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Title: Coldinduced expression of diapause in Praon volucre: fitness cost and morphophysiological characterization

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Physiological Entomology (2010) 35, 301–307

DOI: 10.1111/j.1365-3032.2010.00743.x

Cold-induced expression of diapause in Praon
volucre: fitness cost and morpho-physiological
characterization
H E R V E´ C O L I N E T , F R E´ D E R I C M U R A T O R I and T H I E R R Y H A N C E
Earth and Life Institute, Centre de Recherche sur la Biodiversit´e, Universit´e catholique de Louvain, Louvain-la-Neuve, Belgium

Abstract. Cold exposure (2 ◦ C for 7 days) in constant darkness at mummy stage
induces diapause expression in 9% of the Praon volucre Haliday population.
Diapausing parasitoids show a significant delay in emergence time compared with
nondiapausing counterparts. A diapause-mediated polyphenism is observed in mummy
colour, with diapausing mummies being clearly darker than nondiapausing ones. The
diapause status of dark mummies is confirmed by a significant reduction in metabolic
rate. Diapausing parasitoids also display specific morphological characteristics: they
are heavier (fresh and dry mass) and accumulate larger fat reserves than nondiapausing
counterparts. The diapause status is associated with a fitness cost in terms of adult
longevity. There is no evidence of diapause-related change in supercooling ability.
Key words. Diapause, fitness, mass, metabolic rate, mummy colour, parasitoid,
supercooling point.

Introduction
Insects display extensive variations in the way that they
respond to environmental factors. Such plasticity allows
individuals to adapt to variable habitats and to survive
unpredictable environmental circumstances. For example, a
wide variation in development time can produce either
continuous or discrete batches of emerging insects (Danks,
2006). Examples of discrete emergence include polymodal
(Finch & Collier, 1983; Hagen & Lederhouse, 1985) and
prolonged diapause (Tauber et al., 1986; Danks, 2002). In
many temperate species, occasional individuals that vary
greatly from the mean in terms of development time,
size, diapause incidence/duration or stress resistance can
be found as a result of adaptation to unexpected risk
(Danks, 1983). Developmental delays can serve different
functions, such as energy saving, protection against harsh
conditions, synchronizing lifecycle with food resource or
optimizing reproduction time (Tauber et al., 1986; Danks,
1987, 2002). In a previous study, five Aphidiine parasitoid
species were exposed to low temperature aiming to extend
their shelf-life for mass rearing purposes. In Praon volucre
Correspondence: H. Colinet, Earth and Life Institute, Centre de
Recherche sur la Biodiversit´e, Universit´e catholique de Louvain, Croix
du Sud 4-5, B-1348 Louvain-la-Neuve, Belgium. Tel.: +32 10 47 34
91; e-mail: herve.colinet@uclouvain.be
© 2010 The Authors
Physiological Entomology © 2010 The Royal Entomological Society

Haliday (Hymenoptera: Aphidiinae), a small proportion of the
population displays an unusual developmental pattern, with
some adults emerging a long time after the peak of emergence
(Colinet & Hance, 2010).
Under natural cold conditions, most Aphidiine arrest their
development and overwinter in quiescence or diapause inside
the cuticle of a dead host (termed a mummy) (Hance
et al., 2007). Some studies use mummy colour to distinguish
nondiapausing mummies from diapausing ones, with the latter
tending to be darker (Brodeur & McNeil, 1989a; Krespi
et al., 1994; Polg´ar et al., 1995). Unfortunately, none of these
studies provide developmental data clearly associating the dark
phenotype with a development arrest. Contrasting conclusions
are raised by Legrand et al. (2004a, b) who are unable to
associate the dark colour of Aphidius rhopalosiphi mummies
with diapause. Whether or not colour is a reliable phenotypic
marker for diapause in other Aphidiinae clearly requires further
investigation.
Phenological synchrony between parasitoids and hosts is
essential for parasitoid population growth (Hance et al., 2007).
In temperate regions where host availability follows seasonal
fluctuations of abiotic conditions, diapause usually serves
to synchronize parasitoids with host availability (Tauber
et al., 1986; Polg´ar & Hardie, 2000). Besides the role of
synchronization, diapause also facilitates survival over harsh
periods by affording higher tolerance to extreme environmental
conditions (Tauber et al., 1986); for example, by increasing
301

302 H. Colinet et al.
cold-hardiness (Langer & Hance, 2000). Depending on the
species, cold-hardiness and diapause may or may not be linked
to each other and this question is the subject of much debate
(Denlinger, 1991; Hodkova & Hodek, 2004).
Although diapause is probably essential for maintaining
parasitoid populations under natural conditions, it may also
be costly in terms of fitness. In the parasitoid Asobara
tabida, there is negative relationship between the time spent
in diapause and egg load (Ellers & Van Alphen, 2002).
In parasitoids, the stock of energy reserves is limited
and determined by resources provided by host and is
therefore critically important for diapausing individuals. Even
if diapausing insects typically enter a state of metabolic
depression (Hahn & Denlinger, 2007), a substantial part of
their energy reserves (i.e. lipids) is consumed during prolonged
diapause (Ellers & Van Alphen, 2002). Because longevity
correlates with fat reserves in parasitoids (Ellers, 1996; Colinet
et al., 2007a), it may be affected after diapause. To compensate
for the energy cost of diapause, insects often accumulate energy
reserves prior to diapause (Tauber et al., 1986; Danks, 1987).
However, not all species show a diapause-associated increase
in energy reserves or body size and, so far, studies related to
this question are available for only a few species (Hahn &
Denlinger, 2007).
In the present study, the aphid parasitoid P. volucre
is studied using previously applied conditions (Colinet &
Hance, 2010) to reveal the atypical late-emerging individuals.
These individuals are suspected to be in diapause and
are characterized in comparison with typical ones. Because
metabolic depression is a highly conserved characteristic of
diapausing insects (Hahn & Denlinger, 2007), respirometry is
used to determine whether these late-emerging individuals are
in diapause. Whether or not the phenotype ‘dark mummy’ can
be associated with the diapausing status is studied and the
development times (i.e. to emergence) are measured to validate
diapause status. Supercooling ability, comprising an index
of cold-hardiness in some insects, is analyzed in mummies.
Mass and lipid reserves are measured to check whether the
late individuals accumulate more fat reserves. Finally, adult
longevity is determined to check whether it is affected after
diapause.

Materials and methods
Insect rearing conditions
The green peach aphid Myzus persicae Sulzer (Hemiptera:
Aphidinae) was used as a host for parasitoid rearing and
laboratory cultures. Individuals were collected in agricultural
fields around Louvain-la-Neuve (Belgium) in 2000. Aphids
were reared in cages (0.3 m3 ) on sweet pepper (Capsicum
annuum L.) under long-day conditions (LD 16 : 8 h) at 20 ◦ C
and 60% relative humidity. The parasitoid P. volucre was
collected in fields at Fleurus (Belgium) in June 2008 and
was reared in the laboratory under the same conditions.
Experiments were conducted during November to February
2008–2009.

Thermal conditions
Three-day-old mummies were directly transferred from
20 ◦ C to constant 2 ◦ C in thermo-regulated cooled incubators
(Model 305; LMS Ltd, U.K.) with saturated relative humidity
and continuous darkness (Colinet & Hance, 2010). Mummies
were cold-exposed for 7 days and then brought back to 20 ◦ C
to complete development to adult emergence. Because the
mummy to adult development threshold is 5.5 ◦ C in P. volucre
(Sigsgaard, 2000), individuals do not develop at a constant
2 ◦ C, and 7 days at 2 ◦ C does not induce mortality (Colinet
& Hance, 2010). Previous observations have shown that lateemerging individuals occurred in a small proportion (Colinet
& Hance, 2010); therefore, a large initial stock of individuals
(2000) was used to ensure that enough replicates would be
available for testing. After 7 days of constant cold-exposure,
a small proportion of mummies (9%) appeared darker than
the others. A control group of mummies not exposed to low
temperature completed development at 20 ◦ C and were all light
in colour. All measurements were compared between the three
mummy categories: untreated control (CO), cold-exposed pale
and dark mummy (PM and DM, respectively).
Emergence time and adult longevity
To determine mummy to adult emergence times, individuals
from each category were isolated in centrifuge tubes (1.5 mL;
Eppendorf, Germany) and maintained in an incubator at
20 ◦ C, under constant darkness. These were checked daily
for adult emergence (n = 133, 72 and 89 for CO, PM
and DM, respectively). Time spent at 2 ◦ C was subtracted
from emergence time in PM and DM mummies because no
development occurs at 2 ◦ C (Colinet & Hance, 2010). At
adult emergence, only water was provided on damp cotton
and survival was monitored daily to determine longevity
(n = 129, 70 and 32 for CO, PM and DM, respectively).
One hundred and fifteen days after mummification, mummies
that had not emerged were dissected under a microscope
to determine whether they contained a dead or living
individual. Emergence times were compared between mummy
categories using survival log-rank analysis with a censoring
factor for non-emerged living mummies (R software for
Statistical Computing, version 2.9.0, package survival, version
2.35; http://www.r-project.org). Because adult longevity was
not normally distributed, comparison across treatments was
performed using a Kruskal–Wallis test followed by Dunn’s
multiple comparison tests using prism, version 5.01 (GraphPad
Software, Inc., San Diego, California).
Mass parameters
Mummies from each category were weighed individually
(Me22, Mettler Toledo, Inc., Columbus, Ohio; ±1 μg) (n =
20 for each category). Fresh mass was measured then
mummies were dried at 60 ◦ C for 3 days in an oven and
then reweighed to measure dry mass. Water mass was
determined as the difference between fresh and dry mass. The

© 2010 The Authors
Physiological Entomology © 2010 The Royal Entomological Society, Physiological Entomology, 35, 301–307

Cold-induced diapause in P. volucre 303
lipid mass was measured after an extraction in chloroform
methanol (2 : 1) as described previously (Colinet et al., 2006).
The data were checked for normal distribution using the
Shapiro–Wilk statistic (α = 0.05). Mass parameters were
compared by one-way analysis of variance (anova) followed
by Student–Newman–Keuls comparisons using prism, version
5.01 (GraphPad Software, Inc.).

The CO2 production values were compared by one-way anova
followed by a Student–Newman–Keuls comparisons test using
prism, version 5.01 (GraphPad software, Inc.).

Results
Emergence time and colour

Supercooling abilitiy
Measurements of supercooling point (i.e. temperature of
crystallization) was performed as described previously (Colinet
et al., 2007b). A mummy was placed inside a 1-mL Eppendorf
tube, fixed to the tip thermocouple (K-type) connected to
a high resolution thermometer (model HI 93531R; HANNA
instruments, Belgium) (± 0.1 ◦ C). A refrigerated Cryostat
circulator (Model FP50-ME; Julabo Labortechnik GmbH,
Germany) was programmed to cool at a rate of 1 ◦ C min−1 .
The supercooling point was taken as the start of the exotherm
produced by the latent heat of freezing (n = 20 for each
category). Normality of dataset was checked using the
Shapiro–Wilk statistic (α = 0.05). Mean supercooling point
values were compared between mummy categories by one-way
anova using prism, version 5.01 (GraphPad software, Inc.).

Metabolic rate
A sensitive flow-through respirometry system (Sable Systems Europe GmbH, Germany) was used to measure CO2
production. Ambient air was pumped and cleaned through a
Drierite/Ascarite column (refs 456071 and 223921; SigmaAldrich, Belgium) at 100 mL min−1 controlled by separate
calibrated flow meters (Intelligent Subsampler TR-SS3 and
Intelligent mass flow control unit MFC-2; Sable Systems
Europe GmbH). The air was directed to a respiration chamber with samples. A group of three P. volucre mummies was
enclosed between two polytetrafluoroethylene tubes to create
a respiration chamber (approximate volume = 0.5 mL), which
was connected to a CO2 analyser (CA-10 carbon dioxide analyzer; Sable Systems Europe GmbH). Mummies were weighed
before tests to ensure mass uniformity between samples. Before
measurements, CO2 in the respiration chamber was purged
using a CO2 -free air flow until no CO2 was detected on the
analyzer. The respiration chamber was then isolated using a
flow switcher (BL-1 baselining unit; Sable Systems Europe
GmbH) for 45 min. At the same time, the CO2 -free air flow
(100 mL min−1 ) was analyzed and used as a reference. After
45 min, the respiration chamber was purged a second time to
analyze the volume of CO2 produced by the three mummies.
Baseline drift of the analyser during recording was corrected
from mean measurements during the 45 min of CO2 freeair flow (expedata, version 1.1.14; Sable Systems Europe
GmbH). The CO2 recordings were converted to volume of CO2
production per hour. Fifteen replicates were tested for each
mummy category (n = 15 × 3 mummies). Normality of the
data was checked using the Shapiro–Wilk statistic (α = 0.05).

Control mummies (CO) were all light coloured. Exposure
to low temperature and darkness induced the expression of
two different phenotypes in mummy colour. Dark mummies
(DM) were easily distinguished (Fig. 1) and only 9% of
individuals (180 of 2000) expressed the dark phenotype.
The difference in mummy colour was associated with a
major difference in emergence time, suggesting that most
individuals from DM group were in diapause (Fig. 2).
Five dark mummies (i.e. 6% of the 89 observed) emerged
within 14–20 days. Twenty-six individuals from DM group
(i.e. 29%) emerged with a substantial delay (45–75 days
post-mummification) compared with individuals from PM
and CO groups, which emerged within 10–14 days postmummification. The remaining individuals from DM group
(65%) that did not emerge after 115 days post-mummification
were dissected and were still alive. Individuals from DM group
emerged significantly later than individuals from the PM
group (log-rank test: χ2 = 188.3, P < 0.001) and from the CO
group (log-rank test: χ2 = 228.4, P < 0.001). No difference in
the emergence pattern was found between PM and CO groups
(log-rank test: χ2 = 0.001, P = 0.975).

Adult longevity
After emergence, adult lifespan differed significantly
between mummies categories (Kruskal–Wallis test: H = 9.52,
P = 0.008). Longevity was significantly shorter for individuals that emerged from DM group than those that emerged from
PM or CO groups (Dunn’s comparison test: P < 0.05; Fig. 3).

Fig. 1. Illustration of the diapause-mediated polyphenism in mummy
colour. Cold-exposed nondiapausing pale (PM) versus diapausing dark
(DM) mummies.

© 2010 The Authors
Physiological Entomology © 2010 The Royal Entomological Society, Physiological Entomology, 35, 301–307

304 H. Colinet et al.
Mass parameters
Mummy fresh, dry and lipid mass differed significantly
between mummy categories, whereas water content was not
different (Table 1). The difference resulted from significant
increases in mass parameters in DM category. Compared with
CO individuals, mummies from the DM group showed a
16%, 29% and 26% increase in fresh, dry and lipid mass,
respectively. For all these mass parameters, individuals from
PM group were not significantly different from the CO group
(Table 1).

Supercooling point
Fig. 2. Proportion of non-emerged individuals in relation to the time
spent after mummification; n = 133, 72 and 89 for untreated control
(CO), pale (PM) and dark (DM) mummies, respectively.

There was no significant difference in the supercooling point
of the three mummy categories (anova: F = 0.10, P = 0.052;
Fig. 4).

Metabolic rate
The rate of CO2 production differed significantly between
mummy categories (anova: F = 7.46, P = 0.002; Fig. 5).
The difference resulted from individuals from the DM group,
which produced significantly less CO2 than individuals from
the PM and CO groups (Student–Newman–Keuls test: P <
0.05). Compared with CO individuals, individuals from the
DM group displayed a 27% reduction in metabolic rate.

Fig. 3. Comparison of adult longevity between untreated control
(CO), pale (PM) and dark (DM) mummies. Bars represents the mean
± SE (n = 129, 70 and 32 for CO, PM, DM, respectively). Symbol
(*) indicates P < 0.05.

This difference was magnified by the finding that, in the
DM category, some individuals displayed a lifespan as short
as 1 day. No significant difference was observed in the
longevity of adults that emerged from PM or CO groups
(Dunn’s comparison test: P > 0.05). There was a negative
relationship between longevity of treated mummies and the
time to emergence (linear regression: F = 10.81, r 2 = 0.31,
P = 0.003 for the DM group and F = 21.85, r 2 = 0.24,
P < 0.001 for the PM group).

Fig. 4. Comparison of supercooling point between untreated control
(CO), pale (PM) and dark (DM) mummies. Bars represents the mean
± SE (n = 20).

Table 1. Comparison of fresh, dry, water and lipid mass between untreated control (CO), pale (PM) and dark (DM) mummies.
Mass parameter

CO

PM

Fresh mass (μg)
Dry mass (μg)
Water mass (μg)
Lipid mass (μg)

384.84
161.50
230.72
63.72

±
±
±
±

12.89a
5.74a
8.0a
2.59a

404.85
165.45
247.89
63.63

DM
±
±
±
±

15.11a
5.08a
8.03a
2.22a

444.75
208.95
235.80
80.10

F
±
±
±
±

11.94b
6.78b
6.05a
2.89b

5.092
19.85
1.332
13.09

P
0.009
< 0.001
0.272
< 0.001

Values are the mean ± SE (n = 20). Different superscript letters in the same row indicate a significant difference as a result of a Student–Newman–Keuls
test post analysis of variance (α = 0.05).

© 2010 The Authors
Physiological Entomology © 2010 The Royal Entomological Society, Physiological Entomology, 35, 301–307

Cold-induced diapause in P. volucre 305

Fig. 5. Comparison of CO2 production between untreated control
(CO), pale (PM) and dark (DM) mummies. Bars represents the mean
± SE (n = 15 × 3). Symbol (**) indicates P < 0.01.

The CO2 levels from the CO and PM groups were similar
(Student–Newman–Keuls test: P > 0.05).

Discussion
Exposing 3-day-old P. volucre mummies to a combination of
cold temperature and constant darkness for 1 week induces
diapause in a small fraction of the population. Similarly,
diapause is induced in a fraction of the Trichogramma
cordubensis population exposed to different cold storage
temperatures and short photoperiod (Garcia et al., 2002). It
is not known whether both cold temperature and darkness
are required for diapause induction in P. volucre; however,
both factors are likely to be important. Photoperiod and/or
temperature are known to be critically important for diapause
induction in other insects (Tauber et al., 1986; Danks, 1987;
Kostal, 2006). In temperate zones, aphid parasitoids typically
enter diapause when photoperiod and temperatures decrease in
the autumn (Brodeur & McNeil, 1989b).
Mummy colour can be used to distinguish diapausing from
nondiapausing mummies in Aphidiinae (Brodeur & McNeil,
1989a; Krespi et al., 1994; Polg´ar et al., 1995); however,
no developmental data are available to provide support for
this statement. The use of mummy colour is reported to be
an unreliable marker for identifying diapause in Aphidiinae
(Legrand et al., 2004a, b). In the case of P. volucre, it
appears that the dark phenotype is associated with a significant
developmental delay/arrest. Moreover, dark mummies show
a significant reduction in metabolic rate, which is typical of
diapausing insects (Tauber et al., 1986; Hahn & Denlinger,
2007). There is a large variability in the degree of metabolic
depression among insect species, although it remains a useful
marker for identifying/confirming diapause status (Hahn &
Denlinger, 2007).
A few individuals (6%) from the dark mummy group do not
show a marked developmental delay, possibly as a result of
incomplete diapause induction. A large variability in diapause
duration is observed among diapausing dark mummies.
A fraction of the dark mummies (29%) emerges at 20 ◦ C with
a substantial delay (44–75 days), whereas the majority (65%)

remain in diapause for at least 115 days. In Aphidius ervi, there
is also a large variability in diapause duration (multipeak),
and this phenomenon remains unexplained (ChristiansenWeniger & Hardie, 1997). The termination of diapause is
a complex process. In some species, it may depend on
stimuli received from outside the insect, whereas, in others,
this is not the case (Tauber & Tauber, 1976). During the
course of diapause, there is generally a decrease in diapause
intensity, as well as continual alterations in the responses
to environmental stimuli. These changes can occur even
if the insect is held under constant conditions, inducing
spontaneous diapause termination (Kostal, 2006; Eizaguirre
et al., 2008). The understanding of how diapause ends is
still incomplete (Kostal, 2006). Indeed, for most species in
which the conditions influencing diapause maintenance are
studied in natural populations, no specific stimulus is found to
actively end diapause (Tauber & Tauber, 1976; Tauber et al.,
1986). It is suggested that there is a required day number
for diapause completion (dependent on heat accumulation)
(Eizaguirre et al., 2008). Variability in required day number
between individuals from the DM group may explain why
some individuals complete diapause before others, although
this point requires further investigation.
Because no feeding occurs during diapause, individuals rely
on accumulated energy reserves to survive for long periods
(Danks, 1987). In addition to the energy cost of diapause,
there is also a high energy demand of post-diapause development (e.g. metamorphosis and emergence). Lipids are the
dominant reserves used during insect diapause (Hahn & Denlinger, 2007). In parasitoids, fats are positively correlated with
mass/size (Ellers et al., 1998; Rivero & West, 2002; Colinet
et al., 2007a, b). Therefore, heavier mummies with larger lipid
stores are more likely to survive the diapause and post-diapause
energy demands. In the present study, diapausing mummies
are heavier and contain more lipid than their nondiapausing
counterparts. Morphological differences between diapausing
and nondiapausing phenotypes are observed in insects; however, not all species show a diapause-associated increase in
energy reserves or body size (Tauber et al., 1986; Danks,
1987; Hahn & Denlinger, 2007). In P. volucre, diapausing dark
mummies are 29% heavier (dry mass) and contain 26% more
lipid than nondiapausing mummies. The larger nutrient store
of diapausing mummies may be viewed as a consequence of
diapause or a condition to enter diapause. In the first case,
diapause-destined individuals would accumulate more nutrient
reserves during larval development than nondiapause-destined
individuals. For example, diapausing individuals accumulate
almost two-fold more lipid than nondiapausing individuals in
Sarcophaga crassipalpis (Adedokun & Denlinger, 1984) or
Culex pipiens (Mitchell & Briegel, 1989). In the second case,
diapause-destined individuals would enter diapause only if a
critical body mass or lipid reserve is reached. Under diapauseinducing conditions, undersized larvae of the blow fly Calliphora vicina fail to enter diapause and continue their development to adult (Saunders, 1997). Similarly, there is a threshold
body weight below which Psacothea hilaris larvae are incapable of entering diapause (Munyiri et al., 2004). Aphidiinae
parasitoids diapause as nonfeeding pre-pupae; therefore, the

© 2010 The Authors
Physiological Entomology © 2010 The Royal Entomological Society, Physiological Entomology, 35, 301–307

306 H. Colinet et al.
accumulation of mass and fats before diapause is probably
critically important. Whether these phenotypical modifications
are a consequence of diapause or a condition to enter diapause
requires further investigation.
Diapausing P. volucre have a reduced longevity and a
negative relationship exists between the time to emergence and
longevity in diapausing wasps. This corroborates the finding
that diapause is an energy-consuming process associated with
significant fitness costs in a parasitic wasps (Ellers & Van
Alphen, 2002). Because post-diapause feeding may conceal
the energy-dependent fitness cost occurring during diapause
(Tauber et al., 1986), all emerging adults receive only water.
Longevity of parasitoids is directly linked to the amount of
accumulated fats (Ellers, 1996). Therefore, the accumulation
of fat reserves prior to diapause may serve to compensate
for energy consumption during diapause as well as for the
associated fitness costs.
Depending on the species, cold-hardiness may or may not
be associated with diapause and this question is the matter of
much debate (Denlinger, 1991; Hodkova & Hodek, 2004). In
some insects, a reduction of supercooling ability (used as an
index of cold-hardiness) is observed in diapausing individuals (Hodkova & Hodek, 2004). However, there is no general
pattern for a reduced supercooling point being related to diapause. In parasitoids, only a few studies compare supercooling
ability in relation to diapause. A clear difference is observed
between the supercooling point of diapausing (−26.7 ◦ C) and
nondiapausing (−16.2 ◦ C) Colpoclypeus florus larvae (Milonas
& Savopoulou-Soultani, 2005). With another parasitoid (Nasonia vitripennis), supercooling point values are almost identical between nondiapausing (−27.3 ◦ C) and diapausing larvae
(−25 ◦ C) (Rivers et al., 2000). In two different Aphidinae
species, only slight differences are observed: a 3 or 1 ◦ C difference between both phenotypes in Aphidius ervi and Aphidius
rhopalosiphi, respectively (Langer & Hance, 2000). In the case
of P. volucre, no differences are detected, which implies that
an increasing supercooling ability is not part of the diapause
syndrome in P. volucre. Nevertheless, it is possible that other
indicators of cold-hardiness, such as cold shock or chilling
tolerance, may be modulated by diapause.
In the present study, a small proportion of diapause
is induced in P. volucre exposed to cold and darkness.
Diapausing mummies show a significant delay in emergence
and a reduced metabolic rate compared with nondiapausing
counterparts. Diapausing parasitoids also display specific
morpho-physiological characteristics: they are darker, heavier
and contain more lipid reserves. Supercooling ability is
similar between diapausing and nondiapausing individuals.
The eco-physiological reasons for diapausing mummies being
darker than nondiapausing counterparts remain unknown. The
difference may result from increased cocoon thickness in
diapausing mummies (Krespi et al., 1994) or from differential
chemical tanning reactions. From an adaptive point of view,
dark colour may play an important role in the thermoregulation
of mummies (Hance et al., 2007) or it may provide crypsis for
predators during winter (Tauber et al., 1986). Moreover, the
reason why some individuals completed diapause before others
is intriguing. Diapause duration is a highly variable trait in

parasitoids (Carvalho, 2005) and very little is known about
termination of diapause in Aphidiine (Christiansen-Weniger
& Hardie, 1997). This heterogeneity might be related to
variability in the required day number (Eizaguirre et al., 2008).
In Aphidiinae, there is also a large plasticity in the induction
diapause. It is observed that only a part of a population enters
diapause, whereas the other part remains active or undergoes
quiescence during winter (Langer & Hance, 2000). Variability
in diapause termination as well as the finding that only a
fraction of the P. volucre population enters diapause under a
low temperature stimulus may comprise a ‘spreading the risk
strategy’, which would ensure an optimal exploitation of the
unpredictable host resources during winter.

Acknowledgements
We thank J. Shirriffs from the Centre for Environmental Stress
and Adaptation Research in Melbourne (Australia) for providing constructive comments on the manuscript. This study was
supported by ‘Fonds de la Recherche Scientifique – FNRS’.
This paper is number BRC 147 of the Biodiversity Research
Centre.

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Accepted 15 April 2010
First published online 28 June 2010

© 2010 The Authors
Physiological Entomology © 2010 The Royal Entomological Society, Physiological Entomology, 35, 301–307


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