# Polyethism in an Oriental Hornet (Vespa orientalis) Colony

Authors: Stanislav Volynchik; Marian Plotkin; David J. Bergman; Jacob S. Ishay
Journal: Scholarly Research Exchange
 (2009)

---

## Abstract

The present study describes the daily activities of the Oriental hornet (Vespa orientalis) workers older than 48 hours as observed in an Artificial Breeding Box (ABB) in our laboratory at the peak of the active season. These workers were picked randomly from a population of a single nest that was enabled to free egress into the field. The study points to the existence of polyethism in the nest of V. orientalis with the adult worker hornets, separating them into three groups in accordance with the frequency and nature of their exits from the nest, and the tasks which they perform. This polyethism was not age-dependent.  Recordings were made of the following vespine activities: the frequency of exits to the field during various hours of the day and the various roles undertaken by the hornets. Also investigated was the assignment of tasks among the hornets and the preference given to some tasks over others. The general organization of traffic and other movements in the colony is discussed.


---

## Body

## 1. Introduction

The Oriental hornet
(Vespa orientalis) is prevalent throughout the countries of the
Mediterranean basin and also in the Near East as far as India [1]. After a
period of hibernation, the queen seeks out a site suitable for founding a colony.
In the Tel-Aviv district, the Oriental hornet queen prefers a site comprised of
khamra (red loam) or kurkar (calcareous
sandstone) soil. The primary nest is built by the queen alone and is
known as an “embryonic nest.”While attending the initial larval brood, the queen continues to build additional
comb cells until the brood comb contains 20–30 cells. After
about a month, the initial workers that eclose relieve the queen of various
duties like the provision of food for the developing brood, cleaning of the
nest, and further building and expansion of the nest, leaving to the queen its
main role of ovipositing into the comb cells, one egg per cell comb [2]. While
the queen's ovipositing activities all take place within the nest, without any
need for her to leave the nest, the workers in the course of the active season
engage in various activities that are strongly interconnected and focus mainly
on tending the brood. Additional duties of the workers entail further building
or enlargement of the nest accomplished by the accretion of plant organic
matter or inorganic matter like khamra soil; these are masticated by the
building hornet and then deposited as horizontal stripes of building material
which ultimately give rise to the hexagonal comb cells that make up the combs
in the nest. When the nest is (inadvertently) exposed to light, an envelope is
built around it as a protective measure.With the increase in the nest population, digging activities are undertaken to enlarge
the volume of the nest cavity. This is typical forV. orientalis as well
as other social wasp species like Paravespula germanica. If the dug particle is large, the building
worker lifts it with its mandibles and throws it outside the nest. However, if
the dug particles are small, that is, if the terrain is mostly sandy, the
digging hornet collects and fastens the individual particles into a particle of
appropriate size, lifts it in the air, and disposes of it some 5–10 meters from
the entrance.At the nest
entrance, we find hornets whose primary duty is to stand guard and prevent foreigners,
like hornets from other nests, or various harmful organisms from endangering
the nest. To this end, the sentinel hornet probes with its antennae any hornet trying
to enter the nest. These sentinel hornets are replaced by others every few
minutes and they probably succeed in repelling parasites. Continuing with our
listing of hornet daily activities, we note that in warm days, there are hornets at the nest
entrance that assume a rather tall stance and perform ventilatory activity to
regulate the nest temperature [3]. The worker hornets engage in flight activity
for the purpose of (1) introduction of building materials, to which end the
hornets fly from place to place, taste the materials in the field, and decide
whether they are suitable as building materials, (2) foraging flights in the
course of which the hornets collect food materials, (3) flights outside the nest
which are intended to get rid of waste materials, including soil dug up for
enlarging the nest, uneatable parts of insects, dead larvae, and so forth. All
the aforedescribed activities take place during the main active season which
lasts several months [4–8].At the end of
the active season, from September until December, there is a drop in the
intensity of activities in the nest and also a drop in the overall size of the
worker population. Instead, during this period, there is eclosion of the sexual
forms, first the drones and then the queens. Once the eclosed queens accumulate
in their body a sufficient supply of storage materials, they commence their
wedding flight, and after copulation, the fertilized queens return to the nest,
remaining there till the onset of rains, whereupon they depart, seeking a dark
and dry spot to hibernate there until the spring.The purpose of
the present study was to try to ascertain whether adult hornet workers persist throughout
in one single task or rather there is a shift in tasks in accordance with age
or nest requirements.


## 2. Materials and Methods

The study was carried out in two stages.
Stage one: observations were made on hornets of varying ages between July and
the end of August, 2006. Stage two was undertaken in 2007, during August and
September, and involved 4 different groups of the same age, namely, hornets
that had just eclosed.The experiments pertaining to this study
were undertaken on the Oriental hornetV. orientalis (Hymenoptera,
Vespinae). A nest was located at the end of May-June in the
Tel-Aviv University Botanical garden and after initial observations at the
beginning of June, the nest was transported in an enclosure to the Vesparium,
namely, an Artificial Breeding Box (ABB), whose dimensions were 40×35×20 cm. The
original nest bore 50 workers at different ages, apart from a viable queen and a
comb with 85 cells containing brood at varying ages, including about 25 pupae.
The nest was anesthetized and treated as previously described [9]. All its population
was kept sealed for a week in the ABB. Soil comprised two-thirds of the ABB's
volume (intended to stimulate the hornets to dig as in a natural nest), and
above this soil we laid the comb collected from the field and its adult
population in a vertical position. As long as the ABB was kept sealed, the workers continued to
amend the comb and ensure that the cell walls remained as in nature, that is, with
an orientation toward the gravitational force (Figure 1).Figure 1
Scheme of an Artificial Breeding Box (ABB) and
its attachment to a terminal of the Vesparium: (1) ABB; (2) embryonic comb; (3)
glass wall; (4) red loam soil; (5) glass pipe; (6) Vesparium wall. The ABB measures40×35×20 cm, and is made of
plywood, except for one wall made of glass that enables observation into the
nest. The red loam soil was introduced to stimulate digging activity (Volynchik
et al. [13]).During Stage 1, and right after the nest
anesthetization in the field, we marked 35 adult workers aged 48 hours or more with
labels bearing 5 colors and a wide range of numbers (Figure2). The labels
(Opalith plattchen, Germany) bore the colors white, yellow, red, green, and
blue, while the numbers ranged from 1 to 99, as customary in labeling the queens
of bees. The tagging was effected with special glue on the back of the thorax.
One week later, the outlet from the ABB and the Vesparium was opened, enabling
free egress from the ABB and the vesparium to the field and back. Our tagging
technique enabled us to identify the departing and returning hornets both by color
and number, as well as by recording the departure and entry times. Our results
pertaining to stage 1 are based on observations of 23 out of 35 hornets that
survived and were active throughout this stage. In all, we recorded 2517
egresses of hornets from the ABB.Figure 2
V. orientalis workers labeled by a tag
on their thorax. Each tag has its own color and number so that these can be
read as the labeled hornets pass singly through the nest outlet. Bar = 1 cm (Volynchik
et al. [13]).During Stage 2 in the nest kept inside the
ABB in the Vesparium, which contained workers at various ages, we introduced
gradually 4 groups of young hornets (about one day posteclosion) which were gathered
from another nest kept in the laboratory. Each of the four groups (comprised of
8–16 individuals)
was assigned and marked by different colors and numbers. The observation
periods of the four groups were as follows: first group—from July 31 to September
2, second group—from August 5 to
September 20, third group—from August 22 to
September 20, and fourth group—from September 9
to September 25. All four groups were viewed in the course of 10 to 16 working
days, with close to 7000 egresses from the nest recorded during this stage. The
length of observation per group was dependent on and determined by the survival
rate in the group, that is, the longer the survival, the longer the observation
period, and vice versa.Additionally,
we observed also other activities undertaken by the adult hornets, like feeding
of the larvae and various digging and hunting functions, recording the
distribution of all these activities among different members of the nest.Statistical analysis of the findings was carried
out by applying one-way ANOVA and paired and independent samplet-test. P value of P<.001 was determined as the significant level.


## 3. Results

In the first
few days after eclosion, the hornet workers were photophobic and did not exit
the nest. Most of them engaged in intranest duties, such as connecting the comb
cell walls, mending the margins of the comb, cleaning activities, and also in attempts
to accustom themselves to the nest environment. The hornets start exiting the
nest on the third day after eclosion but the majority exits the nest only on
the fourth day after eclosion.The hornets
that exit the nest bring back food, water, and building materials, clear the
nest from garbage and enlarge it by digging activity, throwing out the dug soil
and stone particles. Such assignment of duties in dependence with age is
indicative of polyethism between young and old hornets.


### 3.1. Stage 1

During Stage 1
entailed, a follow-up on 23 tagged hornets that age-wise comprised a
heterogeneous group. Our observations showed that already at the onset of
extranest activities, there was disparity in the frequency of exits, with some
hornets exiting at very high frequency for brief (up to one minute) flights.
Such brief flights, usually associated with exits of digging hornets, occupied
51% of the total exit flights throughout the observation period. As for digger
hornets, their digging activity may repeat 50–60 times without
stop, but thereafter they rest for 10–15 minutes before
resuming the same digging activity. During the second week of observation, we
noted two digger hornets (3w and 4y) that were very active and
performed more than 60% of the total exits in the tagged group. Of the two,
hornet 3w was seen to exit to the field every 1.9±0.4 minutes (n=259), whereas
hornet 4y exited every 2.7±0.6 minutes (n=160). During the same period,
the remaining hornets in the group exited the nest once every 21.7±14.1 minutes
(n=262). In fact, throughout the observation period, there were 3 hornets in
the first subgroup that were persistent and very active in their digging
activity, namely, 3w, 4y, and 7y (Figure 3). Their
mean numbers of
exits per hour during the observation period were, respectively, 13.7±7.2 (n=452), 10.2±5.04 (n=411),
and 10.5±4.8 (n=438). We need to point out again that this subgroup of hornets engaged
primarily in digging activity, namely, the removal of grains of sand from the
ABB, and that they persisted in this activity for 5 whole weeks—from the end of
July to the end of August. All the remaining hornets exited the nest at a much
lower frequency, and engaged in a wide range of activities, including the
introduction of building materials or food into the nest, but mostly interior
nest duties, like tending to the developing brood.Figure 3
A graph showing the mean frequency of exits
from the nest per hour of 23 labeled worker hornets of different ages (Stage 1)
during 5 weeks of observation. From the standpoint of flight rate, three
subgroups are recognizable here which differ significantly from each other (P<.001). The first subgroup is of hornets whose primary activity is digging; the
second subgroup is of hornets whose flight rates are changeable, that is, of
workers with a variegated range of activities which entails the supply of food
and building materials with additional intranest duties, including digging; and
the third subgroup is of worker engaged in various activities both outside and
inside the nest but absolutely not in digging (Volynchik et al. [13]).The second subgroup of hornets (5y, 2y, 8y, and 5b)
incorporated digging activity on certain days with other duties that did not
require high frequency of exits from the nest like the introduction of food or
building materials into the nest, but rather included the feeding of the
larvae, construction and mending of the nest, and maintaining a steady nest
temperature. The frequency of exits per hour by these hornets ranged between
4.7±4.4 and 5.9±5.8 (n=446).The third
subgroup, the largest of the three (16 hornets), specialized in various
intranest duties but typically did not participate in digging activities
entailing brief exits from the nest. In this subgroup, the number of exits per
hour ranged between1.08±1.05 and 2.3±1.5
(n=770). Thus, the differences in exits from the nest by the three subgroups
proved statistically significant (F=239.04; P=1.08−14).Our
observations on the group that was heterogeneous insofar as age showed that
apart from the juvenile age polyethism (i.e., hornets up to 2-3 days of age
display activity only within the nest), there is also age-independent
polyethism characterized by the frequency of exits from the nest and the
primary duties performed.The two main
roles undertaken by the hornets under observation were digging activities and
the introduction of captured bees into the nest. Occasionally, however, the
digging hornets cease digging and turn to another duty. Regarding the loading
and unloading of soil, the entire process lasts only about a minute, and here
there were three such hornets in the observed group.As for the
introduction of captured bees into the nest, our observations showed that during
two weeks (in the end of August), the nest contained 18 labeled hornets, half
of which commenced bringing entire bees from the field. Follow-up of these
bee-collecting hornets for about 30 hours showed that three of them actually
brought in about 70% of the collected bees, or 23 out of 33. Again, it was the
same already-mentioned three hornets that were brought in the greatest catch of bees.The hornet
that brought an entire bee into the nest enabled the other hornets remaining
behind in the nest to break up the apine prey into small bits, which were ultimately
fed to the developing larvae. As already mentioned, there were three hornets that
were constantly engaged in capturing bees in the field, but apart from bees,
also other captured insects were introduced into the nest, such as the green
larval stages of butterflies or grasshoppers. The latter “catches,”
however, were much less frequent than the apine ones and interestingly, the
other insects were not brought in by the hornets that brought in the bees.


### 3.2. Stage 2

In Stage 2 of the study, undertaken in August-September of 2007
(the peak of the hornet activity), the observations were repeated, but now the
follow-up was carried out on four groups of homogeneous age, which eclosed in
another nest and were incorporated consecutively into the experimental colony.
These hornets were labeled a day post eclosion and were constantly observed
till termination of the experiment.With Group 1, follow-up was carried out on 13 worker hornets during 14
working days (between July 31 and September 2). Mean exits per hour of these
hornets were6.7±4.0 (n=1798). Yet the
exit frequency of two digger hornets (15r and 8r) markedly exceeded
this mean, attaining up to 30–40 exits per hour
with an overall mean of 16.09±7.3
and 13.5±7.7, respectively (Figure 4(a)). There were
also two extremely “passive” hornets (16r and 14r) who never
exited more than 5 times an hour with mean exits of 2.3±2.2
and 2.49±2.06, respectively. The majority (9 of 13)
hornets in Group 1 displayed a variable rhythm of activities in that following
marked flight activity, there were periods when most of the time they remained
inside the nest. The variance in the frequency of exits of the three subgroups
was statistically significant (F=52.04; P=5.17−6).Graphs showing the results of Stage 2
pertaining to four worker groups of homogeneous age (a)–(d) that were
labeled and introduced in the experimental colony about one day after eclosion.
The graphs show the overall mean flight rate for each of the four groups throughout
the observation period. Here, too, as in Figure3, each group is comprised of 3
subgroups in terms of the frequency of exits from the nest, with the difference
statistically being significant (P<.001)
(see Volynchik et al. [13]).
(a)(b)(c)(d)Figure 5
A graph depicting the daily activities of
individual hornets representing the aforementioned three subgroups. Thus,4y from the first subgroup is a digging hornet with a high rate of exits from the
nest throughout the observation period; 6y from the second subgroup is a
hornet showing changeable activity, that is, days with high frequency of
flights alternating with days of low flight frequency; and 1y from the
third subgroup is a hornet that rarely exits the nest and understandably does
not engage in digging activity (see Volynchik et al. [13]).With Group 2, 15 days of follow-up were carried out on 8 hornets between
August 5 and September 20. Mean of exits recorded per hour was5.4±3.9 (n=1088). A high degree digging activity was observed in one
hornet—4y (Figure 4(b)). Another four workers exited at a much lower frequency (1y, 3y, 5y, and 10y), while the remaining three (6y, 7y,
and 8y) were intermediary in terms of exit rate.Group 3 involved 15 hornets that were labeled on August 19 and were
observed during 10 days between August 22 and September 20. We recorded in this
group 1569 exits with mean hornet departures from the nest being6.6±5.5 times per hour. Already from the initial days of observation,
there was a clear dichotomy into three subgroups with respect to flight
activities and other functions performed. Thus, the first subgroup was composed
of two digger hornets (16w and 17w) whose rate of exits was 19.9±8.2 and 16.7±6.8
per hour, respectively (Figure 4(c)). These two digger hornets dug more or less
intensively throughout the observation period. The second subgroup was
comprised of 7 workers (1w, 2w, 5w, 7w, 10w, 14w, and 18w) whose main tasks were the aforedescribed, various,
intranest activities. The exit frequency in this subgroup was accordingly
rather low (about 2-3 exits per hour). The third subgroup was comprised 6
hornets that displayed behavioral flexibility in that they combined interior
duties with searches for food and building materials, and even participated in
the digging activities. The mean values of their flight frequencies are
intermediary between the other two subgroups with the differences between the
three subgroups statistically different (F=264.96; P=1.17−10).Group 4 (composed of 16 workers) was followed up during 16 working days
between September 9 and September 25. In the course of observations, the
hornets of this group exited a mean6.1±4.6
times per hour (n=2474). Again, as in the previous groups, we divided the group
into three subgroups whose differences in respect to flight frequency proved
statistically significant (F=610.16; P=1.4−13). The
first subgroup entailed three digger workers (15y, 16y, and 27y)
that exited the nest 13–15 times per hour
(Figure 4(d)). The second subgroup was comprised of 5 hornets (19y, 20y, 25y, 28y, and 30y) whose exits from the nest amounted to
about half those of the first subgroup. The remaining 8 hornets comprised the
third subgroup and these hornets performed on the average, not more than three flights
per hour.We need
mention here also another role played by hornets of subgroups 2 and 3 of the
four major groups, namely, the hunting activity, mainly the predation on bees.
In this regard, our observations showed that of the 13 hornets of group 1, only
four were seen introducing captured bees into the nest, and of these, the most
successful was23r, that brought in about two-thirds of the catch. Of
the 15 hornets of group 3, only 4 engaged in bee hunting. Of the 16 hornets of
group 4, only two (17y and 24y) were seen introducing bees into
the nest. From all of the
above, we gather that bee hunting is a role in itself in the nest of V.
orientalis, taken up by only a few hornets and not in equal measures.


## 3.1. Stage 1

During Stage 1
entailed, a follow-up on 23 tagged hornets that age-wise comprised a
heterogeneous group. Our observations showed that already at the onset of
extranest activities, there was disparity in the frequency of exits, with some
hornets exiting at very high frequency for brief (up to one minute) flights.
Such brief flights, usually associated with exits of digging hornets, occupied
51% of the total exit flights throughout the observation period. As for digger
hornets, their digging activity may repeat 50–60 times without
stop, but thereafter they rest for 10–15 minutes before
resuming the same digging activity. During the second week of observation, we
noted two digger hornets (3w and 4y) that were very active and
performed more than 60% of the total exits in the tagged group. Of the two,
hornet 3w was seen to exit to the field every 1.9±0.4 minutes (n=259), whereas
hornet 4y exited every 2.7±0.6 minutes (n=160). During the same period,
the remaining hornets in the group exited the nest once every 21.7±14.1 minutes
(n=262). In fact, throughout the observation period, there were 3 hornets in
the first subgroup that were persistent and very active in their digging
activity, namely, 3w, 4y, and 7y (Figure 3). Their
mean numbers of
exits per hour during the observation period were, respectively, 13.7±7.2 (n=452), 10.2±5.04 (n=411),
and 10.5±4.8 (n=438). We need to point out again that this subgroup of hornets engaged
primarily in digging activity, namely, the removal of grains of sand from the
ABB, and that they persisted in this activity for 5 whole weeks—from the end of
July to the end of August. All the remaining hornets exited the nest at a much
lower frequency, and engaged in a wide range of activities, including the
introduction of building materials or food into the nest, but mostly interior
nest duties, like tending to the developing brood.Figure 3
A graph showing the mean frequency of exits
from the nest per hour of 23 labeled worker hornets of different ages (Stage 1)
during 5 weeks of observation. From the standpoint of flight rate, three
subgroups are recognizable here which differ significantly from each other (P<.001). The first subgroup is of hornets whose primary activity is digging; the
second subgroup is of hornets whose flight rates are changeable, that is, of
workers with a variegated range of activities which entails the supply of food
and building materials with additional intranest duties, including digging; and
the third subgroup is of worker engaged in various activities both outside and
inside the nest but absolutely not in digging (Volynchik et al. [13]).The second subgroup of hornets (5y, 2y, 8y, and 5b)
incorporated digging activity on certain days with other duties that did not
require high frequency of exits from the nest like the introduction of food or
building materials into the nest, but rather included the feeding of the
larvae, construction and mending of the nest, and maintaining a steady nest
temperature. The frequency of exits per hour by these hornets ranged between
4.7±4.4 and 5.9±5.8 (n=446).The third
subgroup, the largest of the three (16 hornets), specialized in various
intranest duties but typically did not participate in digging activities
entailing brief exits from the nest. In this subgroup, the number of exits per
hour ranged between1.08±1.05 and 2.3±1.5
(n=770). Thus, the differences in exits from the nest by the three subgroups
proved statistically significant (F=239.04; P=1.08−14).Our
observations on the group that was heterogeneous insofar as age showed that
apart from the juvenile age polyethism (i.e., hornets up to 2-3 days of age
display activity only within the nest), there is also age-independent
polyethism characterized by the frequency of exits from the nest and the
primary duties performed.The two main
roles undertaken by the hornets under observation were digging activities and
the introduction of captured bees into the nest. Occasionally, however, the
digging hornets cease digging and turn to another duty. Regarding the loading
and unloading of soil, the entire process lasts only about a minute, and here
there were three such hornets in the observed group.As for the
introduction of captured bees into the nest, our observations showed that during
two weeks (in the end of August), the nest contained 18 labeled hornets, half
of which commenced bringing entire bees from the field. Follow-up of these
bee-collecting hornets for about 30 hours showed that three of them actually
brought in about 70% of the collected bees, or 23 out of 33. Again, it was the
same already-mentioned three hornets that were brought in the greatest catch of bees.The hornet
that brought an entire bee into the nest enabled the other hornets remaining
behind in the nest to break up the apine prey into small bits, which were ultimately
fed to the developing larvae. As already mentioned, there were three hornets that
were constantly engaged in capturing bees in the field, but apart from bees,
also other captured insects were introduced into the nest, such as the green
larval stages of butterflies or grasshoppers. The latter “catches,”
however, were much less frequent than the apine ones and interestingly, the
other insects were not brought in by the hornets that brought in the bees.


## 3.2. Stage 2

In Stage 2 of the study, undertaken in August-September of 2007
(the peak of the hornet activity), the observations were repeated, but now the
follow-up was carried out on four groups of homogeneous age, which eclosed in
another nest and were incorporated consecutively into the experimental colony.
These hornets were labeled a day post eclosion and were constantly observed
till termination of the experiment.With Group 1, follow-up was carried out on 13 worker hornets during 14
working days (between July 31 and September 2). Mean exits per hour of these
hornets were6.7±4.0 (n=1798). Yet the
exit frequency of two digger hornets (15r and 8r) markedly exceeded
this mean, attaining up to 30–40 exits per hour
with an overall mean of 16.09±7.3
and 13.5±7.7, respectively (Figure 4(a)). There were
also two extremely “passive” hornets (16r and 14r) who never
exited more than 5 times an hour with mean exits of 2.3±2.2
and 2.49±2.06, respectively. The majority (9 of 13)
hornets in Group 1 displayed a variable rhythm of activities in that following
marked flight activity, there were periods when most of the time they remained
inside the nest. The variance in the frequency of exits of the three subgroups
was statistically significant (F=52.04; P=5.17−6).Graphs showing the results of Stage 2
pertaining to four worker groups of homogeneous age (a)–(d) that were
labeled and introduced in the experimental colony about one day after eclosion.
The graphs show the overall mean flight rate for each of the four groups throughout
the observation period. Here, too, as in Figure3, each group is comprised of 3
subgroups in terms of the frequency of exits from the nest, with the difference
statistically being significant (P<.001)
(see Volynchik et al. [13]).
(a)(b)(c)(d)Figure 5
A graph depicting the daily activities of
individual hornets representing the aforementioned three subgroups. Thus,4y from the first subgroup is a digging hornet with a high rate of exits from the
nest throughout the observation period; 6y from the second subgroup is a
hornet showing changeable activity, that is, days with high frequency of
flights alternating with days of low flight frequency; and 1y from the
third subgroup is a hornet that rarely exits the nest and understandably does
not engage in digging activity (see Volynchik et al. [13]).With Group 2, 15 days of follow-up were carried out on 8 hornets between
August 5 and September 20. Mean of exits recorded per hour was5.4±3.9 (n=1088). A high degree digging activity was observed in one
hornet—4y (Figure 4(b)). Another four workers exited at a much lower frequency (1y, 3y, 5y, and 10y), while the remaining three (6y, 7y,
and 8y) were intermediary in terms of exit rate.Group 3 involved 15 hornets that were labeled on August 19 and were
observed during 10 days between August 22 and September 20. We recorded in this
group 1569 exits with mean hornet departures from the nest being6.6±5.5 times per hour. Already from the initial days of observation,
there was a clear dichotomy into three subgroups with respect to flight
activities and other functions performed. Thus, the first subgroup was composed
of two digger hornets (16w and 17w) whose rate of exits was 19.9±8.2 and 16.7±6.8
per hour, respectively (Figure 4(c)). These two digger hornets dug more or less
intensively throughout the observation period. The second subgroup was
comprised of 7 workers (1w, 2w, 5w, 7w, 10w, 14w, and 18w) whose main tasks were the aforedescribed, various,
intranest activities. The exit frequency in this subgroup was accordingly
rather low (about 2-3 exits per hour). The third subgroup was comprised 6
hornets that displayed behavioral flexibility in that they combined interior
duties with searches for food and building materials, and even participated in
the digging activities. The mean values of their flight frequencies are
intermediary between the other two subgroups with the differences between the
three subgroups statistically different (F=264.96; P=1.17−10).Group 4 (composed of 16 workers) was followed up during 16 working days
between September 9 and September 25. In the course of observations, the
hornets of this group exited a mean6.1±4.6
times per hour (n=2474). Again, as in the previous groups, we divided the group
into three subgroups whose differences in respect to flight frequency proved
statistically significant (F=610.16; P=1.4−13). The
first subgroup entailed three digger workers (15y, 16y, and 27y)
that exited the nest 13–15 times per hour
(Figure 4(d)). The second subgroup was comprised of 5 hornets (19y, 20y, 25y, 28y, and 30y) whose exits from the nest amounted to
about half those of the first subgroup. The remaining 8 hornets comprised the
third subgroup and these hornets performed on the average, not more than three flights
per hour.We need
mention here also another role played by hornets of subgroups 2 and 3 of the
four major groups, namely, the hunting activity, mainly the predation on bees.
In this regard, our observations showed that of the 13 hornets of group 1, only
four were seen introducing captured bees into the nest, and of these, the most
successful was23r, that brought in about two-thirds of the catch. Of
the 15 hornets of group 3, only 4 engaged in bee hunting. Of the 16 hornets of
group 4, only two (17y and 24y) were seen introducing bees into
the nest. From all of the
above, we gather that bee hunting is a role in itself in the nest of V.
orientalis, taken up by only a few hornets and not in equal measures.


## 4. Discussion

The appearance of polyethism in its various forms is typical for all
genera of social insects but particularly for Hymenoptera and Isoptera,
including the social wasps. Actually, there are various types of the phenomena,
such as age- (temporal), caste-, sexual-, and size-dependent polyethism [4, 5, 10–12].The Oriental hornet (V. orientalis)
is a digging hornet and a significant part of its activity entails brief
flights out of the nest (digging exits) which are carried out practically
throughout the active season. Our observations on natural nests in the field
have revealed that from August to the beginning of September, the traffic at
the nest entrance in large colonies can amount to more than 500 exits per 5
minutes [13], and most of these exit flights are associated with activities to
enlarge the nest. Our findings during Stages 1 and 2 of our study show that
only a minority of the adult workers engage constantly in digging activity, whereas the other hornets
leave the nest more infrequently and for longer periods of time, bringing back
food and building materials, and mostly performing various tasks inside the
nest. Accordingly, the adult hornet population can be divided into three groups
by the frequency of their exits from the nest (i.e., the roles which they
perform). Moreover, the differences between the three groups are statistically
significant, and the observed polyethism is not age-dependent. Age polyethism appearing in V.
orientalis is during the first two days after eclosion, when the workers
remain in the nest and perform various intranest functions like
thermoregulation and feeding of the brood, and only on the third day, they do some of the new
hornets commence exiting the nest and participating in activities like digging,
cleaning the nest, and bringing back food and building materials. This phase of
the phenomenon could appropriately be called juvenile age polyethism.It would seem that activities of the hornets
also dependent on the size of the population. Thus, at the peak of the active
season, when the population is largest and cell-building is maximal, about 40%
of the workers engage in intensive digging activities. However, when unlabeled
hornets were randomly removed from the nest, the labeled hornets that were
engaged in digging ceased this activity for several days and switched to other
duties, like feeding the brood and bringing in food, until the number of
workers increased sufficiently to enable the workers to return to their previous
role. Apparently, the enlargement of the nest by digging is under certain
conditions a secondary activity, whereas tending to the brood and to the nest
proper is preferable
activities. A similar occurrence of polyethism and work assignment dependent on
size of the population has been reported in various other species of wasps [14–19].
As for the appearance of age polyethism entailing initial work within the nest
and subsequent activity also outside the nest, this has been widely observed in
other species of social insects. Elsewhere, various species of termites are
characterized by age, caste, and sexual polyethism [20–23], while ants display age and caste polyethism [24–27]. It is also
known that honey bees (Apis mellifera) show a tendency to perform
different types of activity at different ages [28–32]. Investigation
of age-dependent division of labor in five species of Vespa (simillima, analis, crabro, mandarinia, and tropica)
[33] has shown no relationship between age and labor.To recap, we have shown in the present study that at the peak of the active season
in all four of our experimental groups, there was already during the initial
days of flight activity a clear division of the hornets into three subgroups
depending on the rate of flight exits. This division was statistically
significant and not dependent on age.Thus, in the first subgroup, entailing 52
labeled hornets in all four groups, 8 hornets (about 15%) engaged in digging
activities, with several scores of exits per hour for brief flights of up to
one minute. This percentage of digging hornets was fairly constant in all four
groups, and these hornets displayed digging activity only, with brief rest
periods, during two thirds of the observation time, while in the remaining time,
they engaged in digging coupled with prolonged flight periods of 5–30 minutes.In the second subgroup, 23 of 52 hornets
(about 45%) engaged in enlarging the nest by digging (as in the first
subgroup), and this is
during 15–20% of the
observation time, whereas in the remaining time (on other days), they displayed
a variegated spectrum of activities combining the bringing of food and building
materials with intranest activities like thermoregulation, guard duty, and nursing
of the comb brood. Here, the digging-related flight exits comprised no more
than a quarter of the total types of exit flights.In the third subgroup, comprising 21 of 52
hornets or about 40% of the labeled population, the hornets also engaged in the
various aforementioned activities but with the principal difference that in
this subgroup, none of the hornets engaged in digging activities.When reviewing all three subgroups, we note
that the second one displays the greatest behavioral flexibility despite a
certain correspondence between it and the first and third subgroups insofar as
the range of roles performed. Figure5 depicts the daily activity of individual
hornets representing the three subgroups. Thus, 4y (of subgroup 1) is a
digging hornet with a high rate of exits from the nest, 6y (of subgroup
2) is a hornet displaying variable behavior in that a spell of days with high
exit rates from the nest is replaced by a run of days with exit rates, and 1y (of subgroup 3) is a hornet that scarcely left the nest and did not engage in
digging.The present findings confirm that the
proportional percentage of each of the subgroups within the described hornet
colony can change in the course of the active season, being dependent both on
the stage of development of the colony as well as on the requirements or needs
of the nest. For instance, there is indeed correlation between size of the
hornet population and the number of hornets that engage in digging activity. We
have observed that on certain days in the peak of the season, when there was an
excess of workers, the proportion of labeled hornets that actively engaged in
digging attained up to 40%. Concurrently, our attempt to reduce colony size to
a third of its size resulted in a marked reduction in digging activities. At
the end of the season (October), when there is no further need for enlarging
the nest, the percentage of digging hornets becomes accordingly low.Our observations clearly show an assignment
of specific roles to various members of the hornet colony and this, despite an absence
of any specific morphological characteristics in association with any specific
role. Insofar as their work is
within the nest, we failed to notice any of the hornets performing a particular
duty at a particular time, but rather their tasks were changeable with time. Also,
we did not find any evidence that role assignment in the adult hornet
population (i.e., more then 3 days after eclosion) was age-dependent.



---

*Source: 243436-2009-03-03.xml*