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Co-adaptation of Apis cerana Fabr. and Varroajacobsoni Oud

Werner Rath

To cite this version:Werner Rath. Co-adaptation of Apis cerana Fabr. and Varroa jacobsoni Oud. Apidologie, SpringerVerlag, 1999, 30 (2-3), pp.97-110. �hal-00891571�

Review article

Co-adaptation of Apis cerana Fabr.and Varroa jacobsoni Oud.

Werner Rath

Department of Horticulture, Faculty of Agricultural Production,Maejo University, Sansai, Chiang Mai, Thailand

(Received 20 August 1998; accepted 11 February 1999)

Abstract - The research on bee and mite biology over the past 20 years has uncovered numerous detailsof the A. cerana-V. jacobsoni co-adaptation which are systematically summarized here. A. ceranaacquired a high degree of hygienic efficiency with a differentiated set of behavioural traits that wedescribe in this review in a broad sense to include grooming of mites by adult bees, uncapping andremoval of infested brood and entombing of infested brood. Approximately 20 % of the reproducingmite population can be eliminated by entombing of lethally parasitized drone pupae. In their equallyeffective infesting behaviour the parasites explore the most suitable adult and larval host individualsfor safe phoretic positions, the favourable caste and suitable age. A. cerana compel V. jacobsoni toreproduce exclusively on drone brood hosts. This limited reproduction, in combination with char-acteristics of the population dynamics of the host, are key factors which limit mite populations to tol-erable levels. © Inra/DIB/AGIB/Elsevier, Paris

Apis cerana / Varroa jacobsoni / co-adaptation

1. INTRODUCTION

Varroa jacobsoni Oudemans (Acari: Var-roidae) females were first collected byJacobson from its host, the Asian honeybeeApis cerana Fabricius, in the year 1904 inJava, Indonesia, and were subsequentlydescribed by the zoologist Oudemans. Thebee parasite received increased attention by

scientists only after it became a major peston its acquired host, Apis mellifera L., inEurope. The early research on the life cycle,biology and impacts of V. jacobsoni wasalmost exclusively conducted on the newhost A. mellifera. However, it appeared thata profound understanding of V. jacobsoniand the parasite-host relations had to be

E-mail: rath@chiangmai.a-net.net.th

based on the original relationship betweenA. cerana and V. jacobsoni. While the earlyreports from Koeniger et al. [21, 22] attractedattention to the original host, the two publi-cations of Peng et al. in 1987 [28, 29] onthe successful grooming behaviour ofA. cerana worker bees were received with

great excitement. The research of Peng [28]nurtured the hope of finding new ways tocontrol V. jacobsoni on A. mellifera. Even asthe quantifications of Peng et al. [28] arerevised by more recent research [4, 10, 17,30], this first publication on the interactionbetween V. jacobsoni and A. cerana is stillsubstantial and frequently cited.

The research on the co-adaptationsbetween A. cerana and V. jacobsoni overthe past 18 years remains modest comparedto the extensive attention to V. jacobsoni onA. mellifera, e.g. as reviewed in 1994 byMilani [26]. Still lacking are data on thepopulation dynamics of A. cerana andV. jacobsoni in their natural area of distribu-tion, and data on the reproductive biology ofV. jacobsoni on drone brood of A. cerana.Many of the well-studied aspects of the mitebehaviour in A. mellifera hosts are almostunknown from the original host A. cerana;e.g. the cognitive abilities and stress-relatedbehaviour of A. cerana, and the behaviour inbrood cells, reproductive cycles, phoreticchoice and dispersal of the mites.

Interspecific co-adaptation is a theoreti-cal concept that describes the phenomenaof a mutual adaptation of two species. Theinterpretation of observable phenomena asco-adaptation is closely connected to theconcept of co-evolution which implies theassumption that the ancestors of A. ceranaand V. jacobsoni exerted selective forces onone another [34]. The following sectionswill summarize host and parasite traits thatmutually influence their respective fitnessand accordingly can be considered as co-adaptive traits. The current knowledge onthe relationship between A. ceranae andV. jacobsoni is grouped here under threecategories: 1) hygienic traits versus infesting

traits; 2) population-dynamic aspects; and3) physiological aspects. The use of the word’hygienic’ is used here in the broad senseto include grooming behaviour, the uncap-ping and removal of parasitized brood, andthe entombing of mites in cells, as opposedto Rothenbuhler’s use of ’hygienic behav-iour’ to describe solely the uncapping andremoval of diseased brood [37]).

2. HYGIENIC TRAITS OFA. CERANA VERSUS INFESTINGTRAITS OF V. JACOBSONI

2.1. The senses of bees and mites

It is obvious that bees and mites musthave elaborate senses to perceive eachother’s presence in different places and occa-sions. V. jacobsoni depends on specificsenses to locate its host, to move to phoret-ically safe places on adult bees [14, 32], tofind phoretic feeding places and even to dis-tinguish between adult bees of different agesand physiologic states (on A. mellifera: [6, 7,24]). The sensory capabilities of V. jacob-soni provide it with information that directsits behaviour to the correct worker or dronebrood host which allows optimum timingof reproduction in A. cerana colonies. Apartfrom the sensory capabilities, the mites inter-act with each other during feeding on adultbees and in brood cells (on A. mellifera:[ 15]). Adult female mites are able to recog-nize host bees in close proximity, and theyfrequently react by jumping rapidly to attachthemselves phoretically to their host. How-ever, apart from some experimental evi-dence on A. mellifera hosts and assumptionsdeduced from general observations, there islittle information on the neurophysiologyof V. jacobsoni, the qualities of its sensesand the stimuli that regulate its behaviourto safe phoresy, successful dispersal andeffective reproduction on A. cerana hosts.

A. cerana must also have specific sensesto detect mites on its body or in brood cells.These specific senses are a prerequisite to

pursuing the hygienic actions of grooming,the uncapping of cells and the removal ofinfested brood. Most of the neurobiologyof the bees in relation to their perception ofthe mites is obscure and several observa-tions are not congruent. For example, it iscommon to observe exposed V. jacobsonifemales on the thorax of A. cerana workerswithout any grooming reaction by the bees.Nestmates may even touch the exposedphoretic mites with their antenna or mouthparts without any further reaction to the par-asite. Additionally A. cerana are not dis-turbed at all by certain non-parasitic phoreticmites that are almost as large as V. jacobsoni[12]. In southern Thailand, individual A. cer-ana workers were found with more than tenmites of the non-parasitic pollen-feedingNeocypholaelaps sp., and the bees did notshow any grooming response (pers. obs.).On the other hand, the bees are disturbedand react by self grooming or by performinggrooming dances when V. jacobsoni leavephoretically safe positions and move aroundon their hosts.

Some investigations suggested that olfac-tory senses are important in mite detection,e.g. A. cerana were found to be more sensi-tive in the detection of experimental mitesthat originated from another host colony([10, 36], conc. adhering scents: [27]. It isnot known how bees sense that certainsealed brood cells are infested with V. jacob-soni, or contaminated with artificially intro-duced particles [30, 36]. This remarkablesense, which also occurs in A. mellifera,induces the uncapping and removal ofmicrobial-infested and dead sealed brood

[37, 41, 42]. In an early investigation on theabilities of A. mellifera to detect sealedworker brood infested with American foul-brood, Schulz-Langer [40] found that beesdetected and uncapped such infected cellsrapidly when the volume of the cell con-tents was altered rather than when the cellscontained strong smelling substances. Theresearcher concluded that physical stimulisuch as responses to the volume of the cells

may play a greater role in the detection of

diseased brood than olfactory stimuli. It isalso uncertain whether nursing bees per-ceive qualitative information on the condi-tion of sealed bee brood via pheromone sub-stances, which might enable the bees torecognize healthy or infested, parasitized ordead pupae, and consequently tolerate themor reject them.

2.2. Groomingof V. jacobsoni by A. cerana

A. cerana worker bees perform a highlydifferentiated set of behavioural traits thatcan be summarized as ’grooming’, ’uncap-ping’, ’removing’ and ’entombing’ [19, 21,22, 28, 29, 33].

The grooming action of A. cerana towardphoretic V. jacobsoni received much atten-tion following a research report by Peng atal. [28]. The researchers observed thatA. cerana bees groomed themselves andalso performed grooming dances to recruitindividual or several nestmates to engagein social grooming. During self grooming,A. cerana bees brush different parts of their

body. In social grooming, the nestmatestouch the thorax, propodeum and abdomenof the infested bee with their antenna, front

legs and mandibula. The action of groomingis not specifically directed at the phoreticmite; rather, it appears that distinct areasare automatically searched [13, 32]. Thegrooming activity may cause the parasite toleave the host or the bees may succeed in

damaging the mite with their mandibula.Most of the damage that A. cerana beesinflict on V. jacobsoni occurs on parts ofthe legs [10, 17, 30].

Table I compares the results from four

experimental investigations on the groomingactivity of A. cerana. The success rates ofchasing the mites from their host bee arereported to range from 61.7 to 99.6 %, whilethe success rate of actually killing the mitesranged from 52 to 61 % (table I). The quan-tification of grooming success rates and theresults on the detection of mites in sealed

brood are influenced by the source of themites [10, 36]. Mites that originated fromA. mellifera colonies were detected by thealien scents of the mites, and mites that werecollected from brood cells were not fit for

phoresy [30]. According to Peng et al. [28]the ability to groom mites indicates that A.cerana "evolved both a physiological anda behavioural adaptation to the parasitismof V. jacobsoni". These researchers sug-gested that the bees sense the mites becauseof "irritation ... caused by salivary compo-nents injected by the mouthparts of the mitesduring feeding". In fact, A. cerana workersreact most intensively to mites that justarrive and move on their body, and the beesgroom less or not at all when mites staycalmly in their phoretic feeding positionsin lateral intersternite folds (pers. obs.).

In the studies cited in table I, it was rarelypointed out that the experimental results onthe grooming success rate would leave vir-tually no chance for survival for the para-sites [17]. In contrast to the experimentalresults on grooming success rates [28], cal-culations derived from a model of the pop-ulation dynamics of V. jacobsoni by Frieset al. [16] indicate that the grooming ofA. cerana workers would cause a daily mitemortality of 1 %, and restrict the number ofreproductive cycles of mother mites to0.75 cycles. The daily mortality of I %would reduce an initial mite population of tenV. jacobsoni to five mites only after 70 days.

The parameter on reproductive cycles, asassumed by Fries et al. [16] for A. cerana, isnot appropriate since the authors based theircalculations on rates of drone rearing andseasonal production of drones in temperateA. mellifera colonies. The populationdynamics of drones in A. cerana colonies,however, differ considerably from A. mel-lifera (see section 3).

2.3. The phoretic strategyof V. jacobsoni

V. jacobsoni has a very peculiar bodyshape and associated traits that enable themite to evade the bees’ grooming attacks[25]. The adult V. jacobsoni females have anelliptical and concave shaped dorsal shieldof approximately 1.5 x I mm. The concavedorsum is matched by a convex ventral side,which permits a close fit to the rounded sur-faces of the host body. This morphology isof foremost importance in the avoidance ofthe bees’ grooming attempts, and may be aphysical adaptation which enabled the Var-roidae to be phoretic, in contrast to the lae-lapid mites (see below). A. cerana workerscan open their mandibula to approximately2 mm (in northern Thailand); thus, the sizeof the dorsal shield of V. jacobsoni is lessprotective than the function of the con-cave-convex body shape. Grooming work-ers have no chance to grab the parasite oncean adult female V. jacobsoni reaches a posi-

tion on the host where its concave ventrumfits closely to the rounded bee body. In thisrespect, the peculiar shape of V. jacobsoni isa morphological adaptation to the bees’intensive grooming. The only opportunityfor A. cerana workers to catch a mite is tobite it from the side, with one mandibula onthe dorsal and the other on the ventral side ofthe mite. Therefore, the space between theedge of the mite dorsal shield and the hostbody surface is of crucial importance. Thelanceolate lateral and posterior setae of theVarroidae, which are especially prominent inthe genus Euvaroa sp., are structures which

support the closing of the gap between themite dorsal shield and the host surface.

Two observations may underline the sig-nificance of mite morphology to groomingsuccess. First, female V. jacobsoni changethe convexity of their ventrum into a flat orconcave condition when they enter thereproductive stage. Usually such gravidmites stay in brood cells and are not sub-

jected to grooming by bees. It was observedfrequently that gravid mites which werereleased onto A. cerana workers were caughtand damaged quickly by grooming work-ers because the mites had lost their close fitto the host body surface when the gapbetween the edge of the dorsal shield andthe host surface was widened. Under natu-ral conditions, gravid mites that leave broodafter the bees uncapped the cell [30, 36]most likely become victims of groomingbees. The second observation concerns theaccidental occurrence of Tropilaelapsclareae mites in A. cerana colonies [13, 49].Although T. clareae comes into close con-tact with A. cerana it survives only on Apisdorsata and Apis mellifera hosts [13].T. clareae have a phoretic strategy whichis different from the slow moving, ’close-fit’ approach of the Varroidae. These lae-lapid mites escape grooming attempts byvery rapid movements on the host. Theirelongated body facilitates fast forwardmovements but at the same time offers anarrow and protruding dorsum which A. cer-ana obviously finds convenient to grasp.

The ’rapid escape’ strategy seems to workwell for Tropilaelaps ssp. on A. dorsata andA. mellifera but fails completely when con-fronted with the grooming skills of A. cerana[49]. Likewise, the Varroidae never estab-lished themselves on A. dorsata hosts.

2.4. Preferencesfor phoretic positions

V. jacobsoni mites prefer certain phoreticpositions on A. cerana, where they are safefrom the hosts’ grooming attempts andwhere they encounter a chance to feed. Thebees groom only certain body areas, so itseems that the mites are indirectly guidedto the safe positions by the groomingattempts of the bees. On A. cerana work-ers, 87 % of V. jacobsoni choose phoreticpositions between the second and third lat-eral tergites (anatomically the first abdom-inal segment is included in the propodaeum)[32]. On A. cerana drones, this preferencewas even more restricted to the second lat-eral intertergit on the left side. The left-sidedpreference for the phoretic position was veryconsistent in 52 out of 54 cases and there isat present no explanation for this phe-nomenon ([14, 30, 32], for A. mellifera: [6]).It seems that A. cerana drones do not prac-tice self grooming nor do they participatein social grooming, accordingly a higherpercentage of mites can be found in exposedpositions on the thorax or abdomen ofdrones when compared to workers [20, 32].At the colony level, V. jacobsoni popula-tions were found on approximately 75 % onthe worker bees and approximately 25 %on drones. Cage experiments revealed a sim-ilar distribution pattern [30].

2.5. Uncapping, removal,and entombing behaviour of A. cerana

A. cerana are able to detect parasitizedcapped worker brood. Their first reactionconsists in the uncapping of the invaded

worker cells, and is probably followed bya closer inspection of the brood. If the mitesleave the opened cell, the workers acceptthe brood as healthy and re-seal the cell [31,33, 36]. If the mites stay in the brood cell,the bees cannibalize or remove the workerbrood. This uncapping and removalbehaviour is not a specific adaptation againstV. jacobsoni, it is a general hygienic traitbecause the bees also react by uncappingand removing the brood when the broodcells are inoculated with particles of differ-ent materials [33, 36] or when the larvae aresuffering from microbial infestations [37,41]. Rosenkranz et al. [36] assumed that thebees detect the specific odour that adheres tothe mites, or that the bees are sensitive tovibrations in the brood cells. Although theparticles (chitin from A. cerana) used in theexperiments of Rosenkranz et al. [36] didnot carry V. jacobsoni odours, they werestill recognized. It might be that the beesreact to metabolic products of the workerbrood, such as brood pheromones, that arereleased in varying quantities in relation tothe health condition of the brood [31, 48].

When freeze-killed sections of workerbrood were offered to an A. cerana colony inThailand, the bees repeatedly uncapped andremoved all dead worker brood within 48 h

[30, 31]. In the same experiment, the beesdid not remove the freeze-killed drone brood

(less than 10 % removal after 13 days). Sim-

ilarly, the bees did not react to mite-infesteddead drone brood. The A. cerana workersleft the dead drone brood capped, thusentombing the parasites and any brood dis-ease [19, 22, 23, 30]. Therefore, the beesmay use two strategies: uncapping + removaland entombing. Both strategies are poten-tially successful; uncapping + removal elim-inates infesting agents from the hive, andentombing seals off these agents [42].A. cerana drones are especially sensitive toinvasion by two or more V. jacobsoni. Inexperiments in which brood cells wereexperimentally infested with mites, only28.6 and 14.3 % of drones eclosed whentwo mites and three mites, respectively,invaded the cell [30, 32] (table II). Morethan half of such multiple-infested dronesdid not show any visible signs of an arrestedor otherwise handicapped development.Instead, it seemed that they were not ableto mobilize sufficient energy to open theirown hard cocoon caps. In the natural situa-

tion, A. cerana bees are not inclined toremove dead drone brood rapidly. Mites aswell as other pests that affect the dead drone

pupa are entombed. Koeniger et al. [23]reported accumulations of 10-28 deadV. jacobsoni per cell together with the deaddrone pupa. These cells were invaded bytwo to five mother mites, considering thatone invading mother V. jacobsoni producedfour offspring [5, 39].

Multiple infestation of A. cerana dronecells is not a rare event. Boot et al. [5] foundthat 19.9-25.8 % of infested A. cerana dronecells were invaded by two mites and6.6-9.0 % were invaded by three V. jacob-soni (data collected from North and SouthVietnam, respectively). By combining thedata on eclosure rates of multiple-infestedcells [30] and the frequency of multipleA. cerana drone cell infestations [5], it canbe derived that out of 100 reproducingV. jacobsoni, 19-26 mites are killed byentombing, up to one quarter of the repro-ducing mite population. Although the mul-tiple invasion of cells might facilitate geneticvariation of V. jacobsoni by the increasedchance for interbreeding [ 18], it imposes asignificant limiting effect on the populationgrowth by the resulting risk of entombing.Entombing has scarcely been researched,but it may be the most effective limitation ofV. jacobsoni population growth. The aggre-gated distribution of mites in drone broodcells is most likely a dynamic function ofthe correlation between the amount of inva-sible cells and the number of reproductivemites at a given time. Multiple cell invasionsand subsequent entombing of these infesteddrones are most prominent in A. ceranacolonies with small amounts of drone broodand large V. jacobsoni populations.

The drone cell caps of A. cerana are a

very unique structure whose biological sig-nificance, function and relation to V. jacob-soni are still unexplained [19, 20, 31]. Thehard, funnel-like drone cell cap is part ofthe larval cocoon. The original wax cap ofthe drone cell is usually removed by theworker bees after the completion of thecocoon cap. The cocoon cap has an elevatedcentral hole which is necessary for the

exchange of respiration gases [30, 31]. Oneexperiment still requires interpretation inrelation to the uncapping and removalbehaviour. When the cell caps from healthyworker and drone brood in prepupal stagewere experimentally removed, the workerbees accepted the worker brood and re-sealed it with a wax cap, but they rejected

and removed the drone brood [30, 31]. Isthe drone cocoon cap a protective device?What are the ethological reasons for therejection of capless drone brood, and is therean evolutionary connection to the parasiticmites? Hadisoesilo and Otis [19] recentlydiscovered that a close relative of A. cer-ana in Papua New Guinea, Apis nigrocincta,has a soft wax cap only; i.e. the hard cocooncap is missing completely. However, thisobservation needs further investigation asrelated details on the mite parasites in Apisnigrocincta presently are missing.

2.6. Brood cell invasion strategiesof V. jacobsoni

During the invasion of A. cerana dronecells, the mites are exposed and subjectedto detection by nurse bees. V. jacobsonifemales squeeze themselves into the nar-row gap between the host larva and the cellwall. The convex dorsal shield, which isfacing the rounded cell wall, appears to be anadvantageous morphological adaptation. Incomparison, we observed the laelapid mite,Tropilaelaps clareae, staying on top of theirhost larva during cell invasion until the nursebees completed the wax cap and the beelarva began cocoon-spinning movements.When the female V. jacobsoni reach the cellbottom of A. cerana drone cells, they regu-larly submerge themselves in the remainsof liquid larval food [30, 32]. In this sub-merged stage, the peritremes of the miteprotrude visibly from the liquid food, but itis not clear which actual function should beattributed to the peritremes [8]. Based ontheir comparison of the peritremes ofV. jacobsoni and Tropilaelaps spp., Bruce etal. [8] suggested that the adaptive hypothe-ses of the peritremes should be reviewedwith caution.

When V. jacobsoni females come intocontact with the bee larval food, they stop allactive movements and rest in a state of aki-nesis until all the bee food is removed bythe feeding larvae. The mites usually recover

before the bee larvae spin their cocoon;however, in rare cases, dead mites can beencountered fixed to the cell bottom bycocoon threads. The akinesis stage, whichcan be experimentally triggered [30, 32],helps the mite to reduce oxygen consump-tion while submerged and it also reducesthe chances that nurse bees recognizemovements of mites in the infested cells. Itis not clear whether the mites submergethemselves in larval food purposely or acci-dentally, but apparently the mites are welladapted to this unusual condition. The mitebehaviour of proceeding to the bottom ofthe host brood cells and submerging them-selves in the remains of the larval food canbe interpreted as a counter-strategy to thewell-developed removal behaviour of theadult bees.

3. A. CERANA POPULATIONDYNAMICS

A. cerana colonies inhabit a wide rangeof climates ranging from arid to temperatesubtropical and tropical regions [38]. Accord-ingly, the population dynamics of A. cer-ana colonies may vary considerably. Theperiods and amounts of drone brood rear-ing are of foremost importance in the co-adaptation between A. cerana and V. jacob-soni because the reproduction of the mites isrestricted to drone brood owing to the mite’s infertility in A. cerana worker brood cells,and the removal of infested brood by workerbees ([1, 5, 21, 29, 33, 36, 47], see also

table III). However, only a few publisheddata on drone brood rearing seasons andcycles for A. cerana are available. In Japan,Sasaki [39] reported a distinctive dronebrood season of 3 months from mid Marchto mid June for A. cerana japonica. The cor-responding time for A. mellifera drone broodrearing in the same region in Japan was3 months longer than for A. cerana. In theChiangmai region of northern Thailand,A. cerana colonies had one or two dronebrood rearing cycles per year with periods ofon average 59.5 (± 33.4) days and an aver-age drone production of 30 drones percolony per day [30, 32]. The peak number ofsealed drone brood per A. cerana colony isusually below 800 cells [30]. With A. ceranadrone postcapping development times of13.5-14 days [39, 43] a single fertile mitewould have only approximately three to fivereproductive cycles per year. This is a con-siderable limitation which imposes a strongselective pressure on V. jacobsoni to makea maximum reproductive effort within a lim-ited period, and to endure the hosts’ groom-ing efforts during enforced long phoreticperiods with a reproductive diapause of upto 9 months [39].

The situation is different at the colonycommunity level. In evergreen moist tropi-cal habitats, drone brood production is notalways restricted to a definite season. Withina local community of A. cerana colonies,drone brood may be present in singlecolonies and absent in others. For example,in the Suratani region of southern Thailand,

A. cerana drone brood can be found in somecolonies during any season, depending onthe availability of local pollen resources.Sasaki [39] even found that V. jacobsonipropagates well in drone cells of queenless,laying worker colonies of A. cerana japon-ica, with four to five female offspring permother mite. The important issue of mitedispersal among A. cerana colonies has notyet been studied. Furthermore, the influenceof the absconding and migrating habits ofA. cerana colonies on the V. jacobsoni pop-ulation is uncertain. In some areas, such asnorthern Thailand, A. cerana bees migrateregularly during the dry and hot season.

Recent investigations of Boot et al. [5]on the reproductive biology of V. jacobsoniin A. cerana drone brood cells confirmedthe hypothesis that V. jacobsoni face a nat-ural selective pressure to maximize their

reproductive effort within a short time. Incomparison to A. mellifera hosts, the high-est reproductive success of V. jacobsoni withrespect to the number of offspring and thefertility of mites occurred in A. cerana dronecells. Boot et al. [5] found that V. jacobsonifemales produce an average of 4.3 femaleoffspring in A. cerana drone brood cells,and nearly 100 % of the mites that entereddrone cells actually reproduced. Table IIIcompiles some comparative results frominvestigations on the reproductive rates ofV. jacobsoni under natural conditions in fourregions of Asia. The investigations revealthat V. jacobsoni do actually enter A. cer-ana worker cells but, apart from some excep-tional cases, they do not reproduce [5, 11,47]. The invasion of worker brood cells isfacilitated because the removal response ofA. cerana bees is low if the mites originatefrom the very same colony [36].

4. PHYSIOLOGICAL ASPECTS

Boot et al. [5] posed the question as towhy V. jacobsoni enter the worker broodcells of A. cerana if they do not reproducethere at all? These researchers assumed that

the invasion A. cerana worker cells is an

adaptive trait and that V. jacobsoni gainsome prospective advantage from thisbehaviour. This advantage might be physi-ological preparation for postponed repro-duction, or the avoidance of the groomingbehaviour by adult bees. The cause ofV. jacobsoni infertility in A. cerana workercells is not known. The mites encounter a

physiological barrier against reproductionon A. cerana worker brood. V. jacobsonifemales that were transferred after initiallyfeeding on A. mellifera brood to A. ceranaworker brood continued to reproduce [5, 30,47]. This observation supports the hypoth-esis that V. jacobsoni females receive a cer-tain nutritional trigger prior to oogenesisand reproduction [35].

V. jacobsoni have a very short period ofontogenesis which seems to be well syn-chronized with the host [13]. The rapid onto-genesis is supported by the very efficientexploitation of the host haemolymph pro-teins. V. jacobsoni have almost no prote-olytic enzymes and utilize the host proteinsimmediately without prior digestion [45].Tewarson et al. [44] provided evidence thatreproducing V. jacobsoni incorporate thehost proteins (A. mellifera) directly into theoocytes. According to Tewarson and Jany[46], this type of vitellogenesis is an indi-cation of the high degree of parasitic adap-tation which enables the mites to have veryrapid oogenesis and a subsequent increase inreproduction efficiency. An additional fea-ture of the accelerated ontogenesis ofV. jacobsoni is the phenomenon of larvipo-sition (ovoviviparity). The ’egg’ that V. jacob-soni females deposit consists of an eggchorion containing a larva which transformsinto a protonymph before hatching from theegg chorion [ 13]. Also, the protonymphae ofV. jacobsoni directly utilize undegraded hostproteins [45]. The physiological adaptationof V. jacobsoni to the utilization of unde-graded host haemolymph components maypartially account for the reproductive effi-ciency on other hosts besides the A. ceranadrone larvae on which the natural selection

for this adaptation took place. The fertility ofV. jacobsoni on the acquired host, A. mel-lifera, is highly variable, depending on thegenetics of the mites and bees and the localconditions [1, 2, 3, 10, 16]. Recent investi-gations on the mitochondrial DNA sequenceof different V. jacobsoni populations [2, 3]indicate that V. jacobsoni is probably a"species-complex of at least three differentspecies" [2] with significant differences intheir reproductive performance on A. cer-ana or A. mellifera hosts. However, the high-est fertility rates were reported from A. cer-ana drone hosts (table III). V. jacobsonienter worker brood cells of A. cerana and

occasionally begin reproductive efforts, butsuccessful reproduction leading to fertilefemale adult offspring has rarely beenobserved in natural conditions [10, 35, 47].It is likely that only specific components ofthe host haemolymph, which are best pro-vided in A. cerana drones, ensure optimumfertility in V. jacobsoni. Tewarson andEngels [44] revealed that V. jacobsoni evenutilize undegraded bovine serum albumen;however, the further effects of atypical hostprotein components on the reproductionbiology of V. jacobsoni have not been inves-tigated.

5. PERSPECTIVES

The co-adaptations of V. jacobsoni andA. cerana constitute a complex network oftraits that support a dynamic balancebetween the host and the parasite popula-tions. According to field observations [33]and model calculations [16, 25] V. jacob-soni populations in A. cerana colonies arelimited to numbers of less than approxi-mately 800 mites per colony.

The early research reports overstated thesignificance of grooming and brood removalas the major mechanisms that define theresistance of A. cerana. Recent investiga-tions, however, indicate that entombing, incombination with the population dynamicsof A. cerana drone brood and physiologi-

cal characteristics, may be of foremostimportance in the tolerance of A. cerana tothe mite. The recent discovery of distinctgenotypes and biotypes in V. jacobsoni pop-ulations on A. cerana and A. mellifera hosts[2, 3] will require a re-interpretation of thevirulence of V. jacobsoni on the acquiredA. mellifera host. Furthermore, it has becomeclear that basic data is missing on the neu-robiology of bees and mites. In particular,the investigation of the physiological back-ground of the reproductive barriers ofV. jacobsoni on A. cerana worker brood hasthe potential to open new approaches forthe future search of V. jacobsoni tolerance inA. mellifera.

Résumé - La coadaptation d’Apis ceranaFabr. et de Varroa jacobsoni Oud. On peutconsidérer comme équilibrée la relationhôte-parasite entre l’abeille mellifère asia-tique, Apis cerana, et l’acarien ectoparasite,Varroa jacobsoni : les abeilles limitent acti-vement les populations d’acariens à unniveau supportable et la survie de l’acarienn’est pas mise en danger. Cet équilibre bienréglé est les résultats d’un long processusde coévolution au cours duquel les hôtes etles parasites ont développé des caractèresspécifiques qui améliorent leur valeur adap-tative respective. On résume ici diversaspects des caractères coadaptatifs, qui ontété étudiés durant les 20 dernières années,afin de faciliter l’interprétation de la rela-tion hôte-parasite. On peut regrouper lescaractères coadaptatifs en trois grandes caté-gories : i) les aspects comportementaux :caractères hygiéniques vs. caractères d’infes-tation, ii) les aspects de dynamique despopulations, et iii) les aspects physiolo-giques. L’efficacité hygiénique d’A. ceranadépend des capacités sensorielles spécifiqueset d’un ensemble correspondant de carac-téristiques comportementales bien diffé-renciées : toilettage, désoperculation descellules et élimination du couvain infesté et« mise en bière » du couvain de mâles mor-tellement infesté. Les mâles parasités à de

faibles niveaux d’infestation meurent dansleur cellule car ils sont incapables d’ouvrirle dur opercule de celle-ci. On peut déduirede divers travaux que la mise en bière ducouvain hyperparasité réduit de plus de 20 %la population d’acariens susceptibles de sereproduire. La mise en bière a été peu recon-nue par les projets de recherche, mais ellecontribue probablement de façon la plusefficace à limiter la croissance des popula-tions de V. jacobsoni. Les capacités d’infes-tation de V. jacobsoni nécessitent des facul-tés sensorielles spécifiques pour localiser etdifférencier les divers stades de l’abeilleadulte et du couvain. La contre-mesure la

plus évidente contre le toilettage des abeillesest la protection offerte par la morphologieconvexe-concave particulière de V. jacob-soni. À côté de ces adaptations morpholo-giques typiques des Varroidae, les acarienscontrecarrent l’activité hygiénique collec-tive des abeilles par des comportements spé-cifiques qui leur assurent la sécurité de laphorésie et de l’invasion des cellules de cou-vain, la survie à une longue diapause de lareproduction et le succès de la dispersion àd’autres colonies hôtes. Le comportementdes hôtes et les barrières physiologiqueslimitent la reproduction du parasite à l’infes-tation du couvain de mâles. Cette restric-tion est un facteur-clé, puisque le couvainde mâles n’apparaît qu’à certaines périodeslimitées dans le temps. Sous la pression desélection V. jacobsoni doit survivre jusqu’àneuf mois par phorésie sur les ouvrières etatteindre une performance optimale dereproduction durant la période limitéed’accès au couvain de mâles. Des analysesgénétiques ont récemment montré queV. jacobsoni pourrait être un complexed’espèces constitué de groupes génétique-ment distincts avec des différences de ferti-lité sur l’hôte Apis mellifera. Pourtant, mêmeen comparant avec la situation sur l’hôteacquis A. mellifera, V. jacobsoni a la fertilitéla plus forte et les taux de reproduction lesplus élevés sur les mâles d’ A. cerana. Lescaractéristiques des barrières physiologiquesqui empêchent le parasite de se reproduire

sur le couvain d’ouvrières n’ont pas été étu-diées. Le caractère unique, qui consiste àingérer des protéines de l’hôte non digéréeset à les utiliser immédiatement pour sa

propre vitellogenèse, pourrait jouer un rôledans l’établissement d’une barrière physio-logique de la fertilité. L’utilisation de pro-téines de l’hôte non digérées fait partie del’adaptation de V. jacobsoni à une ontoge-nèse raccourcie et une synchronisation ulté-rieure avec la métamorphose de l’hôte. Lacompréhension de l’équilibre hôte natu-rel-parasite entre V. jacobsoni et A. ceranaest une condition préalable à de nouvellesapproches dans la recherche de la toléranced’A. mellifera à V. jacobsoni. Les proces-sus de coadaptation physiologique et sen-sorielle entre V. jacobsoni et A. cerana sonttrès peu étudiés et font actuellement partiedes domaines de recherche les plus pro-metteurs. © Inra/DIB/AGIB/Elsevier, Paris

Apis cerana / Varroa jacobsoni /

coadaptation

Zusammenfassung - Gegenseitige Anpas-sung von Apis cerana Fabr. und Varroajacobsoni Oud. Die Wirt-Parasitbeziehungzwischen der asiatischen Honigbiene Apiscerana Fabr. und der ektoparasitischenMilbe Varroa jacobsoni Oud. kann alsbalanciert betrachtet werden. Dies heißt,daß die Honigbienen die Milbenpopulationaktiv auf eine erträgliche Höhe begrenzenund gleichzeitig das Überleben der parasi-tischen Milbenart nicht gefährdet ist. Diesesfein abgestimmte Gleichgewicht ist dasResultat eines über eine lange Zeitdauerzurückreichenden Prozesses gegenseitigerAnpassung, in dem sowohl Wirt als auchParasit spezifische Eigenschaften entwickelt,die ihre jeweilige Fitness optimieren. Überdie vergangenen 20 Jahre wurden unter-schiedliche Aspekte dieser koadaptivenEigenschaften untersucht, diese werden hierzusammenfassend dargestellt, um die Inter-pretation der Wirt-Parasitbeziehung zuerleichtern. Diese koadaptiven Eigenschaf-ten können in drei Hauptkategorien zusam-

mengefaßt werden: 1) hygienische Eigen-schaften vs. Befallsverhalten; 2) populati-ons-dynamische Aspekte; und 3) physio-logische Aspekte. Die hygienische Effizi-enz von A. cerana hängt von spezifischensensorischen Fähigkeiten ab sowie korre-spondierender gut definierter Verhaltensei-genschaften; Putzen, Entdeckeln von Zel-len und Entfernen befallener Brut und das

Einsargen von tödlich parasitierter Droh-nenbrut. Die parasitierten Drohnen sterbenbereits bei niedriger Parasitierung in ihrenBrutzellen, da sie selbst nicht in der Lagesind, die harten Drohnendeckel zu öffnen.Aus verschiedenen Untersuchungen kannabgeleitet werden, daß das Einsargen über-parasitierter Drohnenbrut die vermeh-rungsfähige Milbenpopulation um mehr als20 % mindert. Das Einsargen wird in For-schungsprojekten nur wenig zur Kenntnisgenommen, wahrscheinlich leistet es aberden effektivsten Beitrag zur Begrenzung derPopulation von V. jacobsoni. Die Fähigkeitzum Befallen von Brutzellen setzt beson-dere Sinnesleistungen voraus, um zwischenden Stadien ausgewachsener Bienen unddenen der Bienenbrut unterscheiden zu kön-nen. Die offensichtlichste Gegenanpassungvon V. jacobsoni gegen das Putzverhaltender Bienen ist der Schutz, den die eigentüm-liche konvex-konkave Körperform der Mil-ben gewährt. Neben den typischen mor-phologischen Anpassungen der Varroidaesetzen die Milben den kollektiven hygieni-schen Maßnahmen der Bienen spezifischeVerhaltensweisen entgegen, die einen siche-ren Aufenthalt auf den Arbeiterinnen,sichere Passage in die Brutzellen, das Über-leben einer langen reproduktiven Diapauseund erfolgreiche Verbreitung zu anderenWirtsvölkern gewährleisten. Das Verhaltendes Wirtes und physiologische Barrierenbegrenzen die Vermehrung der Milben aufden Befall von Drohnenbrutzellen. Diese

Einschränkung stellt einen Schlüsselfaktordar, da die Drohnenbrut nur währendbegrenzter Zeiträume zur Verfügung steht.So steht V. jacobsoni unter dem Anpas-sungsdruck, bis zu neun Monate lang pho-

retisch auf den Arbeiterinnen überleben zukönnen. Darauf folgend muß sie währendder kurzen Zeiten, in denen ihr der Zugangzur Drohnenbrut offensteht, eine optimaleVermehrungsleistung erbringen. Dement-sprechend hat V. jacobsoni sogar im Ver-gleich mit der Situation auf dem angenom-menen Wirt A. mellifera in Drohnenzellenvon A. cerana die höchsten Fruchtbarkeits -und Vermehrungsraten. Die Art der phy-siologischen Begrenzung, die V. jacobsonidavon abhält sich in den Arbeiterinnenzel-len von A. cerana zu vermehren, wurden bis-her allerdings nicht untersucht. Die beson-dere Eigenart, unverdaute Proteine aufzuneh-men und sofort in der Dotterbildung weiterzu verwenden, könnte bei der Ausbildungeiner physiologischen Barriere eine Rollespielen. Der Gebrauch unverdauter Proteinestellt einen Teil der Anpassung von V. jac-obsoni an den nur kurzen für die Entwick-

lung zur Verfügung stehenden Zeitraum unddie daraus folgende Synchronisierung mitder Umwandlung des Wirtes dar. Das Ver-ständnis des natürlichen Wirt-Parasit-

gleichgewichts zwischen A. cerana undV. jacobsoni ist eine Voraussetzung neuerAnsätze in der Suche nach Toleranz gegenü-ber der Milbe bei A. mellifera. Die physio-logischen und sensorischen Koadaptationenzwischen A. cerana und V. jacobsoni sindnur wenig erforscht und stellen zur Zeit dasvielversprechendste Forschungsgebiet indiesem Bereich dar. © Inra/DIB/AGIB/

Elsevier, Paris

Apis cerana / Varroa jacobsoni /

Anpassung / Koadaptation

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