Effect of temperature on the development of Eotetranychus hirsti (Tetranychidae) on fig leaves

The fig spider mite, Eotetranychus hirsti is one of the major pests of fig trees worldwide. The effect of temperature on the developmental time and the survival rate of E. hirsti feeding on fig leaves was determined at six constant temperatures of 15, 20, 25, 30, 32 and 35 ░C. The total developmental time of females (from egg to adult emergence) at the above-mentioned temperatures was 41.29, 24.15, 16.95, 12.35, 10.21 and 10.67 days, respectively. The lower, optimal and upper developmental threshold (Tmin, Topt and Tmax, respectively) and thermal constant (K) of the pest were estimated by ordinary linear and Logan 6 nonlinear models. The lower temperature threshold (Tmin) and thermal constant (K) of the immature stages were estimated to be 9.86 ░C and 239.48 degree-days (DD), respectively. The Topt and Tmax were estimated to be 34.30 and 35.44 ░C, respectively. As the temperature increased from 15 to 30 ░C, the survival rate of immature stages increased from 33.33 to 70.59 %, then decreased and reached 54.91 % at 35 ░C. Temperature-dependent development data, thermal requirements and temperature thresholds can be used to predict the occurrence, number of generations and population dynamics of E. hirsti.


INTRODUCTION
. This phytophagous mite was reported for the first time by Hirst (1926) from India as Tetranychus fici.In 1940 Rahman and Sapra reported it from Pakistan under the same name.Later Pritchard and Baker (1955) transferred this species into the genus Eotetranychus.This mite had been reported from different regions of Iran mainly on fig trees (Beyzavi et al., 2013).In Iran, the common fig (Ficus carica L.) is attacked simul-taneously by E. hirsti and Rhyncaphytoptus ficifoliae Keifer (Diptilomiopidae) where it has negative effects on the fruit yield of this tree especially in summer.Eotetranychus hirsti is the key pest of fig trees in Iran and is mainly controlled by chemical pesticides (Khanjani and Hadad-Irani Nejad, 2006).
Temperature is the main abiotic factor that has profound effects on the life history of mites (Aponte and McMurtry, 1997;Liu and Tsai, 1998;Gotoh and Nagata, 2001;Jafari et al., 2012;Ullah et al., 2012;Lin, 2013;Bazgir et al., 2015) and many experiments have proven that temperature plays a crucial role in developmental rates of arthropods (Shi and Ge, 2010).Temperature sets the limits of biological performance in arthropods; the critical temperatures (T min , T opt and T max ) can be considered for all major life processes, where within a specific range, a temperature change results in a proportional rise or fall of the rate of any given process (Roy et al., 2002).Developmental rate, which is zero at the T min , increases with increasing temperature and reaches its peak at the optimum temperature and then decreases rapidly as the higher threshold is approached (Roy et al., 2002).The relationship between temperature and developmental rate is approximately linear at moderate temperatures and curvilinear near the extremes (Wagner et al., 1984).Knowing the temperature requirements of the various life stages of a target species is an important instrument in forecasting its potential distribution and population dynamics.The ability of a pest to develop at different temperatures determines to a large extent its survival under different climatic conditions, which is important in predicting pest outbreaks (Ullah et al., 2012).
Previous to this study, the biology of E. hirsti was studied at 30 °C by Daneshnia et al. (2013), but there is no available data regarding the effect of different temperatures on immature development and survival rate of this mite.Therefore, the aim of this study was to investigate the effect of broad range of temperatures on development and survival rate of E. hirsti immature stages in a laboratory study.

MATERIALS AND METHODS
All experiments were carried out in the Entomology laboratory at the Department of Plant Protection of Lorestan University, Khorramabad, Iran.

Laboratory experiments
The duration of immature stages of E. hirsti was measured at six constant temperatures: 15, 20, 25, 30, 32 and 35 ºC, with a relative humidity of 50 ± 5 % and a photoperiod of 16:8 (L:D) h.All experiments were performed using arenas consisting of a piece of fig leaf (4 cm in diameter), placed upside down on a water saturated foam mat covered with moist filter paper, inside plastic Petri dishes (6 cm in diameter) with a hole in their center (0.5 cm in diameter).Leaf discs were made with fresh fig leaves.Before the release of the mites, all unwanted organisms were removed from leaves by thoroughly brushing them and by examining under binocular microscope.To keep the freshness of leaves and prevent the escape of mites, the margins of fig leaves were covered with strips of moist cotton.The lids of Petri dishes had a big hole that was covered with fine mesh for ventilation.Each experimental arena was placed in a larger Petri dish (9 cm in diameter) filled with water.Plastic Petri dishes were kept in four incubators (Jal Tajhiz Company, Iran), which were able to control the RH, temperature and photoperiod.For measuring the immature development time of E. hirsti the 150 gravid females were transferred to 60 experimental arenas.After 12 h, the females and extra eggs were removed, and only one egg was kept on the detached fig leaf in each arena.Sixty eggs were used as a cohort at each temperature.Developmental time of all immature stages from egg to adult was checked daily.The presence of an exuvium was the criterion for successful moulting to the next stage.The mites were transferred to new arenas every three or four days.To determine the duration of the immature stages and survival rate of E. hirsti, inspections have been carried out every 12 h under a binocular microscope until the mites reached to adult stage.

Model evaluation
The reciprocal of developmental time in days is denoted as developmental rate.These rates are used in linear and non-linear models.An ordinary linear model was used to predict the developmental rate and estimate the lower temperature threshold (T min ) and thermal constant ( K) of E. hirsti.The T min calculated as -a/b, where a and b are determined by the following linear regression model: where R is the development rate (days), T is the temperature (°C), a and b are the regression coefficients.K is calculated as 1/b (Campbell et al., 1974;Huffaker et al., 1999).
The result of 35 °C was excluded in the linear model due to being out with the linear portion of developmental rate.This omission for the correct estimation of the T min is necessary (De Clerq and Degheele, 1992;Ikemoto and Takai, 2000;Jafari et al., 2012).
A Logan 6 nonlinear model was used to describe the relationship between temperature and developmental rate and to estimate the upper temperature threshold (T max ) and optimum temperature threshold (T opt ) for immature stages of E. hirsti (Logan et al., 1976).The following model, derived by Logan et al. (1976), was used to describe the relationship between the developmental rate of immature stages of E. hirsti and temperature:

∆T
where T is the rearing temperature (°C), Ψ is the maximum developmental rate, ρ is a constant defining the rate at optimal temperature, T max is the lethal maximum temperature, and ∆T is the temperature range over which physiological breakdown becomes the overriding influence thresholds (Logan et al., 1976;Huffaker et al., 1999).This model was chosen because it accurately described the developmental rates for other tetranychid mites (Logan et al., 1976;Bonato et al., 1990;Bounfour and Tanigoshi, 2001).

Data analyses
A Students t-test was used to determine the significant difference between the duration of immature stages of males and females (P < 0.05).The influence of temperatures on immature developmental time of E. hirsti was analysed using a one way analysis of variance (ANOVA).When a significant difference was detected, the means of developmental time were compared using a Duncan multiple ranges test (P < 0.05).The ANOVA and mean comparisons were carried out using the SAS software (Pros GLM, SAS Institute, 2003).The temperature dependent models were analysed using SigmaPlot Software 2001 (SAS Institute, 2007).

Ordinary linear model
The lower temperature threshold (T min ) and thermal constant ( K) estimated by the ordinary linear model for each developmental stage of E. hirsti females are presented in table 2. The T min values ranged from 7.58 to 11.16 °C.The estimated T min for overall immature stages was 9.86 °C.The thermal constant (K) for whole immature stages of E. hirsti was 239.48 DD.The curve of the ordinary linear model, which was fitted to the developmental rate of overall immature stages of E. hirsti, is depicted in figure 1.According to R 2 values, the linear model showed an acceptable fit to the developmental rate of various immature stages of E. hirsti (Table 2).
The Logan 6 model was fitted appropriately to our data.The curve of the Logan 6 model, which was fitted to the developmental rate of overall immature stages of E. hirsti is shown in figure 1

Survival rate of immature stages
Table 3 shows the survival rate of immature stages of E. hirsti at the temperatures tested.The survival rate of immature stages ranged from 33.3 % at 15 °C to 70.59 % at 30 °C.The lowest survival rate was for egg stages at all temperatures tested.The lowest and highest survival rate of eggs was recorded at 15 °C (66.67 %) and 30 °C (85.29 %), respectively.

DISCUSSION
These are the first data showing the influence of temperatures on the developmental times of the phytophagous mite Eotetranychus hirsti.This study shows that the developmental rate of the E. hirsti increases with temperature within a certain suitable range (15 -32 °C) with 35 °C causing a decrease in its developmental rate.In agreement with this finding, it is well known that the relationship between temperature and developmental rate in in-  sects is linear over most of the normal operating, middle range of temperature, but becomes sigmoid over the whole temperature range that permits development (Andrewartha and Birch, 1954;Jafari et al., 2012;Zahiri et al., 2012).Similar trends for some tetranychoid mites were reported (Liu and Tsai, 1998;Roy et al., 2003;Ullah et al., 2012;Lin, 2013;Riahi et al., 2013).
In this research the temperature dependent development of the Iranian population of E. hirsti under the broad range of temperatures generally prevailing in this region was studied and its temperature thresholds determined.This study also revealed that the immature developmental period of E. hirsti is strongly affected by temperature.The developmental periods of E. hirsti are longer that those reported for some species of Eotetranychus.The immature developmental time of Eotetranychus hicoriae (McGregor) at 21.1, 26.7 and 32.2 °C was 24.3, 11.1 and 8.5 days, respectively (Jackson et al., 1983).The duration of immature stages of Eotetranychus carpini borealis (Ewing) was 27.90, 18.4, 14.90 and 12 days at 15, 20, 25 and 30 °C, respectively (Bounfour and Tanigoshi, 2001), which are shorter those observed in our study.Grissa-Lebdi et al. (2002) found an average developmental period of 13.7 and 14.5 days for two strains of Eotetranychus pruni (Oudemans), measured at 24 °C, that are shorter than our finding at 25 °C (16.95 days).The duration of immature stages of Eotetranychus willamettei McGregor was 34.05, 15.43, 12.55 and 10.8 days, at 15, 22, 25 and 28 °C, respectively (Stavrinides et al., 2010).However, the developmental time of E. hirsti at 30 ± 1°C, 60 ± 10 % RH and 12: 12 h (dark: light) photoperiod was 11.56 days (Daneshnia et al., 2013) which is close to our result at the same temperature (12.35 days).This small difference could be explained by a difference in experimental conditions (RH and photoperiod).
Although in the present study the duration of whole immature stages of females was longer than of males, there was no statistically significant difference between the developmental times of females and males.A similar result was reported for Oligonychus mangiferus (Rhaman and Sapra) by Lin (2013).
The importance of critical temperatures and thermal budgets for understanding the phenology of an arthropod has long been recognized (Luypaert, 2014).Lower developmental thresholds and thermal constants are not only good predictors of the timing of life events, but are also very useful indicators for the potential distribution of the pest (Campbell et al., 1974).Our study showed that a total of 239.48 degree-days above the threshold temperature was required for E. hirsti to complete development from egg to adult, which was higher than 113.3 DD for Eotetranychus populi (Koch) (Su Xugen et al., 1996), 136.43 DD for T. urticae (Riahi et al., 2013) and 200 DD for Oligonychus perseae Tuttle, Baker and Abbatiello (Aponte and McMurtry, 1997).In contrast, the reported value of K for E. carpini borealis (379.8DD) (Bounfour and Tanigoshi, 2001) is greater than our finding.
The T min for E. hirsti is estimated to be 9.86 °C, which is lower than 10.50 °C for E. Willamettei (Stavrinides et al., 2010) and 13.79 °C for T. urticae (Riahi et al., 2013).Conversely, Bonato et al. (1990) obtained 7 °C for Eotetranychus carpini (Oudemans) and Bounfour and Tanigoshi (2001) reported 2.29 °C as the T min for E. carpini borealis, which is lower than our finding.The reported T min for the overall immature stages of E. populi was 9.64 (Su Xugen et al., 1996) that is relatively close to our result.
The reported T opt for Tetranychus macfarlanei Baker & Pritchard (24.4 °C) (Ullah et al., 2012) and Bryobia rubrioculus Scheuten (29.3 °C) (Javadi Khederi and Khanjani, 2014) are lower than our result (34.30°C).The T max for the total immature stages of Tetranychus tumidus Banks was determined as 35.2 °C by Liu and Tsai (1998), which is close to our finding (35.44 °C).But, the reported T max for E. willamettei was 31 °C (Stavrinides et al., 2010) that is lower than our finding.
In addition to temperature, another environmental factor that greatly influences life table parameters and population dynamics of mites is relative humidity (RH).Some research discusses the relation between the RH and development of tetranychid mites (such as Kramer and Hain, 1989;Bonato et al., 1995;Abou-Awad et al., 2001), but there is no information about the effect of RH on the development of E. hirsti.Additional studies on the effects of relative humidity on the development of E. hirsti can further increase our knowledge to forecast its population dynamics in cropping systems.Finally, our findings suggested that the examined population of E. hirsti was adapted to the higher temperatures for the development of immature stages.This finding is confirmed by the distribution of this pest that is restricted to tropical zones in Iran.
The fig spider mite, Eotetranychus hirsti Baker & Pritchard (Acari: Tetranychidae) is an important pest of fig trees in Iran and other world fig growing areas

Fig
Fig leaves infested by E. hirsti were collected from fig orchards in Veisian region, Lorestan province, Western Iran, during summer 2014 and transferred to the laboratory.Colonies of E. hirsti, were reared on fig leaves at 27 ± 1 °C, 50 ± 5 % RH and a photoperiod of 16:8 (L:D) h.E. hirsti were maintained under laboratory conditions after two generations and then used for experiments.

TABLE 1 :
Mean (± SE) developmental times (days) of Eotetranychus hirsti at six constant temperatures fed on fig leaves.
Means followed by different letters within the same raw for each sex are significantly different (P < 0.05, Duncan's test).

TABLE 2 :
The estimated lower temperature threshold ( T min ), thermal constant ( K) and the regression equation for various immature stages of Eotetranychus hirsti females fed on fig leaves by ordinary linear model.
FIGURE 1: Fitting ordinary linear and Logan 6 nonlinear models (lines) to observed values of the development rate for overall immature stages of Eotetranychus hirsti (dots) reared on fig leaves at six temperatures tested.

TABLE 3 :
The survival rate of Eotetranychus hirsti immature stages at six constant temperatures fed on fig leaves.