Review article

Selecting honey bees for resistance to Varroa jacobsoni

John R. Harbo Jeffrey W. Harris

USDA-ARS, Honey Bee Breeding, Genetics and Physiology Laboratory,1157 Ben Hur Road, Baton Rouge, LA 70820, USA

(Received 16 September 1998; accepted 8 February 1999)

Abstract - This report describes a systematic approach to selecting honey bees (Apis mellifera L.)for resistance to Varroa jacobsoni Oudemans. The equation (P1 (ab)n = P2) describes the growth ofthe mite population in a colony of honey bees that has a constant supply of worker brood. P1 and P,are the initial and final mite populations, a is population change while mites are in brood cells, b ispopulation change outside brood cells and n is the number of reproductive cycles of the mite. By com-paring the growth of mite populations in each colony (P2/P1), one can determine which bees aremore resistant to mites. The values of a, b and n provide details about the growth of the mite popu-lation by identifying which portion of the mite’s reproductive cycle was affected. Selection shouldbe based on specific characteristics of bees rather than on general changes in mite populations. Whenspecific characteristics of bees affect different components of the reproductive cycle of the mite, it maybe possible to combine the characteristics to produce bees that are more resistant to mites.© Inra/DIB/AGIB/Elsevier, Paris

Apis mellifera / mite / Varroa jacobsoni / resistant population / selective breeding

1. INTRODUCTION

This report is a review of our approach toselect honey bees (Apis mellifera L.) forresistance to Varroa jacobsoni Oudemans(Acari: Varroidae). We outline a systematicapproach which should be effective in anyselection program that involves character-istics of honey bees that are measured at thecolony level.

We define mite resistance as the ability ofa colony of honey bees to impede the growthof a population of V. jacobsoni. With thisdefinition, a highly resistant colony of beeswould cause a mite population to declineand then to either disappear or be maintainedat a very low level. This is the breedingobjective. However, during the breedingprocess, especially at the beginning, abreeder may select colonies that measur-

* Correspondence and reprintsE-mail: jharbo@asrr.arsuda.gov

ably slow the growth of their mite popula-tions but would eventually die from the miteinfestation. These susceptible colonies withresistant characteristics can be important inthe selection process, especially in difficultselection projects (such as resistance toV. jacobsoni), and accurate measurementsare needed to identify the colonies that areslightly better than others. Therefore, selec-tive breeding of bees for resistance toV. jacobsoni relies on our ability to accu-rately measure and describe the growth ofmite populations within colonies of bees.

The first step in selective breeding is tochoose a population of bees within whichto work. It is best if this population containssome colonies with the desired phenotype(in this case resistance to mites), but thepopulation may express only low levels ofresistance, or even undetectable levels. Forexample, the importation of bees from fareastern Russia [ 11, 51 ] is the beginning of abreeding program that began with stock thatalready contained some of the desirablequalities.

In bee breeding, man can do things thatnature cannot do. These are: 1) special mat-ing schemes (e.g. single drone insemina-tions or inseminating many queens with thesame mixture of semen); 2) the ability tofocus selection on one characteristic at a

time; and 3) the ability to select susceptiblecolonies that contain characteristics of resis-tance (colonies that would eventually die innature). There is no need for colonies to diein a breeding program that selects bees forresistance to mites.

The use of single-drone inseminationsmay be important at the beginning of a selec-tion program, especially when selecting forcolony traits that are present at low fre-quencies. When a queen is mated to semenfrom a single drone, worker bees in thecolony all have identical genetic materialfrom their father, who is represented byidentical spermatozoa that are now in thespermatheca of the queen. Thus the workerbees in such colonies are more closely

related than normal sisters and have a relat-edness of 0.75. The use of single-drone mat-ings makes it easier to detect colony char-acteristics that may be masked by multiplemating, and it amplifies the differencesamong colonies [26, 55].

Advantage may shift to multiple-droneinseminations during the mid and latterstages of selective breeding [28]. The rea-sons are that: 1) queens survive longer wheninseminated with multiple drones [8, 26];2) more effective selection schemes can beused (such as mating a group of queens withthe same mixture of semen); and 3) daugh-ter queens from a multiply mated queen aremore variable and would therefore reducethe rate of inbreeding at a time when thespecific characteristic of selection is well-established.

We used the following sequence in ourapproach to breed bees for resistance toV. jacobsoni:

1) develop techniques for measuring pop-ulations of bees and mites and for measuringcharacteristics that are associated with resis-

tance;2) identify specific characteristics that

are related to the growth of mite popula-tions;

3) determine if these characteristics areheritable;

4) enhance heritable characteristics withselective breeding;

5) assemble resistant components intoproductive, mite-resistant bees.

2. EVALUATION PROCEDURE

2.1. Establishing heritability

It is important to calculate heritability(h2) of a desirable characteristic beforebeginning a program of selective breeding.Heritability (h2) is the proportion of theobserved variance (among a group of beecolonies in this case) for which differencesin heredity are responsible [35]. The esti-mate of h2 is a pragmatic measurement that

predicts breeding success. If a characteristichas an h2 close to 1, then the characteristiccan be rapidly changed with selective breed-ing. If h2 approaches 0, selective breedingwill probably fail. As a general rule, it isreasonable to attempt selective breeding ifh2 > 0.25.

We used sibling analysis [10] to estimateheritability of various characteristics of beesthat may be associated with the growth ofmite populations (table 1, [27]). The heri-tability test consisted of 28 colonies withunrelated queens that had not been selectedfor resistance to V. jacobsoni. The related-ness of the colonies was established byinseminating groups of four queens with asingle mixture of semen that had been col-lected from the drones produced by onequeen. Seven such queens that served asdrone mothers for the experiment were unre-lated to each other and were unrelated to

the queens that were inseminated. This pro-duced seven groups of four colonies, witheach colony related as a full sister to theother three colonies in its group and unre-lated to the other 24 colonies [27].

2.2. Three levelsof evaluation and measurement

We always measured the growth of amite population within a colony of bees thatwas being evaluated for resistance. A fieldtest is the core of the evaluation with the

honey bee colony serving as the experi-mental unit in the analyses. The evaluationrequires at least 10 weeks and measuresgrowth of mite populations in a group ofhoney bee colonies and provides a frame-work for measuring other characteristics atappropriate times during the course of the

test. Characters that are affected by adultbees such as hygienic and grooming behav-ior cannot be measured until the populationof adult bees becomes the progeny of thetest queen.

We have described our procedure forfield evaluation in detail [25, 27, 29]. Ingeneral, uniform populations of bees andmites are established by collecting about30 kg of mite-infested bees into a large cageand then subdividing the bees into coloniesthat contain about 1 kg of bees, a queen to betested and broodless combs. We calculatedthe initial mite populations by sampling beesfrom the large cage and by knowing theweight of the bees that we put into eachcolony. We simplified the growth model byusing only worker-sized combs in thecolonies.

Growth of the mite population was testedat three different levels of detail. The firstlevel was the most general and was simplythe change in the mite population from astarting point (time zero in a field test) toan ending point (in the first group in table II,

the ending point was 70 days later). Themite populations in each colony at the begin-ning and end of the test were designated asP1 and P2, respectively. Population growthfor the experimental period was thus P2/P1.

At the second level, we explain how amite population went from P1 to P2 in terms ofthe three components of the mite’s reproduc-tive cycle that can affect the growth of a mitepopulation. The equation is P1 (ab)n = P2.Population growth of mites is (ab)n. Thesethree components (described in fig-ure 1) are: 1) the change in the mite popu-lation while mites are in the brood cell (a),which is the average number of adult femalemites that leave a group of brood cells (adultdaughters + living foundresses) divided bythe number of foundresses that entered those

cells; 2) the change in the mite populationwhile mites are on adult bees (b), which isthe number of adult female mites that enterbrood cells divided by the number that hademerged from brood cells in the previouscycle; and 3) the number of days needed tocomplete one reproductive cycle of the mite

(t), which is converted to n, the number ofreproductive cycles during the evaluationperiod. All three factors are colony aver-ages. Therefore, ab (a times b) is the averagepopulation change per reproductive cycle,and multiplying (ab)" times the initial pop-ulation (P1) equals the final mite population(P2). If any four of the five values areknown, the remaining one can be calculated.

The product ab is the average rate ofpopulation growth per reproductive cyclefor a short test. During our short field tests(10-15 wk) in which colonies begin with rel-atively low mite populations, the growth ofthe mite population probably follows theclassic exponential growth curve, P2 = P1 ert.In this equation r is the intrinsic rate ofincrease per unit time and t is the duration ofthe growth period. Exponential growth ofV. jacobsoni populations occurs only forshort periods of time and before the popu-lation reaches a carrying capacity. If r isdefined as ’per reproductive cycle’ and t = n(which is the number of cycles in a test),then r = In (ab).

The third level of evaluation describes a

specific characteristic that is measurableand heritable. We concluded [27] that this isthe level at which one should select for resis-tance to V. jacobsoni. Each specific char-acteristic (described below) affects one ofthe three components described above (a,b or t). When specific characteristics affectthe same component (for example suppres-sion of mite reproduction and hygienicbehavior both affect a), their combinedeffect may not be additive and may not bebeneficial. However, when specific charac-teristics affect different components, theywould probably combine to produce acolony with increased resistance to the mite.

2.3. Specific characteristicsof resistance to V. jacobsoni

2.3.1. Duration of capped period

A brood cell is normally capped =8 daysafter a fertilized egg is laid, and it remains

capped for ≈12 days until the worker beeemerges as an adult. However, among agroup of colonies, there is significant vari-ation in the average duration of the cappedperiod, and this length of time is a herita-ble characteristic [2, 24, 27, 42-44]. Biene-feld [2] also showed that the duration of theprecapping period was affected by the geno-type of the nurse bees. However, the dura-tion of the capped period was affected bythe genotype of the adult bees in the colonyduring the capped period but not by thegenotype of the bees that had nursed them aslarvae.

Büchler and Drescher [6] found a positiverelationship between the duration of thecapped period and the mite population infield colonies. Their data showed that about25 % of the variation in mite populations intheir colonies could be explained by differ-ences in the duration of the capped brood.

2.3.2. Suppression of mite reproduction

Mites that do not reproduce in the broodcell are found in nearly every colony. How-ever, the frequency of non-reproducingmites in European honey bees is normallybelow 40 % [9, 14, 16, 21, 32-34, 36, 37,40, 47]. We define non-reproducing mites asmites that enter the cell to reproduce but1) produce no progeny, 2) produce malesonly, 3) produce progeny too late to mature,or 4) die in the cell before they can repro-duce. Because non-reproduction of miteswas found to be a heritable characteristic ofbees [27], we call it suppression of mitereproduction. This characteristic is directlyrelated to a and has been linked to resis-tance to V. jacobsoni by many investigators[1, 9, 15-17, 23, 38, 52, 53, 58, 62].

Data suggest that there may be two com-ponents that suppress mite reproduction.We (table I) evaluated both the immediatebrood effects of this characteristic reportedby Camazine [9] and a delayed expression ofthis characteristic. In the delayed expres-sion, suppression of mite reproduction wasnot evident when mites went through their

first reproductive cycle in colonies thatwould ultimately suppress mite reproduc-tion about 2 months later. Fuchs [19]attributed this delayed effect to attributesthat the mite attained before it entered abrood cell. Based on sibling analysis intable I, mite reproduction was heritable dur-ing our first observation. Therefore, larvaeand/or pupae suppressed mite reproductionand this immediate effect, described byCamazine, was a heritable characteristic ofbees. Mite reproduction had low heritabilityduring the transition period (early July mea-surement in table I) but was again heritableat the third measurement. This third mea-surement was the delayed effect describedby Fuchs [ 19] and the characteristic that weused as the basis of our selection (table II).

2.3.3. Entrapped mites

We recently observed a higher frequencyof foundress mites that were dead betweenthe cocoon and the cell wall. When thesewere observed in cells with spinning beelarvae, we noticed that the entrapped miteswere often alive. In most cases, the mitesare found at the bottom of the cell, ventralside up, the same posture that they takewhile they are inactive in the brood food.In a 1997 field test, 27 % of the foundressmites in the brood cells were entrapped [30].However, we do not know if this character-istic is heritable. Martin [37] reports thatabout 1 % of the foundress mites become

entrapped, and we found similar levels ofentrapment (0.4 % or 11/2 930 observationsof foundress mites) in unselected colonies ofbees in table II, group 1.

This characteristic is more effective in

reducing mite populations than suppressionof mite reproduction because a non-repro-ducing mite that survives has an ab of 1,while an entrapped mite has an ab of zero.

2.3.4. Hygienic behavior

Hygienic behavior is the rate at whichadult bees remove dead or diseased brood

from capped cells [56]. Spivak [60] hasshown that this characteristic is related toresistance to V. jacobsoni, and table I [27]shows that hygienic behavior (when mea-sured with freeze-killed brood) is heritable(h2 = 0.65). Hygienic behavior is an impor-tant characteristic for mite resistance in Apiscerana [50, 54] and resistance to other dis-eases of bees such as chalkbrood [22, 61]and American foulbrood [56, 57].

2.3.5. Grooming behavior

Physical damage to mites may be causedby the activities of adult bees [59], and thisgrooming behavior is a heritable character-istic of bees (h2 = 0.71) [41]. Caution shouldbe used in deciding what damage to mites isactually caused by grooming bees. Forexample, we found that dents in the dorsalsurface of the idiosoma often occurred while

young adult mites were still in their broodcells [27], and not by the mandibles ofgrooming bees. Although self grooming andnestmate grooming are important mecha-nisms of resistance to V. jacobsoni for Apiscerana F., it is a less important componentof resistance for our western honey bee Apismellifera L. [3, 4, 7, 13, 18, 48, 49].

2.3.6. Proportion of mites in brood

This is a measure of the duration of the

reproductive cycle of the mite [25, 46],which is explained in more detail in section2.4.1. Mites are either on adult bees (in aphoretic stage) or in the brood cells (in apotentially reproductive stage).

The proportion of mites in brood wasfound to be a highly heritable characteristicin bees (h2 = 1.24) [27]. This is sometimescalled invasion of brood cells [5, 20, 39].Perhaps mites with a low proportion of mitesin brood and therefore those with longerreproductive cycles are less attracted to beelarvae and therefore are slower to enterbrood cells. This may be best explained bythe work of Trouiller et al. [63] who describea chemical signal produced by bee larvae

that attracts V. jacobsoni to the brood.Because of a direct effect on t, this charac-teristic may serve well in combination withcharacteristics that affect a such as sup-pression of mite reproduction or hygienicbehavior.

2.4. Describing the reproductionof mites

The most basic calculation is the overall

growth of the mite population in eachcolony. This is simply the final mite popu-lation divided by the initial mite population(P2/P1). By rearranging the equation (P1(ab)n = P2), the overall growth of the mitepopulation (P2/P1) equals (ab)n. The nextstep is to solve for ab by calculating n.

2.4.1. Measuring n

Before we calculated the number of

reproductive cycles (n), we needed to cal-culate the duration of the reproductive cycleof the mites in each colony. The durationof a reproductive cycle is the average timefrom when a mite enters a cell until it entersanother cell (time spent in the cell plus timespent on adult bees). Not all mites will havethe same time for their reproductive cycleand the length may change with the age ofthe mite. However, only the group averageis important.

The duration of the reproductive cycleof the mite is almost entirely related to thevariability of the time that the mites spendoutside the brood cell. There is significantvariation in the duration of the capped periodof bee brood [2, 24, 42] but this variationis only slightly greater than ± 1 day in Euro-pean honey bees. Mites enter the cell about1/2 day before the cell is capped and remainuntil the capping is removed by the teneralbee in the cell. Therefore, the reproductivestage of the mite is relatively constant at≈12.5 ± 1 days or 1/2 day longer than thecapped period. In contrast, the time spentoutside the cells can vary by more than aweek (summarized by Fries et al. [17]). For

example, if 65 % of the mite population is inthe brood (typical), then the average durationof the mite reproductive cycle in that colonyis 12.5/0.65 or 19 days. If 40 % are in broodcells, the reproductive cycle extends to31 days. A longer reproductive cycle slowsthe growth of mite populations.We estimated the length of the repro-

ductive cycle of mites (t) for each colonyby comparing the number of mites on adultbees with the number of mites in brood cells

(both were measured at the end of the testperiod). The number of reproductive cyclesin the test was then calculated from the esti-mate of the length of the reproductive cyclefor mites in a colony (divide the durationof the experiment by the length of the repro-ductive cycle). However, most of our testswere initiated in bee colonies with no brood.Since mites cannot begin their reproductivecycle until bees have capped brood, themites in a colony that begins with no broodwere each assigned 23 days for their firstreproductive cycle (7 + 12.5 + 3.5, whichwas 7 broodless days + 12.5 days in brood +half of a normal phoretic period).

The worst colony in table II (group 1) isused as an example of calculating n. Thetest ran for 70 days and based on propor-tion mites in brood, the colony had a repro-ductive cycle (t) equal to 16.2 days. There-fore, n = 1 + (70 - 23)/16.2 = 3.9 reproductivecycles.

2.4.2. Calculating ab

As shown in figure 1, ab is the populationgrowth of mites per reproductive cycle. Anab of I equals no change in the mite popu-lation; numbers > 1 indicate populationincrease and < 1 indicate population decline.

For calculating ab the equation becomes:ab = n(P2/P1). Again using the worstcolony in table II, group 1, as the examplewhere n was calculated as 3.9: ab = 3.94.19= 1.45. These data and the calculations canbe handled on nearly any spreadsheet pro-gram. However, when using a computer it issimpler to write 3.9(P2/P1) as (P2/P1)(1/3.9).

However, we know that ab is not alwaysconstant. Fuchs [19] showed that the majorcontrol of mite reproduction depended onevents that occurred before the mite entersthe cell. Harris and Harbo [27] have shownthat colonies that suppress mite reproduc-tion at a high rate may not express that char-acteristic in their first brood cycle when theyhave started with package bees. Thus, wenot only assign the first brood cycle a stan-dard length, we assign a standard ab of 1.22(the group average for unselected bees,group 1 in table II) when evaluating coloniesfor suppression of mite reproduction.

2.4.3. Calculating a and b

When ab is already estimated, only oneof the factors needs to be measured and theother can be calculated. We suggest mea-

suring a, the rate of mite increase from thecell. We define a as the sum of mature

daughters and surviving mothers that exitthe cells divided by the number of foundressfemales that entered those cells. Our a differsfrom the rate of reproduction, which is thetotal number of progeny produced permother mite [15, 31].

For table II, we calculated a by observingmites in brood cells. We examined mite-infested cells when the bees were at the tan

body stage (about 16-17 days after the beeegg was laid). In each cell we counted thenumber of foundresses and the number offemale progeny that were deutonymphs oradults. We allowed each adult daughter andimmobile deutonymph to have a 100 %chance of reaching maturity and emergingfrom the cell, while each mobile deu-tonymph only had a 39 % chance of doingso [21 ]. Therefore, our calculation of a was(the number of live foundress females + thenumber of female progeny that were immo-bile deutonymphs or adults + [0.39 * thenumber of mobile female deutonymphs])/thenumber of live and dead foundress females.Harbo [25] estimated a by three differentmethods, and others have calculated a byvarious techniques [17, 38].

There are also various ways to report a.Most report the number of female progenythat mature in a cell. Some do not includecells with mites that produce no progeny orcells with multiple foundresses. We includethe contents of all infested cells, even thosewith dead foundresses. As described in the

paragraph above, our a included the sur-viving foundress. Since 98 % of the foundressmites usually survive the reproductive phase,our estimates of a are about 1 greater thanthe reproductive rate reported by many oth-ers.

An example of measuring a and then sep-arating a and b from ab is again taken fromthe worst colony in table II, group 1, whereab had been calculated as 1.45. We counted20 mobile female deutonymphs, 38 immo-bile female deutonymphs, 28 adult daugh-ters, 48 live foundresses and two deadfoundresses. Therefore, 121.8/50 equals ana of 2.44. Thus b = abla or 1.45/2.44 = 0.60.

What we call b is not strictly survivor-ship outside the cell. It also includes mitesthat transfer into or out of the colony. Whenmite populations are relatively uniform, per-haps one can assume that immigration andemigration are equal. However, when adja-cent colonies have highly variable popula-tions of mites, drifting or robbing bees willprobably tend to move mites into thecolonies with fewer mites. This is a goodargument for evaluating colonies that beginwith uniform populations of bees and mitesand for ending a field test as soon as possi-ble.

3. CONCLUSIONS

It is certainly possible to breed bees thatare resistant to V. jacobsoni. Table II (group 3)is an example of a group of 23 colonies thatwere selected for their ability to suppressthe reproduction of mites in brood cells.Those colonies (as a group) averaged fewermites at the end of the test than at the begin-ning [30]. However, breeding for resistanceto V. jacobsoni has proven to be more dif-

ficult than most other selection programswith bees, so it requires more precise mea-surements and a more systematic approachif one demands quick results.

There are presently many mechanismsof resistance that show promise. We thinkthat the suppression of mite reproduction,hygienic behavior and proportion of mites inbrood (the tendency for adult mites to notenter brood cells) are presently the threemost promising of the specific characteris-tics that are heritable and should respond toselection.

It is important to note that there are loca-tions around the world where honey beessurvive without a program to control mites.Brazil is probably the best example [45].Moreover, the levels of V. jacobsoni infes-tation in both European and Africanizedbees in Brazil have further declined withtime [12]. This suggests that when a popu-lation of bees is able to coexist with V.

jacobsoni, natural selection will work onmites to make them less virulent, or selectionwill work on bees to make them more resis-tant. Both possibilities are likely. However,data suggest that mite populations in Brazilare less virulent than in most other parts ofthe world and that this virulence may be

decreasing. Perhaps we can use this to ouradvantage. If we select bees to a point wherethey can survive reasonably well in the pres-ence of mites, mites may respond by becom-ing less virulent.

Our strategy is to select bees that willretain an acceptable level of resistance whenoutcrossed to drones from non-resistantcolonies. This would provide a broadergenetic base for resistant bees, would pre-serve the genetic diversity of our honey beepopulations, and would enable natural selec-tion to operate more effectively. In this sit-uation, a bee population would retain mostof its genetic diversity as it gradually incor-porates genes for resistance to mites.

Table II (group 2) is an example of queensfrom a mite-resistant stock that were out-

crossed. The mite-resistant queens, selectedfor suppression of mite reproduction, wereinstrumentally inseminated with drones col-lected from colonies that had not beenselected for resistance to mites. Somecolonies were very good and expressed ahigh level of resistance, but others werehighly susceptible. The group was variable.If a second specific characteristic of resis-tance was incorporated into this stock (suchas a low proportion of mites in brood), agroup of colonies with outcrossed queensmay be more uniform and may be able tohold mite growth to an ab of 1.0 or less.

ACKNOWLEDGMENT

We thank David Dodge, Daniel Winfrey,Shelley Savant and M. Shane Smith for theirassistance. This work was supported, in part, bythe Michigan Department of Agriculture and incooperation with the Louisiana AgriculturalExperiment Station.

Résumé - La sélection d’abeilles melli-fères (Apis mellifera L.) résistantes à Var-roa jacobsoni Oud. Cet article examine lesméthodes de sélection d’abeilles mellifèresrésistantes à l’acarien V. jacobsoni. Un plande sélection est décrit ci-après, qui devraitêtre mis en &oelig;uvre dans tout programme desélection prenant en compte les caractèresdes abeilles mesurés au niveau de la colonie.

1) Développer des méthodes pour mesurerles populations d’abeilles et d’acariens etpour mesurer les caractères associés à larésistance.

2) Identifier les caractères spécifiques enrelation avec la croissance des populationsd’acariens.

3) Déterminer si ces caractères sont hérédi-taires.

4) Renforcer ces caractères par un élevagesélectif.

5) Regrouper les composantes de la résis-tance dans des abeilles productives et résis-tantes aux acariens.

Dans cet article on entend par résistance àV. jacobsoni la sélection chez les abeillesde caractères qui vont retarder la croissancede la population d’acariens établie dans lacolonie. Lors du criblage initial des carac-tères, nous avons utilisé des reines fécon-dées par un seul mâle. Nous pensons quecela a permis de détecter aussi des carac-tères peu fréquents. Les caractères au niveaude la colonie auraient pu être cachés si lesreines avaient été fécondées par plusieursmâles. Notre approche décrit la croissancede la population d’acariens au sein d’unecolonie d’abeilles par l’équation : P1 (ab)n =P2, où P1 et P2 sont les populations initialeet finale d’acariens, a la variation de la popu-lation tant que les acariens sont dans les cel-

lules de couvain, b la variation quand ilssont hors des cellules de couvain et n lenombre de cycles reproducteurs de l’aca-rien. L’équation est valable pour une colo-nie qui a un apport régulier en couvaind’ouvrières de façon à permettre la repro-duction de l’acarien. Seules les femellesadultes d’acariens sont comptées et toutesles mesures sont des moyennes pour la colo-nie. En comparant les taux de croisssancedes populations d’acariens de chaque colo-nie (P2 / P1 ), on peut déterminer la crois-sance réelle dans chaque colonie. Lesvaleurs a, b et n peuvent alors expliquercomment la population d’acariens est passéede P1, à P2 en termes de composantes mesu-rables du cycle reproducteur de l’acarien.Les variations de la population d’acariensdoivent se refléter dans une ou plusieurs deces composantes. Les caractères spécifiquesdes abeilles (qui sont la base de la sélection)peuvent alors être associés à l’une de cestrois composantes. Quand plusieurs carac-tères spécifiques agissent sur une même com-posante (par exemple. la suppresssion de lareproduction de l’acarien et le comporte-ment hygiénique), leur effet combiné n’estpas nécessairement additif ni bénéfique. Sien revanche un même caractère spécifiqueagit en même temps sur différentes compo-santes, ceux-ci se combineraient probable-

ment pour donner une colonie ayant unerésistance accrue à V. jacobsoni.Jusqu’ici notre sélection s’est concentréesur un caractère, la suppression de la repro-duction de l’acarien, qui provoque la nonreproduction des acariens qui pénètrent dansles cellules de couvain. Sur la base de ceseul caractère héréditaire, nous avons produitdes colonies dans lesquelles la populationd’acariens décline durant une période testde 70 j. Deux autres caractères héréditaires,le comportement hygiénique et la propor-tion d’acariens sur le couvain, peuvent avoiraussi une importance pour la sélectiond’abeilles résistantes.&copy; Inra/DIB/AGIB/

Elsevier, Paris

Apis mellifera / Varroa jacobsoni /

population resistante / élevage sélectif

Zusammenfassung - Selektion vonHonigbienen auf Resistenz gegen Varroajacobsoni. Dieser Beitrag befa&szlig;t sich mitMethoden zur Selektion von Honigbienen(Apis mellifera) auf Resistenz gegen denBefall durch Varroa jacobsoni Oudemans.In der Folge wird ein Zuchtplan beschrie-ben, der innerhalb jedes auf der Messungvon Volkseigenschaften fu&szlig;enden Selek-tionsprogramms wirkungsvoll angewendetwerden kann.

1) Entwicklung von Methoden zur Erfas-sung der Populationen von Bienen und Mil-ben und zur Messung der mit Resistenz ver-bundenen Eigenschaften.2) Identifizierung spezifischer mit demAnwachsen der Varroapopulationen inBeziehung stehender Eigenschaften.3) Ermittlung, ob diese Eigenschaften ver-erbbar sind.

4) Verstärkung dieser Eigenschaften durchselektive Zucht.

5) Vereinigung der Resistenzkomponentenin produktiven, milbenresistenten Bienen-linien.

In diesem Bericht wird Varroaresistenzzuchtals Selektion der Honigbienen auf Eigen-schaften verstanden, die das Anwachsen

von Varroapopulationen in den Völkern ver-langsamen. In den anfänglichen Untersu-chungen zum Auffinden von Resistenzei-genschaften benutzten wir Königinnen, dievon nur einem Drohn besamt waren. Wir

gehen davon aus, da&szlig; hierdurch auch weni-ger häufige Eigenschaften ermittelt werdenkönnen. Diese Volkseigenschaften könntenverdeckt werden, wenn die Königinnen mitmehreren Drohnen verpaart sind. UnserAnsatz beschreibt das Wachstum von Var-

roapopulationen in den Bienenvölkern unterVerwendung der Formel P1 (ab)n = P2. Hier-bei sind P1 und P2 die Milbenpopulation zuAnfang und am Ende, a die Änderung derPopulation solange die Milben in den Brut-zellen sind, b die Änderung au&szlig;erhalb derBrutzellen und n die Anzahl von Repro-duktionszyklen der Milben. Diese Gleichungbeschreibt das Anwachsen einer Milbenpo-pulation mit einem konstanten Angebot anBrutzellen zur Vermehrung. Es werden nurdie erwachsenen Milben gezählt, und alleMessungen sind Mittelwerte für die Völker.Durch Ermittlung des tatsächlichen Anwach-sens der Milbenpopulation in jedem Volkkann die Zuwachsrate der Milbenpopula-tionen (P2/P1) ermittelt werden. Die Wertevon a, b, und n können dann erklären, aufwelche Weise die Milbenpopulation sich inBezug auf me&szlig;bare Komponenten desReproduktionszyklus der Varroamilben vonP1 nach P2 entwickelt hat. Änderungen derVarroapopulation müssen auf eine odermehrere dieser Komponenten zurückzu-führen sein. Spezifische Eigenschaften derBienen (die Grundlage der Selektion) kön-nen dann mit einer dieser drei Komponentenin Verbindung gebracht werden. Falls meh-rere Eigenschaft eine dieser Komponentenbeeinflussen (beispielsweise wirken sicheine Unterdrückung der Milbenreproduk-tion und das hygienische Verhalten beideauf a aus) mu&szlig; sich ihr kombinierter Effektnicht notwendigerweise additiv oder gün-stig verhalten. Falls dagegen eine einzelneEigenschaft verschiedene dieser Kompo-nenten gleichzeitig beeinflu&szlig;t, würden diesekombiniert mit hoher Wahrscheinlichkeit

zu erhöhter Resistenz gegenüber Varroajacobsoni führen.Zur Zeit hat sich unsere Selektion auf eineeinzelne Eigenschaft konzentriert, die Unter-drückung der Vermehrung der Milben. Diesführt dazu, da&szlig; Milben nicht reproduzieren,nachdem sie eine Brutzelle befallen haben.Auf Grundlage dieser einzelnen erblichenEigenschaft haben wir Völker erzeugt, indenen die Milbenpopulation über einen Zeit-raum von 70 Tagen abnimmt. Zwei weitereEigenschaften, das hygienische Verhaltensowie der Anteil der in der Brut befindli-chen Milbenpopulation könnten ebenfallswichtig sein, um Honigbienen auf Resistenzgegen Varroamilben zu selektieren. &copy; Inra/

DIB/AGIB/Elsevier, Paris

Apis mellifera / Milben / Varroa jacobsoni /Populationen / Selektionszucht

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