Introduction
One of the most important pests of almond trees in the Mediterranean area is the poplar lace bug,
Monosteira unicostata
(Mulsant & Rey) (
Maniglia, 1983
;
Liotta & Maniglia, 1994
;
García Marí & Ferragut, 2002
). This species has been recently detected in the wild also in Canada (
Scudder, 2012
) where its presence could pose a serious risk for the production areas of almonds in USA, which is by far the largest world producer (
USDA, 2017
). This insect has been cited attacking other fruit trees such as cherry, peach, plum, pear and forest trees like poplar or willow (
Péricart, 1983
).
Adults of
M. unicostata
overwinter on the bark of trees, on wild plants or under fallen leaves and in spring they move to young almond leaves where they feed and lay eggs on the underside, thus starting infestations (
Talhouk, 1977
;
Liotta & Maniglia, 1994
;
García Marí & Ferragut, 2002
). Once eggs hatch, nymphs live and feed on the lower side of the leaves, passing through five instars before they reach the adult stage (
Talhouk, 1977
;
Péricart, 1983
;
Sánchez-Ramos
et al.
, 2015
). The nymphs are sedentary and gregarious, and if they are disturbed, disperse to regroup later (
Bremond, 1938
;
Vessia, 1961
;
Neal & Schaefer, 2000
). The feeding activity of this insect seriously damages the leaves that weaken, turn yellow and often fall (
Talhouk, 1977
;
Liotta & Maniglia, 1994
). This defoliation results in smaller fruits, which drop prematurely (
Gómez-Menor, 1950
;
Neal & Schaefer, 2000
), and affects negatively to the production of the following year (
Liotta & Maniglia, 1994
). An additional harmful effect is the accumulation of excrements on the leaves, because this produces a reduction of the gas exchange (
Bremond, 1938
;
Gómez-Menor, 1950
).
Very little is known about the reproductive performance and the life table parameters of this lace bug and the scarce information available is based directly on field observations (
Bremond, 1938
;
Vidal, 1939
;
Vessia, 1961
;
Maniglia, 1983
;
Péricart, 1983
;
Moleas, 1987
). Different authors pointed out that overwintering females begin to lay eggs 10-15 days after post-winter emergence (
Vessia, 1961
;
Maniglia, 1983
;
Moleas, 1987
) and they lay 6-15 eggs per day in a single leaf with a total fecundity of 70 eggs (
Vidal, 1939
;
Péricart, 1983
). It has been suggested the possibility of parthenogenesis in this insect because some authors found only females after the overwintering recovery (
Bremond, 1938
;
Vidal, 1939
). On the contrary, other authors have reported pairs copulating after winter recovery, with males present although in lower proportion with regard to females (
Vessia, 1961
;
Moleas, 1987
). According to
Vessia (1961)
, the oviposition can be extended for 20-30 days so the different generations cannot be well differentiated. In accordance with that, it has been indicated that the generations of this insect overlap, so eggs, nymphs and adults coexist simultaneously from June and during the summer (
Péricart, 1983
). This has been corroborated with field data collected by different authors (
Moleas, 1987
;
Pereira
et al.
, 2008
;
Marcotegui
et al.
, 2015
). The number of generations ranges from 2 to 4, which depends on the geographical area (
Péricart, 1983
). Thus, in the north of its distribution area only 2 generations occur, whereas in the south, the number of generations can be up to 4. In Spain, there are 3-4 generations with the third generation being the responsible of the greatest injury because at that moment the number of insects is the highest (
Gómez-Menor, 1950
;
García Marí & Ferragut, 2002
). The variability in the number of generations can be due to differences in the environmental temperature because insects are poikilothermic organisms so this is one of the main factors affecting their developmental and reproductive rates (
Régnière
et al.
, 2012
;
Régnière & Powell, 2013
).
The aim of the present work was to investigate the effect of temperature on reproduction, longevity and life table parameters of
M. unicostata
and to establish the thermal limits for the increase of the populations of this insect. In addition, the possible occurrence of parthenogenesis in this species was checked in the laboratory.
Material and methods
Laboratory rearing of Monosteira unicostata
A colony of
M. unicostata
was established in the laboratory from wild populations collected in mid spring from almond trees,
Prunus dulcis
(Mill.) D.A. Webb, placed at the INIA facilities (Madrid). The insects were then transferred to the laboratory and reared on small black poplar plants,
Populus nigra
L., grown from hardwood cuttings, since this possibility is not feasible for almond tree. The cuttings were collected in winter and they were kept refrigerated at 4 °C in 15-20 cm long fragments. When needed, they were planted and grown at 25°C, 70% RH and a L16:D8 photoperiod until the poplars were big enough (about 60 cm height) to be infested. This took approximately one month. Fifty to sixty adults of
M. unicostata
were used to infest four poplar plants. The infested plants were confined in wooden framed cages (60 cm × 50 cm × 45 cm) whose sides were covered with translucent nylon cloth pieces. The frontal nylon cloth piece was removable to manipulate the insects and plants. To maintain the
M. unicostata
population, new poplar plants were grown and periodically infested with adults from the laboratory rearing when the older ones were seriously damaged.
Plant material
Assays were performed using small rooted cuttings of black poplar. To obtain them, poplar twigs were collected in spring from trees placed in the surroundings of the INIA facilities. Once in the laboratory, the twigs were washed and inspected to discard those with symptoms of infestation, disease or any type of physiological damage. Then, they were divided in small cuttings with a single leaf and a small portion of stem. The cuttings were introduced into nutritive solution (
Moutous & Fos, 1973
) for a week and those that rooted were selected to be used in the developmental assays, thus assuring that the leaves remained alive during the whole experiment. The rooted cuttings with leaves were introduced into plastic containers (7 cm high × 3.5 cm diameter) with nutritive solution through a small hole (8 mm diameter) made in the lid. The hole was then sealed with plasticine.
Adult survival and reproduction
The reproduction and longevity of adults were monitored at constant temperatures of 22, 25, 28, 31, 34 and 37°C in environmental chambers (Sanyo MLR-350, Sanyo, Japan). For all temperatures, the other environmental conditions were 60 ± 10% RH and L16:D8 photoperiod. Lace bug subpopulations were acclimated for at least one month at each temperature. No temperatures below 22°C could be tested because of no availability of poplar twigs during the long whole period needed to complete the assays.
Assays were conducted in rearing units consisting of plastic tubes (15 cm high × 5.5 cm diameter) closed at the bottom with the upper lid of a 5.8 diameter Petri dish and covered at the upper part with a piece of translucent nylon cloth fitted with the help of a rubber band, thus allowing ventilation on the rearing cell.
A container with a rooted cutting with a leaf as described above was introduced intro each rearing unit together with a <24 h adult pair of
M. unicostata
. To obtain males and females, fifth instar nymphs were taken out of the acclimated subpopulations and put inside rearing units together with a poplar rooted cutting. Fifh instar nymphs were identified according to
Bremond (1938)
,
Péricart (1983)
and
Sánchez-Ramos
et al.
(2015)
. The following day, the new adults were sexed according to
Vessia (1961)
, paired and transferred singly to new rearing units with new poplar rooted cuttings. Forty to forty five pairs, up to 1-day-old, were established for each temperature. Then, the leaves were inspected daily under a stereomicroscope and all the eggs laid counted. Adult pairs were transferred periodically to new rooted cuttings before the eggs hatched. Checking was performed daily until males and females died. A stock of males was kept from the original fifth instar nymphs to replace any which died before the females. At each temperature, the percentage of fertile mating, the preoviposition, oviposition and postoviposition periods, the eggs laid per female per day (daily fecundity), the total number of eggs per female (fecundity), and the male and female longevity were estimated. Curves of mean daily fecundity per female were determined. The starting number of fertile females was used over the oviposition period of the cohort to obtain these curves. Survivorship curves were obtained by using the daily proportion of surviving males and females.
Life table parameters
Adult survival and reproduction data at each temperature were used to calculate the life table parameters. The pre-reproductive period and survival and the sex ratio (1:1) were obtained from a previous work (
Sánchez-Ramos
et al.
, 2015
). The intrinsic rate of natural increase,
r
m
, that is, innate capacity of increase in idealized populations under idealized environments, was estimated according to the equation given by
Birch (1948)
:
![]()
where
x
is the age in days,
l
x
the age-specific survival rate of females (probability at birth of being alive at age
x
), and
m
x
the age-specific oviposition rate (mean number of female offspring produced in a unit of time by a female aged
x
). The net reproductive rate,
R
0
, is given by
R
0
= Σ l
x
m
x
; the mean generation time,
T
, in days, is given by
T=ln R
0
/r
m
; the finite rate of natural increase, λ
, is given by λ
=e
rm
; and the population doubling time,
PDT
, is given by
PDT=ln 2/r
m
. The program
r
m
2.0 (
Taberner
et al.
, 1993
) was used to calculate these parameters. This program provides an estimate of the intrinsic rate of natural increase (
r
m
) variance by means of a bootstrap resampling method. The minimum number of replicates used was 500 as recommended by
Meyer
et al.
(1986)
.
Evaluation of the occurrence of parthenogenesis
Following the procedures described above, a new laboratory population of
M. unicostata
was established with adults collected in mid spring from almond trees located at the INIA facilities (Madrid). This population was maintained in an insectary at 28-30°C, 70% RH and a L16:D8 photoperiod, and these conditions were used also for the experiments.
When the new laboratory colony had enough number of individuals, fifth instar nymphs were collected and individually transferred to rearing units described above. The development of the nymphs was checked daily and when they reached the adult stage, they were sexed, obtaining in this way virgin females. After 12 days, oviposition was checked and females that had laid eggs were transferred to new rearing units and two males added. Again, oviposition was checked after 12 days, and then the adults were removed. The plants with the eggs from virgin females and those from the same females after mating were inspected for egg hatch. The proportion of samples with hatched eggs from virgin and mated females was established.
Analytical methods
The effect of temperature on the different reproductive and longevity parameters was analysed by general linear models using temperature as a quantitative factor. The best fitted model of the observed data was used to estimate the preoviposition, oviposition and postoviposition periods, number of eggs per female per day, total number of eggs per female and male and female longevity. The general form of the models is:
![]()
where
y
represents the predicted reproductive or longevity variable,
T
is temperature (°C),
a
i
are parameters associated with the i-th term of the predictive variable and
ε
the residual error. The model with the highest significant term of temperature was selected. In addition, a general linear model was performed with sex as categorical factor and temperature as quantitative factor to test for differences in longevity between males and females. Only significant terms were included in the model. Male and female longevity data were previously transformed by ln(x) to meet the assumptions of parametric statistics. The ln(x) transformation was the one that best modified the distribution of residuals to satisfy the requirements for parametric tests. Also, chi-squared tests were performed to assess the effect of temperature on the percentage of fertile mating. The level of significance was
p
<0.05 in all cases. Analyses were done using Statgraphics
®
Centurion XVI (
StatPoint Technologies, 2009
).
A Maxima function (
Richter & Söndgerath, 1990
) was used to fit the mean daily fecundity per female at each temperature. The equation is
![]()
where
f(t)
represents the mean daily fecundity per female,
α
and
t
are parameters, and
t
is time expressed in days after the start of oviposition, so that
t
= 1 is the first day of oviposition.
Survival of males and females at each temperature were fitted by a Weibull function (
Pinder III
et al.
, 1978
), whose general form is
![]()
where
S(t)
represents the probability of surviving to a given age,
t
is time,
b
is the parameter that describes the scale and
β
is the parameter that describes the shape of the curve. The shape parameter controls the rate of change of the age-specific mortality rate and, therefore, the general form of the survivorship curve (
Pinder III
et al.
, 1978
). In addition, the patterns of survival obtained for males and females at each temperature were compared by means of Wilcoxon and logrank tests. The Wilcoxon test tends to be sensitive to differences that are most evident early in time. In comparison, the logrank test tends to be more powerful in detecting differences late in time (
Martínez & Naranjo, 2010
).
The equation developed by
Lactin
et al.
(1995)
was used to fit the intrinsic rate of natural increase (
r
m
) data obtained for the different temperatures. This model was selected because it provided the best fit when applied to developmental rate data of
M. unicostata
(
Sánchez-Ramos
et al.
, 2015
). The expression of this model is
![]()
where
T
is temperature (°C);
r
m
(T)
is the intrinsic rate of natural increase at temperature
T
;
Tmax
is the supraoptimal temperature at which
rm(T)
= λ
;
is the range of temperatures between
Tmax
and the temperature at which
rm(T)
is maximum;
describes the acceleration of the function from the low-temperature threshold to the optimal temperature; parameter λ
allows the curve to intersect the abscissa at suboptimal temperatures and, thus, allows estimation of a lower threshold. It is the asymptote to which the function trends at low temperatures.
The Maxima, Weibull and Lactin functions were fitted by iterative non-linear regression based on Levenburg-Marquardt algorithm. Models fitting and parameter estimation were obtained using Tablecurve 2D 5.01 (
SYSTAT, 2002
).
Discussion
The reproductive and life table parameters obtained for
M. unicostata
reveal the good adaptation of this lace bug to the prevailing temperatures in spring and summer in its area of distribution. This had been already shown when the effect of temperature was studied on the developmental rate (
Sánchez-Ramos
et al.
, 2015
) and had been anticipated before by different authors (
Moleas, 1987
;
Neal & Schaefer, 2000
). At temperatures ranging between 25 and 37°C, the populations of this insect are able to double their numbers in less than seven days, what explains the high number of individuals observed at the end of the summer and the damage produced.
The results shown here were obtained at regimes of constant temperatures, so some variability might be expected under the real regimes of fluctuating temperatures registered in the field. However, despite this fact, the results are similar, in general, to those reported by other authors coming from field observations. Thus, the preoviposition periods range from a little bit more than 2 days at 34°C to almost 9 days at 22°C. Based on field observations made on almond tree, different authors reported that first eggs were observed around 10-15 days after the emergence of the overwintering adults (
Vessia, 1961
;
Maniglia, 1983
;
Moleas, 1987
). In addition,
Maniglia (1983)
reported preovipostion periods of 9-11 days for the adults of the first generation and 4-6 for the latter ones. These values agree in general with our findings obtained on black poplar because spring populations are subjected to lower temperatures than summer ones and the daily temperatures registered by
Maniglia (1983)
through the season ranged between more than 35°C during the day to around 15°C at night. Regarding the oviposition period,
Vessia (1961)
indicated that it can be as long as 20-30 days according to field observations, what reasonably agree also with the mean values of 14-38 days reported here. However, according to the results by
Vidal (1939)
in pear tree,
Pyrus communis
L., the oviposition period that can be deduced from his data about total fecundity and daily fecundity is no more than 12 days, what is shorter than our results and those by
Vessia (1961)
. Moreover, the fecundity reported by
Vidal (1939)
is also smaller than that obtained in our laboratory assays. Thus, while we have obtained mean values ranging 90-220 eggs per female,
Vidal (1939)
pointed out that the mean total number of eggs laid by a single female is about 70 eggs. In addition, this author indicated that a female lays 6-15 eggs in a day, whereas we have observed that females were able to lay more than 20 eggs in a day at all temperatures tested, mainly around the days of the oviposition peaks showed in Fig. 1. These differences might be due to the different host species studied. Although pear tree is a common host for this tingid, maybe it is not as suitable as almond tree or black poplar for the reproduction of this species. In addition, since the environmental conditions under which the values reported by
Vidal (1939)
were obtained are not indicated, sound comparisons cannot be performed with our results. Moreover, it is possible that
Vidal (1939)
did not consider the whole oviposition period because his values are based in field observations. Curiously, other authors reported that reproduction experiments with females fertilized in captivity were not possible because they died before laying eggs (
Vessia, 1961
).
Different authors pointed out the possibility of parthenogenesis in
M. unicostata
based on field observations after the winter recovery, in which only females were observed and no copulation with males was registered (
Bremond, 1938
;
Vidal, 1939
).
Bremond (1938)
even isolated fifth instar nymphs and checked that the virgin females obtained from these nymphs were able to lay eggs, similarly to what has been obtained here. However, as
Péricart (1983)
pointed out,
Bremond (1938)
did not state if the eggs laid by the virgin females were able to hatch. We have shown here that the eggs laid by females that have not been mated do not hatch and that eggs laid by the same females after copulating with males are viable and hatch. Accordingly, these results suggest that parthenogenesis does not occur in
M. unicostata
. Similarly, different studies reported that eggs laid by unmated females of other tingids like
Stephanitis pyrioides
(Scott) and
Corythucha cydoniae
(Fitch) failed to hatch (
Neal & Douglass, 1988
,
1990
). This is in accordance with the fact that parthenogenesis is not common in Suborder Heteroptera, since it has been proved only in a few species of families like Mesoveliidae, Miridae and Anthocoridae (
Carayon, 1989
;
Glime, 2015
).
For the comparison of the reproductive, longevity and life table parameters among tingids is necessary to consider the differences in suitability of the plants provided as host. For example, the fecundity reported for
C. cydoniae
on
Pyracantha coccinea
M. J. Roemer (
Neal & Douglass, 1990
) and
Cotoneaster dammeri
C. K. Schneid. (
Braman & Pendley, 1993
) and for
Corythucha ciliata
(Say) on
Platanus orientalis
L. (
Kim
et al.
, 1999
) and
Platanus × acerifolia
(Aiton) Willd. (
Ju
et al.
, 2011
) is quite different depending of the host plant species. Similarly, adult longevity of
Stephanitis takeyai
Drake & Maa on
Pieris japonica
(Thunb.) D. Don ex G. Don and
Lyonia elliptica
(C. Wright ex Small) Alain (
Tsukada, 1994
) and that of
C. cydoniae
on
P. coccinea
(
Neal & Douglass, 1990
) and
C. dammeri
(
Braman & Pendley, 1993
) show high variability depending on the host plant. In addition, different lace bugs are adapted to different temperatures and in many cases, reports are provided only for one or a few temperatures, so that is difficult to establish sound comparisons. Taking this into account, it can be concluded that
M. unicostata
reared in black poplar has in general lower fecundities than species such as
S. pyrioides
on
Rhododendron mucronatum
(Blume) G. Don (
Neal & Douglass, 1988
),
C. cydoniae
on
P. coccinea
(
Neal & Douglass, 1990
) or
C. ciliata
on
P. × acerifolia
(
Ju
et al.
, 2011
) and similar ones to species such as
Corythucha associata
Osborn & Drake on
Prunus serotina
Ehrh. (
Horn
et al.
, 1980
),
Gargaphia torresi
Costa on
Gossypium hirsutum
L. (
Da Silva, 2004
),
Corythucha morrilli
Osborn & Drake on
Helianthus annuus
L. (
Rogers, 1977
) and
Ambrosia dumosa
(Gray) Payne (
Silverman & Goeden, 1979
),
Stephanitis pyri
(Fabricius) on
Pyrus malus
L. (
Aysal & Kivan, 2008
) or
Onchochila simplex
(Herrich-Schaeffer) on
Euphorbia esula
L. (
Pecora
et al.
, 1992
). For other species like
Corythucha juglandis
(Fitch) on
Juglans nigra
L. (
Vogt & McPherson, 1986
),
Leptopharsa heveae
Drake & Poor on
Hevea brasiliensis
Müll. Arg. (
Cividanes
et al.
, 2004
) or
Gargaphia sanchezi
Froeschner on
Phaseolus vulgaris
L. (
Van Schoonhoven
et al.
, 1975
) lower values of fecundity have been reported. In addition,
M. unicostata
has in general shorter preoviposition periods compared with
C. cydoniae
(
Neal & Douglass, 1990
;
Braman & Pendley, 1993
),
C. ciliata
(
Kim
et al.
, 1999
;
Ju
et al.
, 2011
),
G. torresi
(
Da Silva, 2004
),
S. pyri
(
Aysal & Kivan, 2008
) or
L. heveae
(
Cividanes
et al.
, 2004
), but they are longer than those reported for
S. pyrioides
(
Neal & Douglass, 1988
) at similar temperatures. With regard to the oviposition period, this is longer in
C. cydoniae
(
Neal & Douglass, 1990
) and
S. pyrioides
(
Neal & Douglass, 1988
) and shorter in
C. ciliata
(
Ju
et al.
, 2011
). In some cases, shorter oviposition periods are observed at high temperatures and longer at low temperatures, like in
G. torresi
(
Da Silva, 2004
) and
S. pyri
(
Aysal & Kivan, 2008
).
Regarding longevity,
S. pyrioides
and
C. cydoniae
live longer than
M. unicostata
(
Neal & Douglass, 1988
,
1990
), although shorter longevity has also been reported for
C. cydoniae
females in a different host plant (
Braman & Pendley, 1993
). Females of
C. ciliata
(
Kim
et al.
, 1999
;
Ju
et al.
, 2011
),
L. heveae
(
Cividanes
et al.
, 2004
) and
S. pyri
(
Aysal & Kivan, 2008
) show similar lifespan values than poplar lace bug females, but the longevity of males was shorter than for
M unicostata
. In the case of
G. torresi
, higher longevity has been reported at low temperatures and shorter at high ones for both males and females (
Da Silva, 2004
). In addition,
S. takeyai
(
Tsukada, 1994
) has longer or shorter longevity than
M. unicostata
, depending on the host plant. Moreover, the differences in longevity observed here between males and females have been also reported in
S. pyrioides
(
Neal & Douglass, 1988
),
C. cydoniae
(
Neal & Douglass, 1990
) and
G. torresi
(
Da Silva, 2004
). The higher energy requirements of the females due to egg production could explain these differences in longevity (
Da Silva, 2004
). However, for other species like
G. sanchezi
(
Van Schoonhoven
et al.
, 1975
),
C. ciliata
(
Ju
et al.
, 2011
),
L. heveae
(
Cividanes
et al.
, 2004
) or
C. morrilli
(
Silverman & Goeden, 1979
) no differences have been observed and even longer longevity is reported for females of
S. pyri
(
Aysal & Kivan, 2008
) or
C. morrilli
(
Rogers, 1977
;
Stone & Watterson, 1985
) compared to males. In the case of
S. takeyai
(
Tsukada, 1994
), differences in longevity between males and females depend on the host plant.
Reports on the life table parameters for tingids are scarce. The values obtained here for
M. unicostata
range from 0.070 at 22°C to 0.194 at 34°C, which are approximately similar to those reported for other species. Thus, for
C. ciliata
, the intrinsic rate of natural increase (r
m
) ranges from 0.048 at 18°C to 0.170 at 28°C (
Kim
et al.
, 1999
); for
S. pyri
, values from 0.057 at 20°C to 0.129 at 32°C have been cited (
Aysal & Kivan, 2008
); and in
C. associata
, the capacity for increase (r
c
, an approximate value of r
m
(
Laughlin, 1965
)) varies from 0.083 after overwintering to 0.141 in the early summer (
Horn
et al.
, 1980
). In the case of
M. unicostata
, the temperature that provided the greatest r
m
was 34°C despite the greatest fecundity was recorded at 28°C. This is because this population parameter considers not only the overall egg production but also other parameters like the oviposition rate (daily fecundity) (Table 1) and the speed of development from egg to adult (
Sánchez-Ramos
et al.
, 2015
) that were the greatest at 34°C. This is similar in
S. pyri,
for which the greatest fecundity was observed at 26°C whereas the greatest r
m
was obtained at 32°C (
Aysal & Kivan, 2008
). However, in the case of
C. ciliata
, both the maximum fecundity and r
m
were obtained at the same temperature (28°C) (
Kim
et al.
, 1999
).
The results obtained here contribute to increase the existing knowledge about the effect of temperature on reproduction and population increase of lace bugs and could be used to forecast risk situations when environmental conditions approach to those more suitable for the development of the populations of
M. unicostata
. In addition, this information would be helpful to predict the importance of this pest in a climate change scenario, which would allow this insect to colonize other crops and growing areas different to those already exploited. Data indicate that this pest has a very high potential to invade areas located north of its present distribution area in the Mediterranean basin and south of its recently discovered new distribution area in Canada. On the other hand, increasing temperatures in southern areas does not probably have an effect on populations of
M. unicostata
. Therefore, it is possible that the distribution area of this insect will widen in the near future.