Comparative biology and life tables of Trichogramma
aurosum on Cydia pomonella at constant temperatures
R. Samara & J. C. Monje & C. P. W. Zebitz &
Received: 6 May 2010 /Accepted: 23 December 2010 /Published online: 2 March 2011
# Springer Science+Business Media BV 2011
Abstract The influence of constant temperatures on
biological parameters of German strains of Trichog-
ramma aurosum Sugonjaev & Sorokina (Hymenop-
tera: Trichogrammatidae) was evaluated in the
laboratory on eggs of Cydia pomonella L. (Lepidop-
tera: Tortricidae). Development time and longevity of
all strains were decreased as temperature increased.
Development time of the strains differed significantly
only when exposed to 15°, 20°, and 25°C. Cumula-
tive fertility and longevity differed significantly at 15°
and 20°C. Realized fertility differed significantly at
all constant temperatures. Emergence rates of all
strains were less than 65% and were decreased even
further as temperature increased. Female-biased sex
ratio ranged from 65% to 100% at all constant
temperatures. The low temperature threshold for T.
aurosum was 10°C and the mean number of degree-
days at 15°, 20°, 25° and 30°C was 175, 183, 173 and
185, respectively. The Bavarian strain tolerated high
temperatures and had the highest parasitization capa-
bility, while the Hessian strain had the lowest
parasitization at all temperatures. Fertility life table
analysis revealed a major effect of temperature on the
population growth parameters. Net reproductive rate
was highest at intermediate constant temperatures in
all strains, with the highest rate recorded for the
Bavarian strains at all constant temperatures. Mean
cohort generation time, and population doubling time
decreased as temperature increased. The daily intrin-
sic rate of increase and finite rate of increase were
positively correlated with temperature. The relevance
of our results is discussed in the context of climatic
adaptation, intraspecific variability and biological
Keywords Codling moth.Cumulative fertility.
Development time.Egg parasitoids.
Ephestia kuehniella.Intrinsic rate of increase.
Life time fertility.Longevity.Sex ratio
The codling moth, Cydia pomonella (L.) (Lepidop-
tera: Tortricidae), is a major pest of apple, pear, peach,
plum, cherry and walnut worldwide (Madsen and
Morgan 1970). Infestation rates have been reported to
Phytoparasitica (2011) 39:109–119
R. Samara (*)
Faculty of Applied Science,
Palestinian Technical University,
Tulkarem, Palestinian Authority
J. C. Monje
State Museum of Natural History,
C. P. W. Zebitz
Institute of Phytomedicine, University of Hohenheim,
70593 Stuttgart, Germany
Palestinian National Agricultural Research Center NARC,
Jenin, Palestinian Authority
reach 95% when no adequate control methods were
applied (Ahmad and Abul-Hab 1977). Control of the
codling moth has been tried in commercial orchards
by pheromone trapping, trunk banding, sanitation,
application of pesticides (Barnes 1957; Madsen and
Morgan 1970), mating disruption (Bloem et al. 1999;
Wang et al. 2001), and biological control (Unruh and
Lacey 2001). At present, the management of the
codling moth has shifted from application of chemical
insecticides to integrated management that includes
horticultural practices such as pruning and thinning.
Although these practices are aimed primarily at
producing a healthy, productive fruit tree, they may
also improve control of the pest through habitat
management (Ahmad and Abul-hab 1977). Emphasis
is placed on the minimum use of pesticides that have
a disruptive impact on the beneficial arthropods
present in apple orchards (Blomefield et al. 1997;
Unruh and Lacey 2001).
The gregarious egg parasitoids of the genus
Trichogramma (Hymenoptera: Trichogrammatidae)
are the most widely used natural enemies worldwide
with several species being mass-produced and sold by
a number of commercial companies (Smith 1996).
Five species of Trichogramma have been reported to
be potentially useful for control of codling moth.
They include T. platneri Nagarkatti (Mills et al.
2000), T. minutum Riley and T. pretiosum Riley (Yu
et al. 1984) in the Nearctic region, while T. dendrolimi
Matsumura and T. cacoeciae (Marchal) (also errone-
ously identified as T. embryophagum Hartig) have
been tested in the palearctic region (Hassan 1993).
Mills et al. (2000) reported 60% reduction of damage
in California walnut and apple orchards through
releases of T. platneri, and Hassan (1993) suggested
T. dendrolimi as a promising biocontrol agent.
Reduction in pest damage, however, does not
necessarily mean that moth population deceased
below the economic threshold at the end of the
growing period. Hence, there is a necessity to seek
additional candidate species. For instance, Pinto et al.
(2002) reported 11 species of Trichogramma attack-
ing tortricid eggs in apple and pear orchards in the
USA. One of them, Trichogramma aurosum Sugon-
jaev and Sorokina, is a holarctic species that occurs
naturally in Middle Europe (Samara 2005), the USSR
(Livshits and Mitrofanov 1986; Lopatina 1983) and
the USA (Pinto et al. 2002). It was collected in
Germany for the first time in 2000. In host preference
experiments it was shown that this species prefers
eggs of the codling moth, C. pomonella, to other
lepidopteran eggs. It may therefore be a potential
candidate for the control of C. pomonella in apple
orchards (Samara et al. 2008a, b). From 2001 to 2003,
a wide collection of this species was carried out in the
German Federal Republic from eggs of Nematus
tibialis Newman (Hymenoptera: Tenthredinidae) on
Robinia pseudoacacia (L.), in order to obtain strains
that could be used for pre-introductory research.
Temperature plays a major role in the activity and
metabolic processes of poikilothermic organisms such
as insects (Suverkropp et al. 2001). Suitability of
Trichogramma spp. for their use as bioagents is
dependent on their ability to tolerate and adjust to
adverse abiotic conditions (G.A. Pak, 1988, thesis,
Agricultural Univ. of Wageningen). According to Pak
and van Heiningen (1985), climatic adaptability is one
of the criteria for the comparative selection of a strain
through laboratory experiments. Many studies were
conducted at constant temperatures to assess the
biological characteristic of Trichogramma species
(Maceda et al. 2003; H. E. Sakr, 2003, thesis, Univ.
of Hohenheim; Schöller and Hassan 2001).
The impact of constant temperatures was investi-
gated on the biology, parasitization potential and
population growth parameters of different German
strains of T. aurosum, in order to obtain suitable
candidates that can possibly be used against C.
pomonella in different parts of the country.
Materials and methods
Insect rearing A stock culture of the codling moth C.
pomonella was maintained in the laboratory according
to Bathon et al. (1991). Strains of T. aurosum collected
from the field (Table 1) were reared and maintained on
eggs of the factitious host Ephestia kuehniella Zeller
(Lepidoptera: Pyralidae). The factitious host was
reared following the procedure described in Cerutti et
al. (1992). Strains were placed in culture tubes (70×
20 mm) closed with a plastic lid, which had a small
hole for aeration. The tubes were kept at ca 25°C, 65±
5% r.h. and a photoperiod of 18:6 h (L:D) in a climate
cabinet during pupal development of the parasitoids.
To feed the wasps, a droplet of honey was placed in
the tube prior to or upon their emergence. Emerged
parasitoids were provided with fresh host eggs on an
110Phytoparasitica (2011) 39:109–119
‘egg card’. Egg cards were prepared by sprinkling host
eggs on a drop of gum arabic on a piece of paper index
card (50×15 mm). Females used for the experiments
were 24 h old, mated, fed on honey, and had no
experience with hosts prior to the tests.
Effect of preimaginal development, emergence, and
mortality Temperature-dependent development of T.
aurosum from egg to adult emergence was determined
by allowing females to oviposit on 24-h-old C.
pomonella eggs for 24 h. Females of each strain were
placed singly (n=20) in glass test tubes (70×20 mm)
with 10–20 C. pomonella eggs to follow individual
development and emergence. Development duration
was calculated from the time when the test females
were removed. Each group of test tubes was placed in
a separate cabinet at constant temperature regimes of
15°, 20°, 25° and 30°C (all ±0.5°C) with a 16:8 h (L:
D) photoperiod, and a relative humidity of 65±5%.
Development time from oviposition to adult emer-
gence was determined by two visual controls for each
parasitized egg every 12 h (07:00, 19:00). Degree-
days and mean development threshold were deter-
mined according to Smith and Hubbes (1986).
Fertility and longevity Mean cumulative female fer-
tility and relative cumulative parasitized eggs per day
were determined under the same conditions as given
in the previous paragraph. Test females were obtained
from egg cards with parasitized E. kuehniella eggs in
a small glass vial (40 mm long, 15 mm diameter), 3–
6 h before the experiments. A single mated female,
maximum 16 h old, was placed on a circular filter
paper that covered the bottom of a glass petri dish
(70 mm diameter, 10 mm deep). One minute drop of
undiluted honey was provided in the middle of the
petri dish to feed the adult during this test. Fresh
codling moth eggs (10–20 eggs per dish per day)
were supplied daily until the natural death of the
female wasps. Twenty replications were performed
for each wasp strain and for each temperature
treatment. Longevity (number of days) until death of
the adults was also recorded. Hatched C. pomonella
larvae from unparasitized eggs were removed daily
from the vials to avoid parasitized eggs being
consumed in the vials. Cumulative female fertility is
defined as the total number of successfully parasitized
eggs by a female over the full life span (as evidenced
by black coloration of the eggs). The number of
parasitized host eggs as well as the number of adults
emerging from the host eggs and the number of
unhatched eggs were counted. We refer to the number
of live female progeny per female in each age interval
as age-specific fertility (mx) (Southwood 1978).
Cumulative realized fertility is the number of eggs
successfully parasitized by a female wasp over the
first 3 days of adult life (Kuhlmann and Mills 1999),
since most Trichogramma sp. die within this period in
nature if they are not able to find a food source. The
parameters usually estimated from fertility life tables
are the net reproductive rate (R0); the intrinsic rate of
increase (rm), which is a measure of the growth rate of
a population per female (Pak and Oatman 1982); the
mean cohort generation time (Tc); the doubling time
(Dt) (the time required for a population to double its
numbers); and the finite rate of increase (λ). All were
estimated according to Southwood (1978), Maia et al.
(2000) and Nagarkatti and Nagaraja (1978).
Data analysis Female longevity, cumulative fertility,
realized fertility, and development time data were
transformed to log10(x+1), whereas the data of
emergence rate and sex ratio were arcsine-
transformed. The transformed data were then ana-
lyzed by ANOVA test using the General Linear
Models (PROC GLM) procedure (SAS Institute
1996). The Student-Newman-Keuls (SNK) procedure
was used to separate the means.
Table 1 List of the collected Trichogramma aurosum strains, their locations, latitude, longitude and time of collection
CodeLocality LatitudeLongitudeTime of collection
Baden-Württemberg, Stuttgart (Southwest)
Hesse, Worms (West)
Bavaria, Munich (Southeast)
Lower Saxony, Göttingen (North)
Berlin, Schöneberg (Northeast)
48° 42′ North
49° 39′ North
48° 08′ North
51° 32′ North
52° 28′ North
9° 13′ East
8° 21′ East
11° 35′ East
9° 55′ East
13° 22′ East
Phytoparasitica (2011) 39:109–119111
Significant differences in cumulative fertility among the
various strains were detected at 15°C. Cumulative
fertility was highest for Ta4 and Ta20, whereas Ta10
had the lowest values among all strains at 15°C
(Table 2). The highest cumulative fertility was mea-
sured in strain Ta19 at 20° and 25°C. At the extreme
temperature (30°C), cumulative fertility of Ta13 was
significantly higher than that of the rest of the strains
(Table 2), which did not differ significantly. Female
longevity did not differ significantly at 25° or 15°C in
all T. aurosum strains. At the highest temperature
(30°C), females of all strains died after a maximum of
3 days. Females of Ta19 were able to live longest at all
temperatures (Table 2). A significant difference was
recorded at 30° and 20°C for Ta19 female longevity.
Realized fertility for T. aurosum was calculated in
response to temperature impact. Temperature had a
significant impact on the female realized fertility for all
strains studied. Ta20 had the highest realized fertility at
15°C, and Ta4 had the highest significant realized
fertility at 20°C. At intermediate and high temperatures
(25° and 30°C), both Ta19 and Ta13 had the highest
significant differences, respectively (Table 2). Parasit-
ism reached the highest values in the first 3 days at
high temperature, whereas at the low temperature,
parasitism ranged between 20% and 40% for the above
mentioned strains. Mean development time was similar
among all populations of Trichogramma sp. at 30°C,
ranging from 8.6 to 9.2 days (Table 2). Significant
differences were found among the strains at the
remaining temperatures studied. No development was
recorded for any strain at 35°C (preliminary experi-
ments). The development rate from egg to adult of
both female and male parasitoids increased with
Table 2 Effect of different constant temperatures on the female mean cumulative fertility, longevity, realized fertility, development
time, sex ratio, emergence rate and degree-days of five strains of Trichogramma aurosum (Data are means ± SE)
zLog transformed data were used for the mean values
yArcsine transformed data were used for the mean proportion
*Within columns, data followed by a common letter do not differ significantly (P<0.05, ANOVA, Student Newman Keuls procedures)
ND = there was no development, and no hatching was recorded
112Phytoparasitica (2011) 39:109–119
y = -0,0053x + 0,0574
R2 = 0,03
y = 0,0277x + 0,0044
R2 = 0,9985
Developmental rate / day
y = 0,0277x + 0,0041
R2 = 0,9916
y = 0,027x + 0,0028
R2 = 0,9864
y = 0,0271x + 0,0052
R2 = 0,9789
15 2025 30 35
Fig. 1 Effect of constant temperatures on the development rate per day of different strains of Trichogramma aurosum
Phytoparasitica (2011) 39:109–119113
increasing temperature from 15° to 30°C (Fig. 1). The
low temperature development threshold was 9.25°,
9.33°, 9.5°, 10.55° and 9.07°C, for Ta4, Ta10, Ta13,
Ta19 and Ta20, respectively. The sum of degree-days
(D-D, thermal constant) of Ta4, Ta10, Ta13, Ta19 and
Ta20 was 180, 180, 185, 159 and 186 D-D, respec-
tively. At 30°C no progeny hatched from the parasit-
ized eggs of strain Ta19 even though black eggs were
observed. This is an indication that the parasitoids
reached the larval stage, but died during the prepupal
stage. Therefore, the low temperature development
threshold was higher and the sum of D-D was shorter
than in the other strains. An inverse relationship was
found between rearing temperature and development
time (Fig. 1) and a direct relationship between rearing
temperature and development rate. Sex ratio was
significantly affected by temperatures. Interestingly,
progeny of Ta10 was extremely female-biased (Table 2)
compared with that of other strains, although the
remaining strains had also a high sex ratio (>67%).
Emergence rate of Ta10 was affected by low temper-
ature (15°C). Ta4 had the highest emergence rate at all
temperatures studied (Table 2).
Fertility life table parameters differed significantly
in response to the different temperature treatments.
The net reproduction rate (R0) of Ta4, Ta10 and Ta19
varied from 3.22 to 8.12, 0.14 to 2.67, and 1.76 to
7.40 times, respectively, according to the temperature
variation. The maximum increase in capacity was
reached at 20°C (Table 3). The values for Ta13 and
Ta20 varied from 1.56 to 2.77 and from 0.71 to 4.59
times, respectively; the maximum was reached at 30°
and 25°C, respectively. The intrinsic rates of increase
(rm) for the different strains is shown in Table 3; Ta4
and Ta13 had the highest rate at 30°C, whereas Ta19
and Ta20 had the highest rate at 25°C. Cohort
generation time (Tc) for the studied strains differed
significantly at variant temperatures. The finite ca-
pacity for increase (λ) of the parasitoids increased as
Table 3 Fertility life table
parameters of Trichogramma
aurosum on Cydia pomo-
nella at 15°, 20°, 25° and
30°C (Data are means±SE)
R0= net reproductive rate,
rm= intrinsic rate of in-
crease, Tc= cohort genera-
tion time (days), λ = finite
capacity for increase, Dt=
doubling time (days)
*Within columns, data fol-
lowed by a common letter
do not differ significantly
( P < 0 . 0 5 ,
A N O VA ,
Fig. 2 Survivorship in five strains of Trichogramma aurosum
at four constant temperatures
114 Phytoparasitica (2011) 39:109–119
Number of Survivors (lx)
Age in days
48 1216 202428 32 364044
Phytoparasitica (2011) 39:109–119115
temperature increased. Doubling time (Dt) decreased
with the increase of temperature, the longest time
required occurring at 15°C and the shortest time at
Temperature is one of the most important abiotic
factors affecting the development rate, cumulative
fertility, longevity, sex ratio and emergence rate of
Trichogramma spp. Tolerance of immature stages of
Trichogramma species/strains to high or low temper-
ature extremes has been the subject of several studies
(G. A. Pak, thesis, 1988; Nagarkatti and Nagaraja
1978; Smith and Hubbes 1986). However, most of the
published work was conducted only at constant
temperatures, which are useful for laboratory rearing
and commercial mass production. No significant
differences among strains were observed in longevity
or development time at some temperatures tested,
whereas the duration of development and longevity
decreased as temperature increased. This reduction
could be due to the long rearing of the wasps under
constant temperature (the wasps used in these experi-
ments had been reared for at least 40 generations
under laboratory conditions). It is not well known
whether the long laboratory rearing affects the wasps’
activity and vigor. According to Nagarkatti and
Nagaraja (1978), female fertility of T. confusum
wasps reared for a long time under laboratory
conditions was significantly lower than that of wild
females. However, female longevity is affected by
many factors, such as temperature (Pak and Oatman
1982), humidity (Stinner et al. 1974), host size
(Stinner et al. 1974), and food (Yu et al. 1984).
Almatni (2003, Ph.D. thesis, Damascus Univ., Syria)
found that the high temperature could have caused
sterilization of European strains of T. cacoeciae,
because they stopped laying eggs but lived for a few
more days without laying new eggs. Also a reduction
in female longevity of T. platneri was recorded from
53 days at 10°C and 3 days at 35°C (McDougall and
The highest cumulative fertility for the strains
studied was at intermediate temperatures (20° and
25°C). These results agree with those of G. A. Pak
(thesis, 1988), who found that the number of
parasitized hosts increased with increasing tempera-
ture to a maximum at 20–25°C and declined at 30°C.
Some strains examined in this study showed better
tolerance to high temperatures and others were able to
tolerate low temperatures. The strains Ta4, Ta19 and
Ta20 were able to live up to 6–11 days longer than the
rest of the strains at 15°C, and Ta13 was able to live
up to 1–2 days longer than the rest of the strains at
30°C. Relative cumulative parasitized eggs per day
was temperature-dependent, females reaching 100%
parasitization at the high temperature regimes in a
short time after hatching. On the contrary, females
held under low temperatures reached 100% parasiti-
zation after a longer period of time. The short period
of parasitism can be considered as a specific survival
strategy, because a faster oviposition at higher
temperatures will allow this pro-ovogenic parasitoid
to lay most of its available eggs during a short
lifetime period (Garcia and Tavares 1994). Garcia and
Tavares (1994) found significant differences between
all temperatures for T. cordubensis longevity, which
increased with the decrease of temperature, results
that are similar to ours. However, it was noticed that
the decrease in temperature increased the pre-
oviposition period for all the studied strains. At
15°C the average pre-oviposition period was 3, 5, 2,
1.5 and 2 days for Ta4, Ta10, Ta13, Ta19 and Ta20,
respectively. There was no measurable pre-
oviposition period at the higher temperatures. Accord-
ing to Al-Ahmed and Kheir (2003), temperature is
considered an important factor affecting the duration
of the pre-oviposition period.
Realized cumulative fertility was calculated
according to Mills and Kuhlmann (2000), and found
to be the highest at the high temperature regime. This
parameter was reduced at the lower temperature
regime. The total cumulative fertility values did not
differ from the values of the realized cumulative
fertility at high constant temperatures, but both values
were found to differ when T. aurosum strains were
reared at 15°C. Similar results were reported for T.
minutum (Smith and Hubbes 1986) and Trichog-
ramma spp. (Pak and van Heiningen 1985).
According to Jervis and Copland (1996), there is an
optimal range of temperature for insect development,
beyond which they would be unable to continue
oogenesis and laying eggs or unable to function
appropriately for a long period of time. This could be
due to the increase of respiration rate, i.e., the insects
would be unable to produce fertile eggs due to the high
116Phytoparasitica (2011) 39:109–119
consumption of energy (Mills and Kuhlmann 2000).
Data recorded for sex ratio from the present experiments
agree with some data from the literature but disagree
with other data. Pintureau and Bolland (2001) found that
the percentage of males in the offspring of thelytokous
females increased faster according to temperature in T.
cordubensis than in T. pretiosum; the sex ratio was
higher at low and intermediate temperatures. These
results agree with those of Bowen and Stern (1966).
In general, sex ratio was not affected with an increase
of temperature; female percentage ranged from 65% to
100%.OurdataagreewithHaileetal.(2002), who found
that sex ratio was biased to female production at all
temperatures. It seems that rearing the wasps at lower
temperatures has a significant effect on the biological
characteristics. Crozier (1977) suggested that lower
temperatures could promote fusion of nuclei and
increase the proportion of diploid offspring, which could
explain the high sex ratio at 15°C in all strains studied.
Development time needed by the T. aurosum strains
was shorter as temperature increased; this was in
agreement with Consoli and Parra (1995), who
assumed that this might be due to a more appropriate
metabolic process of the immature stages. The optimal
temperature for development and survival was 25°, and
35°C was assumed to be the upper vital threshold
(McDougall and Mills 1997; G. A. Pak, thesis, 1988).
Total development time was four times faster at 30°
than at 15°C. However, the total mortality was greater
at the higher temperature; these results are similar to
the findings of Hawkins and Smith (1986). Mean
development time at constant temperature ranged
between 32.2 and 8.8 days at temperatures between
15° and 30°C. Although development occurred at 10.8°
C (indicated by the parasitized eggs turning black, i.e.,
development takes place up to the third larval stage),
no emergence was observed during 6 months of
incubation (unpublished data). For all the temperatures
tested, a linear regression model was calculated for egg
to adult development of all Trichogramma strains. As
temperature increased, the duration of development
decreased. Mean development time of female T.
turkestanica on E. kuehniella was 32.9, 18.2, 9.1 and
7.0 days at 15°, 20°, 25° and 30°C, respectively
(Hansen 2000; Pintureau and Bolland 2001).
Survivorship curves according to Southwood
(1978) obtained from our results at different constant
temperatures differed remarkably for the strains
studied. At low constant temperature (15°C) the
mortality acts most heavily on the old females. All
strains at 20°C had a constant mortality per unit time;
whereas Ta10, Ta19 and Ta20 had a constant rate of
mortality, Ta4 had the highest mortality rate of the
young individuals. At 25°C, all strains had a
logarithmic survivor rate where the mortality rate is
constant. At 30°C, all strains had the highest mortality
rate during the young stages (Fig. 2). The Ta13 strain
showed a better adaptation to all temperatures studied,
althoughTa4 and Ta20 also showed promising results
in response to rearing temperature. The relationship
between the temperature of the geographical origins
of the strains and their climatic adaptability was
evaluated. We found that Ta13, which was collected
from southeast Germany—where the average temper-
ature from June to September is 15–19°C—was able
to tolerate the high constant temperature (30°C). Ta4,
Ta19 and Ta20 were able to tolerate intermediate and
low temperatures; in the above mentioned season the
average temperature range in the original locations
was 13–16°, 14–17° and 14–18°C, respectively.
Emergence rate reached the highest values at inter-
mediate temperatures in all strains studied. For T.
annulata and T. pretiosum, it was found to be more
than 89% (Maceda et al. 2003). The reduction in the
emergence rate and the long time required for the
immature stages to develop from egg to adult were
recorded at the low temperature 15°C for all strains
studied. This can be due to the high mortality in the
immature stages. Smith and Hubbes (1986) noticed that
the emergence rate was reduced when the parasitized
eggs were reared for 24 days at 15°C. Fertility life table
studies provide a powerful technique for evaluation of
population dynamics because they provide a detailed
description of age-specific mortality of individuals in
the population. Pratissoli and Parra (2000) found that
the net reproduction rate varied according to the
temperature variation for T. pretiosum. It was the
highest at 20°C for all the T. aurosum strains studied.
The net reproductive rate recorded for T. mwanzai at the
constant temperatures 20°, 25° and 30°C was 7.3, 35.9,
and 31.9 (Lu 1992), and for T. cacoeciae at the same
temperatures it was 48.69, 45.83 and 24.02, respectively
(Uzun and Akten 1992). The finite increase rate was
proportionally related to temperature. For T. pretiosum
the relation between λ and temperature increase
occurred for the range from 18° to 30°C (Pratissoli
and Parra 2000), and for Trichogrammatoidea sp. it was
the highest at 27°C (Baitha and Ram 1998) and in our
Phytoparasitica (2011) 39:109–119117
results it was at 30°C. The mean cohort generation time
(Tc) values show a decreasing trend from 18° to 30°C
(Baitha and Ram 1998). This agrees with our findings,
where the generation time values decreased as the
temperature increased. The differences between the
strains were very obvious, some strains showed good
adaptability to high temperature (Ta13), whereas others
showed good adaptability to intermediate temperatures
(Ta4, Ta10 and Ta19), and some showed adaptability to
a low temperature (Ta20). Cabello and Vargas (1988)
related the reduction of the net reproductive rate at high
temperatures to the production of both males and
females at these temperatures. This can explain our
results, where the R0was reduced at 30°C. Accordingly,
values of daily intrinsic rate of increase (rm) and finite
rate of increase (exp. rm) were affected. Development of
each insect stage is dependent on temperatures; insect
activity such as locomotion and searching behavior is
dependent on the temperature.
This study can provide us with information to select
one or several strains as a suitable candidate for
biological control of the codling moth according to
their climatic adaptability. It is possible to select either
those strains that showed high parasitization rate, higher
longevity and higher tolerance to high temperatures, or
those strains that showed high parasitization activity at
low temperature conditions. However, further studies of
host location and searching, parasitization behavior and
female dispersal are also required before field release
experiments are conducted.
German Academic Exchange Service (DAAD, A/00/19004). Our
thanksare extended to all who helped in the field collection andto
R. Siegel for maintaining the lab culture of E. kuehniella.
The present research was supported by the
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