Review article
Behavioral defenses of honey bees
against Varroa jacobsoni Oud.
Boecking
Otto
a
Marla
Spivak
Zoologie und Bienenkunde der Universität,
42, 53127 Bonn, Germany
Melweg
b
Department of Entomology, 219 Hodson Hall, University of Minnesota,
Institut für Landwirtschaftliche
St. Paul, MN 55108, USA
(Received
15
September 1998; accepted 9 February 1999)
Abstract - Two behaviors of honey bees, hygienic behavior and grooming, are mechanisms of
defense against brood diseases and parasitic mites. Studies have shown that Apis mellifera colonies
remove worker brood infested with Varroa jacobsoni mites from the nest (hygienic behavior), and
groom the mites off other adult bees, but to a limited extent compared to the original host of
V. jacobsoni, A. cerana. Research is reviewed on hygienic and grooming behaviors with respect to
their potential as mechanisms of resistance to V. jacobsoni. Studies related to hygienic behavior
include the removal of experimentally infested and naturally infested brood, measurements of heritability, the uncapping and recapping of cells containing infested pupae, and the detection of infested
brood. Studies on grooming include the process by which a groomer detects and damages a mite
found on itself or on another adult bee, how the behavior is quantified, and problems with these
methods of quantification. Finally, unresolved questions concerning grooming and the effects of
hygienic and non-hygienic behaviors on limiting the population growth of V. jacobsoni are discussed. © Inra/DIB/AGIB/Elsevier, Paris
Apis mellifera / Apis
cerana
resistance / tolerance
/
Varroa jacobsoni / hygienic
1. INTRODUCTION
Beekeeping with Apis mellifera L. bees is
endangered worldwide by the ectoparasitic
mite Varroa jacobsoni Oud. [22, 71].
*
Correspondence and reprints
spiva001 @ tc.umn.edu
E-mail:
behavior /
grooming
behavior /
Phoretic mites infest new colonies by attaching to drifting and robbing bees. The mite
feeds on the hemolymph of adult bees and
brood, and reproduces in the brood. The
daily amount of hemolymph that a mite con-
sumes from a bee (0.25 μL according to
Moritz [86]) probably does not have a negative effect on an otherwise healthy bee.
Bees infested with multiple mites during
their metamorphosis show degenerated fat
bodies, underdeveloped hypopharyngeal
glands and a shortened lifespan [7, 34, 110,
111]. In addition, secondary infections
(viruses and bacteria) can be transferred by
the mite or triggered in the bees’ body and
are the primary cause of bee mortality in
colonies severely infested with V. jacobsoni
[1, 4, 5, 23, 55, 56, 59, 94, 95,113].
Because A. mellifera colonies die from
varroosis within a few years if the mite population growth is not regulated by the beekeeper [10 1, 104] and chemical control has
its problems and limitations [72, 136], it is
of common interest to breed bees for a
higher resistance to this mite. Selection and
breeding are long-term solutions to the present crisis in apiculture. Szabo [ 124] stated
that this solution "... is the most complex,
time consuming, and economically significant research for the beekeeping industry".
Using the known defense mechanisms of
A. cerana Fabr. as a model, we review the
present knowledge about behavioral defense
mechanisms of the honey bee A. mellifera to
the mite V. jacobsoni. These individual
defensive behaviors might form a behavioral complex that contributes to overall
resistance by the bees.
2. MECHANISMS TO REDUCE
DISEASES AND PARASITES
Feral and domesticated honey bee
colonies have evolved elaborate defense
mechanisms to protect both themselves and
their food from pathogen and parasite invasion. The defense mechanisms of individual bees serve to minimize the threat for the
whole colony.
Constitutional defense mechanisms, such
the chitinous cuticle, which serves as a
barrier between internal and external envias
ronments, and the intestinal microflora of
the bee gut, can protect each individual bee
against infectious diseases [35, 42, 44, 45,
57, 75]. Cellular defense mechanisms (hemocytes) and humoral reactions (enzyme and
antimicrobiotic factors) can contribute to
resistance towards infections [29, 57, 76,
77, 134, 137]. The proventricular valve
enables the bees to filter ingested spores,
which serves as a mechanism of physiological resistance to diseases [35, 122]. These
individual responses, coupled with the bees’
short life-span and rapid replacement with
healthy individuals, can limit the spread of
infections between bees within a colony.
Of interest to this article are the behavioral defenses that limit the spread of diseases and parasites. Hygienic and groom-
ing behaviors are examples of such
behavioral defenses. Hygienic behavior is
the main mechanism by which A. mellifera
resists the brood diseases American foulbrood (AFB) (Paenibacillus larvae larvae)
[108, 122] and chalkbrood (Ascosphaera
apis) [43, 44, 117]. Hygienic honey bee
workers have the ability to detect diseased
brood, uncap the wax covering over the
brood cells and remove the infected larvae or
pupae. The impact of hygienic behavior on
the spread of infective diseases is maximized if it takes place before the causative
organism reaches the infectious sporulating
stage [118, 119, 139].
Hygienic behavior of worker honey bees
is determined largely by two behavioral
components, the uncapping and the removal
of dead brood. Few colonies (10 % or less)
in nature demonstrate hygienic behavior
[89, 116, 117]. However, colonies can be
readily selected for the behavior using a
field assay [119], or by direct inoculation
with the pathogen [44, 108] or mites [13,
114].
Grooming behavior enables individuals
and groups of bees within a colony to
remove dust and pollen from their bodies, to
disperse pheromones, and to remove
ectoparasites. Grooming behavior involves
biting and licking with the mouthparts and
movements of the mesothoracic legs. Antennae are carefully cleaned by the antenna
cleaner on the prothoracic legs [21, 58, 138].
Many self-grooming (auto-grooming) activities of bees can be seen on flowers, in flight,
during pollen collection, and in the hive.
Social grooming (allo-grooming) between
bees can be observed within the hive. Allogrooming may be elicited by a grooming
dance, first described in A. mellifera by Haydak [52]. A bee performing this dance is
groomed by a nestmate with the mesothoracic legs [74]. Grooming is an important
mechanism of protection against the invasion of tracheal mites (Acarapis woodi) into
the tracheal tubes in bees known to have
genetically based resistance to this parasite
removal behavior of the bees. When mites
from other colonies (either intraspecific
A. cerana, or interspecific A. mellifera) were
introduced into the brood cells, the removal
rates of mite-infested brood were higher
(about 60 %) than when the introduced mites
were collected from within the test colony
itself (about 10 %).
The original host of V. jacobsoni, A. cerlives in equilibrium with this ectoparasitic mite. The balanced host-parasite relationship between A. cerana and V. jacobsoni
can be explained by the limited reproduction of the mite on this bee species and
behavioral defense mechanisms of the bees
against the mites.
A. cerana workers do not remove miteinfested drone brood owing to the thick cell
capping over the drone cell, a structure
unique to A. koschevnikovi and this species
[96, 100]. Drones which are infested with
multiple mites become weakened and are
not able to open their cell caps from the
inside as they normally do at the time of
emergence. They die together with the mites
inside such cells because the worker bees
do not open these cells from outside, thus
creating ’mite-traps’ in the brood nest [61,
96, 97]. Initial infestation rates with two
mother mites per cell can reduce the emergence rate of the infested drones to less than
30 % [97]. Recently Boecking (unpublished
data) found that A. cerana bees sometimes
additionally close the central pore in the
drone cell cap of such infested pupae from
outside with wax material, entombing the
mites within the brood nest. Consequently
the successful reproduction of V. jacobsoni
in A. cerana drone brood is limited, resulting in low overall rates of parasitism.
The removal of mite-infested worker
brood, or hygienic behavior, is one defense
mechanism of A. cerana towards V. jacobsoni [91, 92, 99]. In some cases, A. cerana
workers open the cappings of mite-infested
brood without removing the bee brood. The
mites then leave the opened cells or are
removed by the bees, and the brood cells
are subsequently resealed with a new wax
cap [96, 100, 105, 106, 127]. Investigations
by Rosenkranz et al. [106] indicated that
the number of infested pupae that are
removed from experimentally infested cells
may be lower than previously reported [91,
92, 100] because the source of mites used for
experimental infestation influences the
Grooming behavior towards phoretic
mites is another known defense mechanism
of the Asian bee towards the ectoparasitic
mite [28, 39, 91, 92, 96, 126]. Auto- and
allo-grooming behavior of A. cerana workers is dependent on the ability of the workers to detect the mite and successfully groom
it from the bee’s body. The bees can readily
be observed grabbing and crushing mites in
their mandibles. If a bee fails to groom herself, she may perform the grooming dance
by rapidly shaking her abdomen. The dance
elicits allo-grooming by nestmates [91].
Thus, mites are disturbed and sometimes
killed by the grooming behavior of A. cerana workers.
[31, 64, 93].
3. DEFENSE MECHANISMS
OF A. CERANA TO V. JACOBSONI
ana,
Fries et al. [39] indithat this behavior may be less effective
in combating mites than previously reported
[28, 91, 92]. Using A. cerana and A. mellifera bees in cage experiments, observation hives and full-size colonies, the authors
found that the proportion of live mites that
had visible damage as a consequence of
grooming was about 30 % in A. cerana compared to 12.5 % in A. mellifera colonies.
Fries et al. [39] considered only instances
of successful grooming in which the mites
were damaged, in contrast to Peng [91 ] and
Büchler [28], who included the movement of
mites from one bee to another bee, and the
observer losing sight of the mites as groom-
Investigations by
cate
ing events.
V. jacobsoni mites are specifically
adapted to their host, having distinct attachsites on the bees’ bodies which make
it difficult for the bees to successfully groom
the mites [33, 96, 98]. If not groomed off,
the mites can survive on the bees for long
periods without reproduction (several months
see Rath [96]) and can disperse to other
colonies while attached to drifting bees.
Infestation levels of up to several hundred
adult female mites in A. cerana colonies
demonstrate that V. jacobsoni does survive
in the colonies and is well adapted to its
original host [2, 96, 98, 100, 106, 127, 140].
ment
It should be emphasized that, in principle,
grooming and hygienic behaviors of A. cerana towards V. jacobsoni are factors which
contribute to a balanced host-parasite relationship. However, because the mite does
not reproduce, or has very limited repro-
duction in worker brood of A. cerana,
grooming and removal behaviors may not
be the most important mechanisms of resistance. Based on simulation studies on the
population growth of the mite. Fries et al.
[38] argued that the limited reproduction of
V. jacobsoni on seasonally occurring drone
brood in A. cerana is sufficient to explain the
bees’ tolerance of the mite.
4. DEFENSE MECHANISMS
OF A. MELLIFERA
TO V. JACOBSONI
When the host-parasite relationship
between V. jacobsoni and A. cerana is used
as a model to search for natural resistance to
the mite by A. mellifern, it is evident that
the defense mechanisms of the Asian honey
bees (grooming and hygienic behavior)
against the mite are also present in European and North American honey bees.
4.1.
Hygienic behavior
A. mellifera also removes mite-infested
pupae from capped brood cells but to a limited extent compared to A. cerana [6, 11,
13, 14, 18, 91, 113]. A. mellifera also
removes brood infested with Tropilaelaps
clareae Delfinado & Baker [19, 102]. In
most published investigations, brood cells
experimentally infested with living V. jacobsoni were used to quantify this behavior;
data on the removal of naturally infested
brood are scant. Africanized bees in Mexico
removed 32 % of brood naturally infested
with mites compared to 8 % by European
bees under the same local conditions [ 132].
A. m. intermissa bees in Tunisia removed
on average 15.5 % of the pupae in naturally
infested cells (Ritter and Boecking, unpublished) and A. m. carnica colonies removed
16.6 % [14]. The mean percentage removal
of brood experimentally infested with one
living mite per cell by 76 colonies not preselected for hygienic behavior was 23.5 ±
18.2 (tested three times during 1997) [17].
Only 9.2 % of these colonies removed more
than 50 % of the infested brood. A. m. ligustica colonies that had been pre-selected for
hygienic behavior in the US (28 colonies
total, 1994-1997) removed an average of
52.1 % (± 25.6) of the experimentally
infested pupae, compared to 17.4 % (± 14.7)
in colonies selected for non-hygienic behavior (19 colonies, 1994-1997) [114, 119].
In contrast to diseased brood, miteinfested larvae and pupae do not necessarily
die. Opened brood cells containing healthy
pupae can indicate part of the process of
removal behavior [30]. In some cases the
caps of mite-infested brood are opened and
then closed again by the bees with a new
wax cap without eliminating the bee brood.
In those cases, the mites may leave these
cells by the temporary opening in the capping [6, 12] as observed in A. cerana. As a
result of opening and closing the cell cap
by the bee, a distinct change in the silk/waxstructure of the inner cell cap can be
observed [ 16]. Careful examination of 643
brood cells experimentally infested with one
living mite revealed that 69 (10.7 %) of the
cell caps showed clear signs that the worker
bees had opened and closed those cells at
least once during the 10 days of the investigation without eliminating the brood.
Although the introduced mite could have
left the cell while it was opened, the mite
was missing in only 4 (5.8 %) of these cells
[17]. Video recordings by Boecking [12]
also confirmed that the removal of miteinfested brood did not always involve a strict
behavioral sequence of detecting, uncapping and then removing the parasitized
brood. In some cases, the cappings of the
mite-infested brood cells were opened by
the bees then sealed again after some time
(e.g. after 150 min). Later, these same cells
were uncapped again and the pupae were
then removed. In most cases, the mites did
not leave the brood cells until the bees had
already removed most of the larval/pupal
body. Individually marked bees were seen
repeatedly detecting and uncapping miteinfested cells. Other bees repeatedly closed
such opened brood cells although the mites
were still present in these cells [12]. Using
infra-red video recordings Thakur et al. [ 128,
129] also observed a few individuals repeatedly uncapping and removing mite-infested
brood cells. These observations indicate that
some bees specialize in these different
behavioral traits. Early studies on hygienic
behavior suggested that hygienic bees were
genetic specialists; mixed groups of hygienic
and non-hygienic workers expressed the
hygienic phenotype only when the hygienic
worker subgroup was large enough to take
care of all the uncapping and removing tasks
[131]. Experimental colonies containing
groups of bees from different patrilines (n 4)
uncapped more dead brood cells (killed by
freezing) compared to pure supersister
=
groups
[63].
The expression of hygienic behavior is
known to be strongly influenced by environmental factors. For example, weak
colonies, or a lack of incoming nectar have
been shown to reduce the removal response
to mite-infested and dead brood cells,
respectively [15,79, 114, 117]. Early experiments on hygienic behavior [130] concluded that young hygienic bees will remove
all diseased brood regardless of nectar availability,
remove
but bees older than about 4 weeks
the larvae only during a nectar flow.
experiments (Spivak, unpublished)
hygienic colonies composed of bees
of 11different age cohorts (ranging from
3-33 days) indicated that the mean ages of
the bees that uncapped and removed freezekilled brood were 15 and 16 days, respectively (n 242 and 299 bees). The mean
ages of the foragers in the same colonies
were significantly older, 22 and 20 days,
respectively (n 79 and 66 bees; t-tests
P < 0.01). This experiment indicated that
hygienic bees are middle-aged; they have
brood-rearing experience but have not necessarily begun foraging.
Recent
on
two
=
=
Recently Boecking and Drescher [17]
heritability value based on the
mother-daughter regression for the removal
of brood experimentally infested with one
2
= 0.18 ± 0.27
living mite per cell was h
found the
(SD). The value for the removal of dead
(killed using the pin-killed brood
2 0.36 ± 0.30. A statistical
assay) was h
brood
=
repeatability of
the measurements was w 0.24 (four test
repetitions, 97 colonies, 11bee yards,
297 records) for the removal of mite-infested
brood, and w 0.46 (six test repetitions,
114 colonies, 11bee yards, 421 records) for
analysis
revealed that the
=
=
the removal of dead brood. These values
emphasize that environmental effects
strongly affect the expression of the hygienic
behavior in the experimental bee populations used. Moreover, the results demonstrate that the rate of removal of miteinfested or dead brood within a particular
colony even under the same environmental
conditions is not always consistent between
assays [54, 103, 115].Quantitative genetic
studies on hygienic behavior using a laboratory bioassay also showed only moderate
estimates for genetic variance, with h
2
0.14 for uncapping 2
and h 0.02 for removing dead brood [73]. In contrast, Harbo and
Harris [50] calculated the heritability of
hygienic behavior, based on the removal of
2 0.65 ± 0.61.
freeze-killed brood, to be h
=
=
=
There is a positive correlation between
the rate of removal of mite-infested brood
and dead brood (freeze-killed or pin-killed).
Colonies pre-selected for high hygienic
behavior on the basis of the freeze-killed
brood assay [115], removed significantly
more brood cells experimentally infested
with mites compared to colonies selected
for low hygienic behavior [114,119].
Honey bees are able to detect even a single cell containing an abnormal, dead or diseased larva or pupa within a healthy brood
[36,112]. Mechanical and chemical
stimuli, including brood pheromones, enable
nest
bees to recognize and distinguish healthy
brood within a colony [60, 78]. However,
it is not known how the bees determine that
a particular larva or pupa is dead or infested
with a mite under a wax-capped brood cell.
Ritter and Boecking (unpublished data)
found the removal of brood experimentally
infested with mites that transferred or triggered a secondary infection of acute paralysis virus (APV) was higher (on average
56.7 %) compared to brood infested with
mites collected from sources without viruses
(on average 12.2 %). The signals that enable
the bees to detect mite-infested live brood
might be different from the signals used to
detect brood that has been killed by diseases
or viruses.
Preliminary experiments indicated that
in contrast to dead mites (frozen, washed in
alcohol or collected from hive debris) and
other introduced, non-living particles (tinfoil
globules, eucalyptus seeds, filter paper),
only live mites in brood cells elicit a removal
response by the bees. Cells infested with
more than one mite elicit a stronger removal
response than cells infested with one mite
[16, 114]. Rosenkranz et al. [106] demonstrated that the removal of mites by A. cerana depends on the alien scent adhering to
the mite, which is not the case for A. mellifera [3, 16]. These results imply that the
odor of the mites itself is probably not an
important cue to A. mellifera.
It is not clear what cues bees use to detect
brood cell infested with a mite. It has been
hypothesized that bees may use acoustical
signals to detect infested brood [112]; however, checking for movements released
through the capping of brood cells containing live, dead (pin-killed) or mite-infested
pupae using a laser interferometer could not
support this hypothesis (Kirchner and
Boecking, unpublished data). Gramacho et
al. [46, 47] showed that the average body
temperature of dead pupae (pin-killed) inside
the brood cell under brood chamber conditions was significantly lower (0.3-0.7 °C)
than the temperature of live pupae. However, the use of an infra-red thermographic
system, which allowed a continuous measurement of capped brood combs without
physical contact, revealed no differences in
temperatures released through the capping of
brood cells containing live, dead (pin-killed)
and mite-infested pupae (B. Görgens,
unpublished data). Since the temperature
above the sealed cell should be perceived
by worker bees, these data imply that differences in temperature inside the brood cell
might not be the cue bees use to detect dead
brood.
a
Pupae treated with hemolymph or body
fluid seem to be a strong stimulus for the
bees to open cells and to remove the treated
pupae. Even colonies pre-selected as non-
hygienic can be induced to express hygienic
using hemolymph or body fluid
([115], Görgens, unpublished data; Gramacho, pers. comm.). Bees may use olfactory
cues to detect diseased or parasitized brood
in the nest. Preliminary field and laboratory
experiments, using proboscis-extension
reflex (PER) conditioning (see methods in
behavior
Bitterman [10]) indicated that individual
bees collected from a colony selected for
hygienic behavior have lower response
thresholds than bees collected from a nonhygienic colony to olfactory stimuli associated with chalkbrood-killed pupae [70].
These results suggest that the differences
between individual hygienic and nonhygienic bees lie in their responsiveness to
olfactory stimuli associated with diseased
brood. However, no published research has
revealed the exact nature of the cues that
enable hygienic bees to detect mite-infested
brood cells.
Mite removal behavior may theoretically
limit the population growth of V. jacobsoni
in three ways [38]: first, immature mites
which have begun to develop in brood cells
are killed, decreasing the average number
of offspring per mother mite. Second,
removal of female mites extends the phoretic
period of those mites which survive the
removal process. Third, mite removal
increases the mortality of mother mites.
Experiments
behavior on the
on
the effects of removal
survivorship of single mites
[11, 12, 15] revealed that most of the adult
female mites that escaped the brood cells
after removal of the infested brood could
invade other brood cells again. In one experiment 104 individually color-marked mites
escaped from brood cells during the removal
of infested brood by the bees; 61.3 % of
these mites invaded new brood cells, and
14.6 % became phoretic on adult bees;
24.6 % were found on the hive bottom board
and 10.9 % of those were killed by the bees.
The overall effect of hygienic behavior on
population growth of the mites within a
colony is still unknown. However, Spivak
and Reuter [120, 121]demonstrated that
colonies bred for hygienic behavior (based
on rate of removal of freeze-killed brood)
had fewer mites than commercial colonies
not selected for the behavior after 1 year
without mite treatment. The same colonies
also had lower incidences of chalkbrood and
AFB, and produced more honey than the
commercial colonies. These results demonstrate the benefits of selecting bee colonies
for hygienic behavior.
4.2.
Grooming behavior
Grooming
of phoretic mites by A. melis
not
as
lifera
pronounced as it is in A. cerana. This difference was first pointed out
by Peng et al. [91] and later confirmed by
other researchers [18, 25, 28, 39, 53, 81, 83,
84, 87, 96, 107, 109, 135]. It is evident that
A. mellifera bees are able to kill mites during grooming activities, since damaged
mites still showing movement were found in
the debris on the bottom board of the
colonies [13, 15, 48, 53, 107, 108]. In these
studies, care was taken to ensure that no
other insect or scavenger could enter the
hive debris to cause the mite damage. The
main type of damage to the mite caused by
successful grooming is amputation or mutilation of one or more legs. Injuries to the
mites’ idiosoma or gnathosoma are rela-
tively rare [67, 107]. Using video-technique,
mellifera bees can sometimes be observed
grabbing and crushing mites in their
A.
mandibles. As on A. cerana, the mite
chooses distinct attachment sites on the bodies of A. mellifera at the 3rd and 4th ventro-lateral tergites of the abdomen, and have
a significant preference for the left side of
the host, which may demonstrate an adaptation of the mite to grooming [15, 18, 20,
33, 96, 98].
Using infra-red video recordings Thakur
al. [ 128, 129] confirmed that bees used
active grooming defenses against the mites
including grabbing and crushing mites in
their mandibles. They observed a few indi-
et
viduals performing repeated grooming activities. General studies of grooming behavior
(independent of its effect on mites) have
shown that some bees in A. mellifera are
grooming specialists; the behavior is genetically determined and age specific [31, 40,
62, 93, 133]. Moore et al. [80] observed one
highly specialized grooming honey bee as it
aged from 4-31 days. This bee groomed other
bees 84 % of the time she was observed, and
never developed into a forager.
It is not known how the bees that groom
detect mites on the bee body. Observation of
behavioral patterns of groomers (Boecking,
pers. obs. of A. cerana and A. mellifera)
indicate that they begin to allo-groom other
infested adults after they perceive the
grooming dance. Occasionally bees were
observed grooming one bee after another in
rapid succession. During grooming, the
receiving bees hold their wings perpendicular to the body axis and the grooming bees
work on those body parts that can not be
reached by auto-grooming. However, the
mites are not necessarily located in the
places where the bee grooms. Mites have
been observed to leave the bee while the
bee was still being allo-groomed, indicating that the groomers may not be able to
detect the mite itself (Boecking, pers. obs.).
Grooming behavior is thought to be a
heritable trait. F1 colonies bred from
colonies that demonstrated high or low mite
grooming ability showed the same ability
as their parent colony [84, 85]. Nevertheless, precise heritability estimates have not
been calculated for this trait. As with
hygienic behavior, the expression of grooming behavior is known to be strongly influenced by environmental factors [69]. Moosbeckhofer [82] observed low numbers of
damaged mites in March (11 %) but significantly higher numbers in June (42 %) in
the same colonies. Consequently, the proportion of damaged mites within a particular colony, even under the same environmental conditions, is not always consistent
between collections.
The grooming potential of a colony can
be measured by counting the number of live
and damaged mites that drop from miteinfested bees onto protected sheets (inserts)
placed under the brood nest. Observing adult
bees that have been inoculated with mites
in observation hives provides information
about the behavioral patterns of groomers
and infested bees, but is not a good measure of the overall grooming ability of the
colony. Data from cage experiments and
from laboratory bioassays can reveal that a
particular genetic line or race of bee reacts
more strongly to mites (shaking and biting
the mites) compared to others bees, but it
is difficult to correlate these data with the
behavior of a whole colony [3, 53, 54, 125].
Investigations by Rosenkranz et al. [107]
showed that the number of damaged mites
can reach relatively high numbers even in
colonies that had not been pre-selected for
this behavioral trait. Mites were collected
from protected inserts every 12 h over
12 days in eight A. m. carnica colonies.
Approximately 46 % of the dead mites
(n = 3917) and approximately 11% of the
live mites (n 889) on the inserts were damaged. A portion of the live undamaged mites
that fell onto the bottom board were able to
reproduce after they were subsequently
introduced into freshly capped brood cells.
=
Although counting the number of damaged mites on an insert within the colonies
indicates the grooming potential of the
colony, the mites may have been damaged
by means other than grooming. Mites that
are already dead may be damaged secondarily, and immature mites might be damaged during the removal of infested brood
(hygienic behavior). Wax moth larvae may
also damage mites [124]. In addition, the
number of mites falling onto the bottom
board is highly correlated with the presence
and stage of brood [37, 65]. Lobb and Martin [66] found a strong correlation between
levels of falling mites and the emergence
of brood. It was estimated that around half of
the falling mites originated from mites that
died within the sealed cell; the other half
died shortly after bee emergence. The number of mites falling from worker brood was
two to three times higher than from drone
brood. Damaged mites that originated from
the emerging brood should not be attributed
to the grooming potential of a bee colony,
although it is difficult to distinguish between
the two.
Boecking and Drescher [17] compared
the number of damaged mites found on the
bottom boards to the estimated mite population that was phoretic on adult bees. In
113colonies containing no brood, the average percentage of damaged mites captured
on the inserts on three successive days was
21.5, 15.6 and 11.7 %. In total, 337 mites
were captured. In contrast, 84 136 mites
were collected after a chemical treatment
of the bees following the last mite collection, averaging 744.6 mites/colony (range
91-2 157 mites/colony). The treatment with
an effective acaricide gave an estimate of
the mite population that was phoretic on the
bees during this investigation. Comparing
the number of damaged mites that dropped
onto the bottom boards to the number of
mites phoretic on the bees, the relative frequency of damaged mites in these colonies
on the 3 days was 0.08, 0.06 and 0.03 %.
5. CONSIDERATIONS
AND UNRESOLVED QUESTIONS
Promoting natural resistance to V. jacobthrough selection and breeding is in its
infancy today. Despite the extensive research
on grooming and hygienic behaviors, it is
still unclear to what extent they are effective mechanisms of defense against the
mites. It is commonly thought that bees that
vigorously shake and bite the mites are the
soni
It is still controversial whether the number of damaged mites on the bottom board is
a useful criterion for successful grooming
behavior. Because the percentage of damaged mites may vary widely from colony to
colony and because of the difficulties in
measuring successful grooming, it is not
clear if grooming behavior is an effective
mechanism of defense against mites [8, 9,
65, 107, 125]. Nevertheless, model simulations showed that control of the mite population depends on the frequency and efficiency of the bees’ ability to groom and that
a moderate increase in the mite death rate
could theoretically help a bee population
become genetically resistant to parasitism
most efficient groomers. However, this disturbance by the bees may cause the mites
to invade brood cells more readily. This possibility remains to be tested. Likewise, it is
thought that colonies that rapidly uncap and
remove diseased and parasitized brood are
more resistant to diseases and mites. It has
been shown that the spread of infective diseases is minimized if hygienic bees remove
the diseased brood before the causative
organism reaches its infectious sporulating
stage [139]. However, Spivak and Gilliam
[117] considered the possibility that nonhygienic behavior in honey bees could benefit a colony. If bees leave diseased brood
under a capped cell, they may avoid contact with the pathogens in the cell. It is not
clear if non-hygienic bees actively avoid
diseased brood, or if they do not respond to
it. Bees from pre-selected non-hygienic
colonies tended to recap partially uncapped
cells that contained freeze-killed brood
[117]. A. mellifera bees also recapped miteinfested brood cells that had been previously
uncapped by other bees [ 12]. In A. cerana,
this non-removal behavior is much more
marked (Boecking, unpublished data); they
uncap and remove worker brood cells
infested with V. jacobsoni, but tend not to
remove drone brood infested with multiple
mites or with bacteria disease. The bees
close the pore of the drone cell cap with
wax from the outside. The non-removal of
infested drone brood and closure of the central pore by the workers isolates the mites
and infectious diseased cells in the brood
by
nest.
V. jacobsoni
[68].
When A. mellifera does remove miteinfested brood, it is not clear at what point
the bees should perform this behavior to
most effectively limit the population growth
of V. jacobsoni. To negatively affect the
potential average number of offspring per
mother mite, removal should begin after the
foundress has laid her full clutch of eggs in
the brood cell. At this point, the bees would
destroy the offspring during removal of the
pupae, and the foundress would have to
enter the phoretic stage again before she
could invade a new brood cell. In highly
infested colonies, it may not be advantageous for the bees to remove all miteinfested worker brood, which could substantially reduce the adult population of the
colony.
Hygienic behavior of honey bees may be
a generalized adaptation for cell reuse and
thus, may present a conflict between risking
infection by removing diseased brood and
the need to clear out comb space for the queen
to lay eggs [117]. A. cerana can use another
strategy to avoid mites; they tend to abscond
from the nest when under high disease or
parasite pressure. Predators (such as wax
moths) then destroy the abandoned combs,
ridding them of the infectious material.
Lines of A. mellifera demonstrate differdegrees of susceptibility to V. jacobsoni
[24, 26, 27, 32, 48, 51, 90]. When colonies
are initiated with equal numbers of mites,
the most resistant colonies can be identified
as those that have the fewest mites at the
end of the test period [8, 25, 51, 90]. However, numerical data on just the population
growth or decline of the mites during the
experimental period do not yield information
on the mechanisms that influenced these
trends. In breeding bees for mite resistance,
it is critical to determine the exact nature of
the traits that confer resistance, and to ensure
they are measurable and heritable (e.g. [49,
ent
50, 51]).
The main genetic parameters which influthe bees’ expression of hygienic
behavior are maternal and heterotic effects
ences
which dominate additive gene effects [54].
In contrast, grooming behavior is mainly
influenced by additive gene effects, such
that a combination of specific maternal and
paternal bee lines is necessary for the breeding of mite-resistant stock. Such a combination that favors hygienic behavior will
not likewise favor grooming ability and vice
versa [54]. It has been documented that
colonies that show a high degree of removal
of mite-infested brood are not necessarily
better groomers [114].
High genetic variability and heterosis do
not seem to enhance resistance to V. jacobsoni, although these factors do increase
colony productivity [41, 88]. In a 2-year
hybrid queen breeding program, characteristics related to disease resistance and in
particular to resistance to V. jacobsoni
(infestation level, duration of the postcapping brood phase, hygienic behavior and
number of damaged mites), line-mixed
colonies showed a consistently lower performance compared to the separate lines.
This result is contrary to the theoretical
expectation of enhanced resistance to diseases with increased genetic variation
between workers [41].
In conclusion, bee breeding strategies for
increased resistance to V. jacobsoni should
be based on knowledge of the mechanisms
of resistance, their potential for decreasing
mite population growth, and their mode of
inheritance. At the same time, the ability of
the mite to adapt to changes in their host
should be taken into consideration.
ACKNOWLEDGEMENTS
We thank John Harbo, Rebecca Masterman
and Scott Camazine for their helpful critiques of
this manuscript. The research by M.S. was funded
by National Science Foundation IBN-9722416
and USDA, Northcentral Region SARE. This is
contribution #99-1-17-0003 from the Minnesota
Agricultural Experiment Station. The research
by O.B. was funded by the Federal Ministry of
Food, Agriculture and Forestry (BML, Bonn 89
HS 020) and the European Union (EUROBEE
AIR3-CT94-1064).
Résumé - Les comportements de défense
des abeilles mellifères contre Varroa
jacobsoni. Les abeilles mellifères présentent deux types de comportements qui sont
des mécanismes de défense contre les maladies du couvain et les acariens parasites.
Des études ont montré que des colonies
d’Apis mellifera L. éliminaient du nid le
couvain d’ouvrières infesté par l’acarien
Varroa jacobsoni (comportement hygiénique) et débarassaient les abeilles adultes
des acariens (comportement de toilettage),
mais dans
une
moindre
mesure
que l’hôte
d’origine, Apis cerana Fabr. En Allemagne
carnica (n 76), non
pré-sélectionnées pour le comportement
hygiénique, ont éliminé en moyenne 23,5 %
(± 18,2 %) du couvain infesté expérimentalement avec un acarien par cellule. Aux
États-Unis des colonies d’A.m. ligustica
sélectionnées pour leur comportement hygiénique (28 colonies au total, 1994-1997) ont
éliminé en moyenne 52,1 % (± 25.6 %) des
nymphes infestées expérimentalement contre
17,4 % (± 14,7 %) chez les colonies non
sélectionnées (19 colonies, 1994-1997). Les
observations concernant l’élimination par
A. m. carnica du couvain infesté montrent
que certaines abeilles peuvent désoperculer
et d’autres refermer l’opercule de cire sur
des cellules de couvain infestées avant que
le couvain ne soit finalement désoperculé
et éliminé ou, dans certains cas, laissé tel
quel. Des observations préliminaires indiquent une certaine spécialisation des abeilles
pour ces tâches. La plupart des fondatrices
d’acariens peuvent s’échapper au cours du
processus d’élimination du couvain et envahir à nouveau d’autres cellules ou devenir
phorétiques sur des abeilles adultes ; un petit
pourcentage peut être mutilé par les abeilles
lors du toilettage. Les acariens immatures
qui ont commencé à se développer sont tués
pendant le processus d’élimination du couvain, ce qui contribue à diminuer le nombre
moyen de descendants par acarien femelle.
L’héritabilité de l’élimination du couvain
infesté a été estimée à h
2 0,18 (± 0,27)
chez A. m. carnica ; ceci montre que l’envi-
des colonies d’A.
m.
=
=
agit fortement sur l’expression
de ce comportement.
On ne sait pas quels signaux utilisent les
abeilles pour détecter le couvain infesté par
V. jacobsoni. On a émis l’hypothèse que les
abeilles hygiéniques avaient un seuil de
réponse plus bas que les abeilles non hygiéniques aux signaux olfactifs associés au couvain parasité (ou anormal pour une autre
ronnement
raison).
mellifera n’a pas
un comportement de
aussi marqué qu’A. cerana.
L’observation d’abeilles engagées dans le
A.
toilettage
toilettage suggère qu’elles sont peut-être
incapables de détecter l’acarien proprement
dit sur le corps de l’abeille qu’elles toilettent,
puisqu’on a observé des abeilles toiletter
alors qu’il n’y avait pas d’acariens et qu’on
a vu des acariens quitter l’abeille pendant
le toilettage. L’héritabilité du comportement
de toilettage n’a pas été estimé. On pense
pourtant que son expression est contrôlée
par des effets additifs de gènes et qu’il est
fortement influencé, comme le comportement hygiénique, par les conditions du
milieu. Le potentiel de toilettage est généralement mesuré en dénombrant les acariens
morts et mutilés tombés sur des feuilles collantes placées sous le nid à couvain. Pourtant
des acariens mutilés ne sont pas toujours le
résultat d’un toilettage efficace. Des acariens morts (dans les cellules de couvain
d’ouvrières ou sur les adultes) peuvent avoir
été mutilés dans un second temps et les larves
de fausse-teigne, les fourmis ou d’autres
nécrophages sont capables d’infliger des
mutilations aux acariens.
En comparant le nombre d’acariens mutilés au nombre d’acariens phorétiques dans
113 colonies, on a trouvé une fréquence relative d’acariens mutilés inférieure à 0,09 %.
On ne sait pas si le nombre d’acariens mutilés trouvés sur le plancher est un critère utile
pour évaluer l’efficacité du comportement de
toilettage.
L’influence globale des comportement
hygiénique et de toilettage sur la limitation
de la croissance de la population et la survie
des acariens dans les colonies d’A. melli-
fera reste incertaine. Au cours du toilettage
les acariens sont secoués vigoureusement
mordus. On n’a pas testé si cela facilitait
l’envahissement d’autres cellules par les
acariens. Sélectionner des colonies pour le
comportmeent hygiénique diminue la loque
américaine et le couvain plâtré et peut aider
à limiter les populations d’acariens. Mais
les coûts et les bénéfices de l’élimination
du couvain parasité et malade (comportement hygiénique) par rapport au maintien
de ce couvain dans des cellules operculées
(comportement non hygiénique) n’ont pas
été totalement explorés. Lorsqu’on sélectionne des abeilles pour la résistance aux
acariens, il est fondamental de déterminer
la nature exacte des caractères qui confèrent la résistance et de s’assurer qu’ils sont
mesurables et héritables. © Inra/DIB/AGIB/
Elsevier, Paris
et
Apis mellifera / Apis cerana / Varroa
jacobsoni / comportement hygiénique /
comportement de toilettage / résistance /
tolérance
Zusammenfassung - Abwehrverhalten
von Honigbienen gegen Varroa jacobsoni
Oud. Honigbienen besitzen zwei verschie-
dene Verhaltensweisen zur Abwehr von
Brutkrankheiten und parasitischen Milben,
das hygienische Verhalten und das Putzverhalten. Untersuchungen haben gezeigt,
daß Völker von Apis mellifera mit Varroa
jacobsoni befallene Arbeiterinnenbrut aus
dem Brutnest (hygienisches Verhalten) und
Milben von anderen Bienenarbeiterinnen
entfernen können. Allerdings erfolgt dies in
weit geringerem Ausmaß als bei dem
Ursprungswirt von V. jacobsoni, A. cerana.
Nicht vorselektierte Völker von A. m. carnica (n 76) in Deuschland entfernten im
Mittel 23.5 % (± 18.2) von mit jeweils einer
Milbe künstlich infizierten Brutzellen.
Dahingegen entfernten in den USA auf
hygienisches Verhalten selektierte Völker
von A. m. ligustica im Mittel 52.1 %
(± 25.6 %, insgesamt 28 Völker; 1994-1997)
der experimentell infizierten Puppen. Im
=
Vergleich hierzu entfernten die auf geringes hygienische Verhalten selektierten Völ-
ker 17.4 % (± 14.7; 19 Völker, 1994-1997).
Beobachtungen der Entfernung von milbeninfizierter Brut durch A. m carnica deuten
darauf hin, daß nach Öffnung des Deckels
der infizierten Zellen durch einige der Arbeiterinnen andere ihn wieder schließen können, bevor die Brutzelle endgültig entdeckelt
und die Brut entfernt wird oder sie in einigen
Fällen auch intakt gelassen wird. Einige
Beobachtungen deuten weiter auf einen
gewissen Grad von Spezialisierungen der
Arbeiterinnen für diese Aufgaben hin. Die
meisten Milbenweibchen können während
des Entdeckelungsvorganges entkommen
und wieder in andere Zellen eindringen oder
auf Bienenarbeiterinnen aufsitzen; ein geringer Prozentsatz wird durch das Putzverhalten der Bienen verletzt. Unausgewachsene
Milbennachkommen werden während des
Entfernens der Brut getötet, hierdurch vermindert sich die mittlere Anzahl von Nachkommen pro Milbenweibchen. Die Erblichkeit des Entfernens von befallener Brut
bei A. m. carnica beträgt h
2 = 10.18 (± 0.27).
Dies zeigt an, daß die Ausprägung dieses
Verhaltens stark durch Umgebungsfaktoren beeinflußt wird (aber siehe Harbo [51]).
Es ist nicht bekannt, auf Grund welcher
Anzeichen die Bienen mit V. jacobsoni
befallene Brutzellen entdecken. Es wird
allerdings die Hypothese vertreten, daß
hygienische Bienen eine verringerte
Ansprechschwelle für die mit parasitierter
oder anderswie unnormaler Brut zusammenhängenden geruchlichen Reize haben.
Das Putzverhalten ist bei A. mellifera nicht
so ausgeprägt wie bei A. cerana. Da beobachtet wurde, daß sie auch an Stellen putzen,
an denen keine Milbe vorhanden war, oder
daß die Milben während des Putzens die
geputzte Biene verlassen konnten, wird
angenommen, daß die Bienen die Milben
auf dem Körper der von ihnen geputzten
Biene nicht direkt wahrnehmen können. Die
Heritabilität für das Putzverhalten wurde
bisher nicht bestimmt, man kann aber annehmen, daß dieses wie das hygienische Verhalten durch additive Geneffekte bestimmt
und stark durch Umgebungsfaktoren beeinDas Putzpotential wird üblicherweise durch Auszählen von verletzten
Milben auf klebrigen Bodeneinlagen in den
Bienenbeuten bestimmt. Allerdings müssen
verletzte Milben nicht immer das Ergebnis
von erfolgreichem Putzverhalten sein. Es
können auch Milben, die in Brutzellen oder
auf Arbeiterinnen bereits zuvor gestorben
waren, erst später verletzt worden sein.
Zusätzlich können auch Wachsmottenlarven, Ameisen und andere Aasfresser die
Milben nachträglich verletzen. In einem
Vergleich von verletzten Milben und der
Anzahl von Milben auf den Arbeiterinnen
betrug der Anteil verletzter Milben weniger als 0.09 %. Es ist daher nicht klar, ob
der Anteil verletzter Milben auf Bodeneinlagen zur Bestimmung eines erfolgreichen
Putzverhalten geeignet ist. Es bleibt weiterhin unklar, welchen Gesamtbeitrag das
hygienische Verhalten und das Putzverhalten zu einer Begrenzung des Populationswachstums der Milben in Völkern von
A. mellifera leisten können. Es wurde bisher
nicht untersucht, ob das heftige Beißen und
Schütteln während des Putzverhaltens die
Milben dazu bewegt kann, eher die Brutzellen zu befallen. Die Zucht von Bienenvölkern auf verstärktes hygienisches Verhalten kann den Schaden durch Amerikanische Faulbrut und Kalkbrut mindern,
und es kann helfen die Milbenpopulation
zu verringern. Allerdings wurden Nutzen
und Kosten der Entfernung parasitierter oder
erkrankter Brut (hygienisches Verhalten)
gegenüber dem Belassen dieser Brut unter
dem Zelldeckel (nicht-hygienisches Verhalten) bislang nicht vollständig aufgeklärt.
In der Zucht von milbenresistenten Bienen
ist es von grundlegender Bedeutung, die
Natur der verantwortlichen Verhaltensweisen aufzuklären und sicherzustellen, daß
diese meßbar und erblich sind. © Inra/DIB/
AGIB/Elsevier, Paris
flußt wird.
Apis mellifera / Apis cerana / Varroa
jacobsoni / hygienisches Verhalten /
Putzverhalten / Resistenz / Toleranz
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