute to this trend. We investigated the effect of host size on mite-fly interactions (Macrocheles subbadius and Drosophila nigrospiracula). Mites strongly preferred to infect larger flies in pair-wise choices, and larger flies were more likely to be infected and acquired more mites in infection microcosms. Preferences of parasites resulted in size-biased infection outcomes. We discuss the implications of this heterogeneity in infection on parasite overdispersion and fly populations.

Understanding the Enemy: A Review of the Genetics, Behavior and Chemical Ecology of Varroa destructor, the Parasitic Mite of Apis mellifera

Varroa destructor (Mesostigmata: Varroidae) is arguably the most damaging parasitic mite that attacks honey bees worldwide. Since its initial host switch from the Asian honey bee (Apis cerana) (Hymenoptera: Apidae) to the Western honey bee (Apis mellifera) (Hymenoptera: Apidae), Varroa has become a widely successful invasive species, attacking honey bees on almost every continent where apiculture is practiced. Two haplotypes of V. destructor (Japanese and Korean) parasitize A. mellifera, both of which vector various honey bee-associated viruses. As the population of Varroa grows within a colony in the spring and summer, so do the levels of viral infections. Not surprisingly, high Varroa parasitization impacts bees at the individual level, causing bees to exhibit lower weight, decreased learning capacity, and shorter lifespan. High levels of Varroa infestation can lead to colony-wide varroosis and eventually colony death, especially when no control measures are taken against the mites. Varroa has become a successful parasite of A. mellifera because of its ability to reproduce within both drone cells and worker cells, which allows populations to expand rapidly. Varroa uses several chemical cues to complete its life cycle, many of which remain understudied and should be further explored. Given the growing reports of pesticide resistance by Varroa in several countries, a better understanding of the mite's basic biology is needed to find alternative pest management strategies. This review focuses on the genetics, behavior, and chemical ecology of V. destructor within A. mellifera colonies, and points to areas of research that should be exploited to better control this pervasive honey bee enemy.

Endosymbiotic male-killing Spiroplasma affect the physiological and behavioural ecology of Macrocheles-Drosophila interactions

While many arthropod endosymbionts are vertically transmitted, phylogenetic studies reveal repeated introductions of hemolymph-dwelling Spiroplasma into Drosophila. Introductions are often attributed to horizontal transmission via ectoparasite vectors. Here, we test if mites prefer to infect Spiroplasma poulsonii MSRO infected flies, and if MSRO infection impairs fly resistance against secondary mite (Macrocheles subbadius) attack. First we tested if mites prefer MSRO+ or MSRO- flies using pair-wise-choice tests across fly ages. We then tested whether mite preferences are explained by changes in fly physiology, specifically increased metabolic rate (measured as CO₂ production). We hypothesize that this preference is due in part to MSRO+ flies expressing higher metabolic rates. However, our results showed mite preference depended on an interaction between fly age and MSRO status: mites avoided 14-days old MSRO+ flies relative to MSRO- flies (31% infection), but prefered MSRO+ flies (64% infection) among 26-day old flies. Using flow-through respirometry, we found 14 day-old MSRO+ flies had higher CO₂ emissions than MSRO- flies (32% greater), whereas at 26 days old the CO₂ production among MSRO+ flies was 20% lower than MSRO- flies. Thus, mite preferences for high metabolic rate hosts did not explain the infection biases in this study. To assess changes in susceptibility to infection, we measured fly endurance using geotaxis assays. Older flies had lower endurance consistent with fly senescence, and this effect was magnified among MSRO+ flies. Given the biological importance of male-killing Spiroplasma, potential changes in the interactions of hosts and potential vectors could impact the ecology and evolution of host species. Importance Male-killing endosymbionts are transmitted mother to daughter and kill male offspring. Despite these major ecological effects, how these endosymbionts colonize new host species is not always clear. Mites are sometimes hypothesized to transfer these bacteria between hosts/host species. Here we test if 1) if mites prefer to infect flies that harbour Spiroplasma poulisoni MSRO and 2) if flies infected with MSRO are less able to resist mite infection. Our results show that flies infected with MSRO have weaker anti-mite resistance but the mite preference/aversion for MSRO+ flies varied with fly age. Given the fitness and population impacts of male-killing Spiroplasma, changes in fly-mite interactions have implications for the ecology and evolution of these symbioses.

Towards disrupting Varroa -honey bee chemosensing: A focus on a Niemann-Pick type C2 transcript

We focused our study on the 12 recently identified putative odorant carrier proteins in the ectoparasitic mite, Varroa destructor. Here we show, via an exclusion of the chemosensory appendages (forelegs and gnathosoma) that transcripts of five of the 12 genes were significantly lower, suggesting that they are likely involved in carrying host volatiles. Specifically, three transcripts were found to be foreleg-specific while the other two transcripts were expressed in both the forelegs and gnathosoma. We focused on one of the highly expressed and foreleg-specific transcript Vd40090, which encodes a Niemann-Pick disease protein type C2 (NPC2) protein. Effects of dsRNA-mediated silencing of Vd40090 were first measured by quantifying the transcript levels of genes that encode other putative odorant carrier proteins as well as reproduction related proteins. In addition, the impact of silencing on mites behaviour and survival was tested. Silencing of Vd40090 effectively disrupted Varroa host selection, acceptance and feeding and significantly impaired the expression of genes that regulate its reproduction in brood cells, resulting in reduced reproduction and survival.

Varroa destructor from the Laboratory to the Field: Control, Biocontrol and IPM Perspectives-A Review

Varroa destructor is a real challenger for beekeepers and scientists: fragile out of the hive, tenacious inside a bee colony. From all the research done on the topic, we have learned that a better understanding of this organism in its relationship with the bee but also for itself is necessary. Its biology relies mostly on semiochemicals for reproduction, nutrition, or orientation. Many treatments have been developed over the years based on hard or soft acaricides or even on biocontrol techniques. To date, no real sustainable solution exists to reduce the pressure of the mite without creating resistances or harming honeybees. Consequently, the development of alternative disruptive tools against the parasitic life cycle remains open. It requires the combination of both laboratory and field results through a holistic approach based on health biomarkers. Here, we advocate for a more integrative vision of V. destructor research, where in vitro and field studies are more systematically compared and compiled. Therefore, after a brief state-of-the-art about the mite's life cycle, we discuss what has been done and what can be done from the laboratory to the field against V. destructor through an integrative approach.

Can supplementary pollen feeding reduce varroa mite and virus levels and improve honey bee colony survival?

Varroa destructor is an ectoparasitic mite of immature and adult honey bees that can transmit several single-stranded RNA viruses to its host. Varroa reproduce in brood cells, and mite populations increase as colonies produce brood in spring and summer. Mite numbers also can sharply rise, particularly in the fall, by the migration of varroa into hives on foragers. Colonies with high levels of varroa and viruses often die over the winter. Feeding colonies pollen might keep virus levels low and improve survival because of the positive effects of pollen on immunity and colony growth. We compared varroa and virus levels and overwinter survival in colonies with (fed) and without (unfed) supplemental pollen. We also measured the frequency of capturing foragers with mites (FWM) at colony entrances to determine its relationship to varroa and virus levels. Colonies fed supplemental pollen were larger than unfed colonies and survived longer. Varroa populations and levels of Deformed wing virus (DWV) rose throughout the season, and were similar between fed and unfed colonies. The growth of varroa populations was correlated with FWM in fed and unfed colonies, and significantly affected DWV levels. Increasing frequencies of FWM and the effects on varroa populations might reduce the positive influence of supplemental pollen on immune function. However, pollen feeding can stimulate colony growth and this can improve colony survival.

Inventory of Varroa destructor susceptibility to amitraz and tau-fluvalinate in France

Varroa destructor is one of the greatest threats for the European honeybee, Apis mellifera. Acaricides are required to control mite infestation. Three conventional chemical acaricide substances are used in France: tau-fluvalinate, flumethrin and amitraz. Tau-fluvalinate was used for over 10 years before experiencing a loss of effectiveness. In 1995, bioassay trials showed the first mite resistance to tau-fluvalinate. In some countries, amitraz was widely used, also leading to resistance of V. destructor to amitraz. In France, some efficiency field tests showed a loss of treatment effectiveness with amitraz. We adapted the bioassay from Maggi and collaborators to determine mite susceptibility to tau-fluvalinate and amitraz in France in 2018 and 2019. The lethal concentration (LC) which kills 90% of susceptible mite strains (LC₉₀) is 0.4 and 12 µg/mL for amitraz and tau-fluvalinate, respectively. These concentrations were chosen as the determining factors to evaluate mite susceptibility. Some mites, collected from different apiaries, present resistance to amitraz and tau-fluvalinate (71% of the mite samples show resistance to amitraz and 57% to tau-fluvalinate). As there are few active substances available in France, and if mite resistance to acaricides continues to increase, the effectiveness of the treatments will decrease and therefore more treatments per year will be necessary. To prevent this situation, a new strategy needs to be put in place to include mite resistance management. We suggest that a bioassay would be a good tool with which to advise the policymakers.

Varroa destructor: A Complex Parasite, Crippling Honey Bees Worldwide

The parasitic mite, Varroa destructor, has shaken the beekeeping and pollination industries since its spread from its native host, the Asian honey bee (Apis cerana), to the naïve European honey bee (Apis mellifera) used commercially for pollination and honey production around the globe. Varroa is the greatest threat to honey bee health. Worrying observations include increasing acaricide resistance in the varroa population and sinking economic treatment thresholds, suggesting that the mites or their vectored viruses are becoming more virulent. Highly infested weak colonies facilitate mite dispersal and disease transmission to stronger and healthier colonies. Here, we review recent developments in the biology, pathology, and management of varroa, and integrate older knowledge that is less well known.

Rethinking recognition: social context in adult life rather than early experience shapes recognition in a social wasp

Social recognition represents the foundation of social living. To what extent social recognition is hard-wired by early-life experience or flexible and influenced by social context of later life stages is a crucial question in animal behaviour studies. Social insects have represented classic models to investigate the subject, and the acknowledged idea is that relevant information to create the referent template for nest-mate recognition (NMR) is usually acquired during an early sensitive period in adult life. Experimental evidence, however, highlighted that other processes may also be at work in creating the template and that such a template may be updated during adult life according to social requirements. However, currently, we lack an ad hoc experiment testing the alternative hypotheses at the basis of NMR ontogeny in social insects. Thus, to investigate the mechanisms underlying the ontogeny of NMR in Polistes wasps, a model genus in recognition studies, and their different role in determining recognition abilities, we subjected Polistes dominula workers to different olfactory experiences in different phases of their life before inserting them into the social environment of a novel colony and testing them in recognition bioassays. Our results show that workers develop their NMR abilities based on their social context rather than through pre-imaginal and early learning or self-referencing. Our study demonstrates that the social context represents the major component shaping recognition abilities in a social wasp, therefore shedding new light on the ontogeny of recognition in paper wasps and prompting the reader to rethink about the traditional knowledge at the basis of the recognition in social insects. This article is part of the theme issue 'Signal detection theory in recognition systems: from evolving models to experimental tests'.

Mite bombs or robber lures? The roles of drifting and robbing in Varroa destructor transmission from collapsing honey bee colonies to their neighbors

When honey bee colonies collapse from high infestations of Varroa mites, neighboring colonies often experience surges in their mite populations. Collapsing colonies, often called "mite bombs", seem to pass their mites to neighboring colonies. This can happen by mite-infested workers from the collapsing colonies drifting into the neighboring colonies, or by mite-free workers from the neighboring colonies robbing out the collapsing colonies, or both. To study inter-colony mite transmission, we positioned six nearly mite-free colonies of black-colored bees around a cluster of three mite-laden colonies of yellow-colored bees. We then monitored the movement of bees between the black-bee and yellow-bee colonies before, during, and after mite-induced collapse of the yellow-bee colonies. Throughout the experiment, we monitored each colony's mite level. We found that large numbers of mites spread to the black-bee colonies (in both nearby and distant hives) when the yellow-bee colonies collapsed from high mite infestations and became targets of robbing by the black-bee colonies. We conclude that "robber lures" is a better term than "mite bombs" for describing colonies that are succumbing to high mite loads and are exuding mites to neighboring colonies.

Natural biocide disrupts nestmate recognition in honeybees

Honeybee colonies are under the threat of many stressors, biotic and abiotic factors that strongly affect their survival. Recently, great attention has been directed at chemical pesticides, including their effects at sub-lethal doses on bee behaviour and colony success; whereas the potential side effects of natural biocides largely used in agriculture, such as entomopathogenic fungi, have received only marginal attention. Here, we report the impact of the fungus Beauveria bassiana on honeybee nestmate recognition ability, a crucial feature at the basis of colony integrity. We performed both behavioural assays by recording bee guards’ response towards foragers (nestmate or non-nestmate) either exposed to B. bassiana or unexposed presented at the hive entrance, and GC-MS analyses of the cuticular hydrocarbons (CHCs) of fungus-exposed versus unexposed bees. Our results demonstrated that exposed bees have altered cuticular hydrocarbons and are more easily accepted into foreign colonies than controls. Since CHCs are the main recognition cues in social insects, changes in their composition appear to affect nestmate recognition ability at the colony level. The acceptance of chemically unrecognizable fungus-exposed foragers could therefore favour forager drift and disease spread across colonies.

Parasite dispersal risk tolerance is mediated by its reproductive value

Parasite dispersal theory draws heavily upon epidemiological SIR models in which host status (susceptible (S), infected (I), or recovered (R)) is used to study parasite dispersal evolution. In contrast to these extrinsically host-centric drivers, in this study we focus on an intrinsic driver, the parasite's reproductive value (predicted future offspring) as a regulator of the extent to which the individual will engage in risky dispersal behaviour. As a model system we use the honeybee Apis mellifera and its ectoparasite, the mite Varroa destructor. Mite reproduction happens exclusively inside cells of bee brood, and newly emerged fecund mites may parasitize either a homocolonial brood cell (low risk dispersal) or emigrate to a new bee colony via phoretic attachment to mature forager bees (high risk dispersal). In an empirical bioassay, prepartum mites (high reproductive value) and postpartum mites (low reproductive value) were offered a choice of newly emerged homocolonial worker bees (low risk), homocolonial pollen forager bees (high risk), or heterocolonial pollen foragers (high risk). A preference for newly emerged bees was earlier and more strongly sustained among prepartum mites. This suggests comparatively greater dispersal risk tolerance among postpartum mites with lower reproductive value. A dangerous bid for dispersal may be adaptive if the individual has already successfully reproduced and the rewards for successful dispersal are sufficiently large.

Are Dispersal Mechanisms Changing the Host-Parasite Relationship and Increasing the Virulence of Varroa destructor (Mesostigmata: Varroidae) in Managed Honey Bee (Hymenoptera: Apidae) Colonies?

Varroa (Varroa destructor Anderson and Trueman) are a serious pest of European honey bees (Apis mellifera L.), and difficult to control in managed colonies. In our 11-mo longitudinal study, we applied multiple miticide treatments, yet mite numbers remained high and colony losses exceeded 55%. High mortality from varroa in managed apiaries is a departure from the effects of the mite in feral colonies where bees and varroa can coexist. Differences in mite survival strategies and dispersal mechanisms may be contributing factors. In feral colonies, mites can disperse through swarming. In managed apiaries, where swarming is reduced, mites disperse on foragers robbing or drifting from infested hives. Using a honey bee-varroa population model, we show that yearly swarming curtails varroa population growth, enabling colony survival for >5 yr. Without swarming, colonies collapsed by the third year. To disperse, varroa must attach to foragers that then enter other hives. We hypothesize that stress from parasitism and virus infection combined with effects that viruses have on cognitive function may contribute to forager drift and mite and virus dispersal. We also hypothesize that drifting foragers with mites can measurably increase mite populations. Simulations initialized with field data indicate that low levels of drifting foragers with mites can create sharp increases in mite populations in the fall and heavily infested colonies in the spring. We suggest new research directions to investigate factors leading to mite dispersal on foragers, and mite management strategies with consideration of varroa as a migratory pest.

Population Growth of Varroa destructor (Acari: Varroidae) in Colonies of Russian and Unselected Honey Bee (Hymenoptera: Apidae) Stocks as Related to Numbers of Foragers With Mites

Varroa (Varroa destructor Anderson and Trueman) is an external parasite of honey bees (Apis mellifera L.) and a leading cause of colony losses worldwide. Varroa populations can be controlled with miticides, but mite-resistant stocks such as the Russian honey bee (RHB) also are available. Russian honey bee and other mite-resistant stocks limit Varroa population growth by affecting factors that contribute to mite reproduction. However, mite population growth is not entirely due to reproduction. Numbers of foragers with mites (FWM) entering and leaving hives also affect the growth of mite populations. If FWM significantly contribute to Varroa population growth, mite numbers in RHB colonies might not differ from unselected lines (USL). Foragers with mites were monitored at the entrances of RHB and USL hives from August to November, 2015, at two apiary sites. At site 1, RHB colonies had fewer FWM than USL and smaller phoretic mite populations. Russian honey bee also had fewer infested brood cells and lower percentages with Varroa offspring than USL. At site 2, FWM did not differ between RHB and USL, and phoretic mite populations were not significantly different. At both sites, there were sharp increases in phoretic mite populations from September to November that corresponded with increasing numbers of FWM. Under conditions where FWM populations are similar between RHB and USL, attributes that contribute to mite resistance in RHB may not keep Varroa population levels below that of USL.

Pathogenesis of varroosis at the level of the honey bee (Apis mellifera) colony

The parasitic mite Varroa destructor, in interaction with different viruses, is the main cause of honey bee colony mortality in most parts of the world. Here we studied how effects of individual-level parasitization are reflected by the bee colony as a whole. We measured disease progression in an apiary of 24 hives with differing degree of mite infestation, and investigated its relationship to 28 biometrical, physiological and biochemical indicators. In early summer, when the most heavily infested colonies already showed reduced growth, an elevated ratio of brood to bees, as well as a strong presence of phenoloxidase/prophenoloxidase in hive bees were found to be predictors of the time of colony collapse. One month later, the learning performance of worker bees as well as the activity of glucose oxidase measured from head extracts were significantly linked to the timing of colony collapse. Colonies at the brink of collapse were characterized by reduced weight of winter bees and a strong increase in their relative body water content. Our data confirm the importance of the immune system, known from studies of individually-infested bees, for the pathogenesis of varroosis at colony level. However, they also show that single-bee effects cannot always be extrapolated to the colony as a whole. This fact, together with the prominent role of colony-level factors like the ratio between brood and bees for disease progression, stress the importance of the superorganismal dimension of Varroa research.

Why do Varroa mites prefer nurse bees?

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.

Population growth of Varroa destructor (Acari: Varroidae) in honey bee colonies is affected by the number of foragers with mites

Varroa mites are a serious pest of honey bees and the leading cause of colony losses. Varroa have relatively low reproductive rates, so populations should not increase rapidly, but often they do. Other factors might contribute to the growth of varroa populations including mite migration into colonies on foragers from other hives. We measured the proportion of foragers carrying mites on their bodies while entering and leaving hives, and determined its relationship to the growth of varroa populations in those hives at two apiary sites. We also compared the estimates of mite population growth with predictions from a varroa population dynamics model that generates estimates of mite population growth based on mite reproduction. Samples of capped brood and adult bees indicated that the proportion of brood cells infested with mites and adult bees with phoretic mites was low through the summer but increased sharply in the fall especially at site 1. The frequency of capturing foragers with mites on their bodies while entering or leaving hives also increased in the fall. The growth of varroa populations at both sites was not significantly related to our colony estimates of successful mite reproduction, but instead to the total number of foragers with mites (entering and leaving the colony). There were more foragers with mites at site 1 than site 2, and mite populations at site 1 were larger especially in the fall. The model accurately estimated phoretic mite populations and infested brood cells until November when predictions were much lower than those measured in colonies. The rapid growth of mite populations particularly in the fall being a product of mite migration rather than mite reproduction only is discussed.

How Honey Bee Colonies Survive in the Wild: Testing the Importance of Small Nests and Frequent Swarming

The ectoparasitic mite, Varroa destructor, and the viruses that it transmits, kill the colonies of European honey bees (Apis mellifera) kept by beekeepers unless the bees are treated with miticides. Nevertheless, there exist populations of wild colonies of European honey bees that are persisting without being treated with miticides. We hypothesized that the persistence of these wild colonies is due in part to their habits of nesting in small cavities and swarming frequently. We tested this hypothesis by establishing two groups of colonies living either in small hives (42 L) without swarm-control treatments or in large hives (up to 168 L) with swarm-control treatments. We followed the colonies for two years and compared the two groups with respect to swarming frequency, Varroa infesttion rate, disease incidence, and colony survival. Colonies in small hives swarmed more often, had lower Varroa infestation rates, had less disease, and had higher survival compared to colonies in large hives. These results indicate that the smaller nest cavities and more frequent swarming of wild colonies contribute to their persistence without mite treatments.

Ecology of Varroa destructor, the Major Ectoparasite of the Western Honey Bee, Apis mellifera

Varroa destructor is the most important ectoparasite of Apis mellifera. This review addresses the interactions between the varroa mite, its environment, and the honey bee host, mediated by an impressive number of cues and signals, including semiochemicals regulating crucial steps of the mite's life cycle. Although mechanical stimuli, temperature, and humidity play an important role, chemical communication is the most important channel. Kairomones are used at all stages of the mite's life cycle, and the exploitation of bees' brood pheromones is particularly significant given these compounds function as primer and releaser signals that regulate the social organization of the honey bee colony. V. destructor is a major problem for apiculture, and the search for novel control methods is an essential task for researchers. A detailed study of the ecological interactions of V. destructor is a prerequisite for creating strategies to sustainably manage the parasite.