Introduction
The Khapra beetle, Trogoderma granarium Everts (Coleoptera: Dermestidae), is considered as one of the most destructive pests of cereals and other stored products in many parts of the world (Stuart et al., Reference Stuart, Barak and Burkholder1994; Ahmedani et al., Reference Ahmedani, Haque, Afzal, Aslam and Naz2009). The import restrictions are supported by the fact that feeding by Khapra beetle larvae reduces the quality, grade, and weight of grain (Burges, Reference Burges2008). It is a destructive feeder and can cause enormous losses (Banks, Reference Banks1977). The barley (Hordeum vulgare L.) is considered a major daily animal feed in the world, as well as an important source of the required carbohydrates for most of the plant-feeding animals (Khodabandeh, Reference Khodabandeh2003; Neelhirajan et al., Reference Neelhirajan, Karunakram, Jayas and White2007).
In recent years, pest control of stored products (including T. granarium) is undergoing a rapid change from an insecticide-based approach to an integrated management. Host plant resistance is an alternative approach for pest management, as it is both economically and environmentally acceptable (Golizadeh & Razmjou, Reference Golizadeh and Razmjou2010). Evaluating the resistance of various cultivars and crop species to pests may offer useful information about their suitability or unsuitability for the target pest species (Tsai & Wang, Reference Tsai and Wang2001; Golizadeh et al., Reference Golizadeh, Abedi, Borzoui, Golikhajeh and Jafary2016). The appropriate use of resistant varieties in pest management techniques requires knowledge of life table and biological parameters of the pests (Golizadeh et al., Reference Golizadeh, Esmaeili, Razmjou and Rafiee-Dastjerdi2014; Hosseininejad & Naseri, Reference Hosseininejad and Naseri2015; Golizadeh & Abedi, Reference Golizadeh and Abedi2016).
Host-plant resistance allows the plants to avoid, tolerate or recover from the effects of insect pest invasion (Tingey, Reference Tingey, Miller, Miller and Berenbaum1986; Panda & Khush, Reference Panda and Khush1995). Growth, development, and reproduction of insect populations strongly depend on the quality and quantity of the food consumed by them (Scriber & Slansky, Reference Scriber and Slansky1981). Poor quality host plants may reduce insect survival rate, size or weight, adult longevity, and fecundity potential, which increase the developmental time of insects (Sarfraz et al., Reference Sarfraz, Dosdall and Keddie2006). Plant species and varieties differ greatly in suitability as host for a specific pest in terms of survival, development, and reproductive rates (Sedaratian et al., Reference Sedaratian, Fathipour and Moharramipour2009). In addition, insect growth is directly attributable to nutrient input (Hwang et al., Reference Hwang, Liu and Shen2008) and the importance of maintaining the balance of nutrients is clear in most insects (Borzoui & Naseri, Reference Borzoui and Naseri2016). Nutritional regulation by an insect represents the integrated outcome of a highly complex set of interacting processes (Rahimi Namin et al., Reference Rahimi Namin, Naseri and Razmjou2014).
Recently, Golizadeh & Abedi (Reference Golizadeh and Abedi2016) evaluated the performance of T. granarium in response to feeding on different wheat cultivars and noted that Gaskojen and Kouhdasht cultivars are relatively susceptible and resistant cultivars for feeding of T. granarium, respectively. Naseri & Borzoui (Reference Naseri and Borzoui2016) studied the life history and nutritional physiology of T. granarium fed with different wheat cultivars and reported that Parsi and Morvarid cultivars were the most unsuitable cultivars for the development and feeding of this pest. Seifi et al. (Reference Seifi, Naseri and Razmjou2015) investigated the nutritional indices and digestive enzymatic activity of this pest on some barley cultivars and demonstrated that Bahman cultivar was the most unsuitable host for T. granarium.
In spite of the economic importance of T. granarium on barley cultivars in Iran and many tropical and subtropical countries of the world (Hosseininaveh et al., Reference Hosseininaveh, Bandani, Azmayeshfard, Hosseinkhani and Kazemi2007; Seifi et al., Reference Seifi, Naseri and Razmjou2015), no published information is available about the population growth parameters of this pest on various barley cultivars. Demographic parameters along with nutritional properties are appropriate indices for the evaluation of susceptibility and resistance of feeding diets. Therefore, our objective of this study was to assess the response of T. granarium to feeding on ten barley cultivars. Findings of this research could contribute to IPM (Integrated Pest Management) programs of T. granarium on barley.
Materials and methods
Barley cultivars
Seeds of ten barley (H. vulgare) cultivars, including Abidar, Bahman, Line20, Line22, Line30, Lisuei, Lokht11, Makuyi, Sahand, and Sahraa were obtained from Agricultural and Natural Resources Research Center of Ardabil, Iran. These cultivars are commonly grown in barley-growing plantations in Iran. The grains were broken and were then used for the experiments.
Insect rearing
The initial population of T. granarium larvae was collected from the stored rice seeds from Karaj, Iran. They were separately reared on seeds of each barley cultivar and maintained at 33 ± 1°C, relative humidity of 65 ± 5%, and an approximate photoperiod of 14:10 (L:D) h, as described by Borzoui et al. (Reference Borzoui, Naseri and Rahimi Namin2015). The breeding cage was a clear cylindrical cage (25 cm in diameter and 20 cm in height) with two holes in its cap covered by fine mesh gauze containing 200 g of each cultivar grains. Before beginning the experiments, the population of T. granarium was reared for three generations on each cultivar and the individuals of the fourth generation were used to conduct the following experiments.
Development time, survival rate, and body weight
All the experiments on each barley cultivar were carried out under the following laboratory conditions: 33 ± 1°C, relative humidity of 65 ± 5 %, and a photoperiod of 14:10 (L:D) h. To obtain T. granarium eggs of the same age on each barley cultivar, 25 male–female pairs of the newly emerged beetles (reared from the same host plant) of the fourth laboratory generation were transferred to oviposition containers. The adult stage of males and females was distinguished based on size and especially antennae features (Bagheri Zenouz, Reference Bagheri Zenouz1997; Kulkarni et al., Reference Kulkarni, Gupta, Kataria and Sathyanarayana2015). The oviposition container consisted of a clear Plexiglas container (20 × 10 × 5 cm3). In order to assess the developmental time and survival rate, 61-day-old eggs of T. granarium were transferred into Petri dishes (diameter of 6 cm and depth of 2 cm) containing the related barley cultivar grains and were used in the experiments. The eggs were inspected daily until hatching time. Once hatching, each newly hatched larva of T. granarium was encoded and released into each Petri dish (diameter of 6 cm and depth of 2 cm). Petri dishes were monitored daily until the preimaginal stages of T. granarium completed development or died. Pupal weight of T. granarium was measured 24 h after pupation on each barley cultivar. Thus, developmental performance of the individuals, duration of immature stages, and their survival were recorded. The weights of 1-day-old female and male T. granarium adults reared on the corresponding barley cultivar were recorded.
Adult life history parameters
One-day-old, virgin female and male T. granarium adults reared on the corresponding barley cultivar were paired. Each pair was transferred into a plastic tube fitted with mesh lids (2 cm diameter and 5 cm height). Experimental tubes were checked daily and the number of T. granarium eggs deposited in each tube was recorded. In this order, each pair was daily transferred to the new tube provided with fresh food and the number of eggs laid by the adults was recorded. We evaluated the fecundity of 25 adult pairs (25 replicates) on each barley cultivar. Moreover, all the eggs collected in this study were maintained for 15 days to determine the number of larvae emerging from these eggs (hatching rate).
Life table parameters
The development time, immature survival rate, and adult fecundity were used for calculation of life table parameters. The life table parameters, age-specific survival rate (l x ), and fecundity (m x ) were calculated for T. granarium fed on ten barley cultivar grains. For this purpose, for each barley cultivar, age-specific survival and age-specific fecundity were used to calculate the intrinsic rate of natural increase (r m) using the following formula (Birch, Reference Birch1948; Carey, Reference Carey1993):
$$\mathop \sum \limits_{x = 1}^{\rm \omega} {\rm e}^{ - r_{\rm m} x} l_x m_x = 1,$$
where ω is the oldest age class, and l x and m x are the proportion of surviving individuals at age x and the number of females offspring per female in the age interval x, respectively. In addition, net reproductive rate (R 0), mean generation time (T), finite rate of increase (λ), and doubling time (DT) were calculated based on Carey's formulae (Carey, Reference Carey1993).
Nutritional indices experiment
Nutritional indices, including the larval weight gain and food consumption by the larvae, were calculated gravimetrically according to the method of Waldbauer (Reference Waldbauer1968). Nutritional indices were evaluated on the basis of dry weight. The weight of each barley cultivar was measured and the cultivar was then transferred into plastic plates (6 cm in diameter and 2 cm depth) for larval feeding. The initial weight of the newly ecdysed sixth-instar larvae was recorded and it was then reared on barley cultivar. Final larval weight and remnant food was recorded until feeding stopped and the prepupal stage was reached. The initial fresh food and the food remnant at the end of each experiment were weighed. The quantity of food ingested was calculated by subtracting the weight of remaining food at the end of each experiment from the weight of fresh food supplied. To obtain the dry weight percentage of food and larvae, 20 specimens for each were weighed, oven-dried at 60°C for 48 h, and then weighed again (Sartorius AG Germany GCA803S, d = 0.001 ct). Nutritional indices of T. granarium larvae were calculated using formulae described by Waldbauer (Reference Waldbauer1968): conversion efficiency of ingested food (ECI) = P/E; relative consumption rate (RCR) = E/A × T; and relative growth rate (RGR) = P/A × T, where A represents the mean dry weight of larvae over unit time (mg), E the dry weight of food consumption (mg), P the dry weight gain of larvae (mg), and T the duration of feeding period (day).
Physical and biochemical traits of barley cultivars
In order to understand any possible correlation between the most important life table parameter and nutritional indices with some physical and biochemical traits of barley cultivars, the percentage of humidity, grain hardiness, and protein content of barley cultivars was determined. Percentage of humidity and hardiness index of barley cultivars were quantified according to the method of AACC (American Association of Cereal Chemists, 1996). Also, protein concentrations of the tested cultivars were measured using BSA (bovine serum albumin) as a standard (Bradford, Reference Bradford1976).
Statistical analysis
All the data were examined for normality by Kolmogorov–Smirnov test using the SPSS v. 16.0 statistical program (SPSS, 2007). Differences in r m, R 0, T, DT, and λ values were tested for significance using the Jackknife procedure (Maia et al., Reference Maia, Alferdo and Campanhola2000). Jackknife pseudo-values computed for life table parameters on ten barley cultivars were analyzed by one-way ANOVA (SAS Institute, 2002) (Meyer et al., Reference Meyer, Igersoll, Mac Donald and Boyce1986; Maia et al., Reference Maia, Alferdo and Campanhola2000). Additionally, developmental data, weights of sixth-instar larvae and pupa, as well as nutritional indices were analyzed by ANOVA with mean separation at 5% level of significance by Tukey test (SPSS, 2007). Correlation between some important life history parameters with physical and biochemical traits of barley cultivars were evaluated through Pearson Correlation test. A dendrogram of barley cultivars based on life table parameters and nutritional indices of T. granarium on tested barley cultivars was constructed by Ward's method using SPSS.
Results
Development time, survival rate, and body weight
The effect of various barley cultivars on developmental time of T. granarium is given in figs 1 and 2. Significant differences were observed in the duration of egg (F = 5.52; df = 9, 469; P < 0.0001), larval (F = 13.37; df = 9, 416; P < 0.0001), and pupal (F = 4.53; df = 9, 385; P < 0.0001) stages between the tested cultivars. Moreover, the immature development time (from egg to adult emergence) was different among the cultivars (F = 12.72; df = 9, 385; P < 0.0001). Regarding the results, T. granarium showed the longest egg incubation period on Line30 cultivar, while the shortest period was on Line20 cultivar. The longest larval period was detected on Line22 and Line30 cultivars and the shortest on cultivar Makuyi. The pupal period was longer on Lokht11 cultivar and shorter on Lisuei cultivar. Also, the shortest development time was on Makuyi cultivar (52.4 ± 0.6 days) and the longest was on Line22 cultivar (67.8 ± 0.78 days) (fig. 2). The descending order of immature survival rates were 71.7, 70. 0, 68.3, 66.7, 65.0, 61.7, 61.7, 60.0, 58.3, and 58.3 % on Makuyi, Lisuei, Line20, Sahraa, Sahand, Bahman, Line30, Lokht11, Abidar, and Line22 cultivars, respectively. The age-specific survival rate (l x ) of T. granarium on different cultivars is shown in fig. 3. Age-specific survival rate generated similar curves among the cultivars. However, female adults were long-lived on Lisuei cultivar and l x curve was more extended on this cultivar.
Egg incubation and larval periods (mean ± SE) of Trogoderma granarium reared on various barley cultivars (the number in parenthesis is sample size).
Pupal period and total developmental time (mean ± SE) of Trogoderma granarium reared on various barley cultivars (the number in parenthesis is sample size).
Age-specific survival rate (l x ) and age-specific fecundity (m x ) of Trogoderma granarium on ten barley cultivars.
Figures 4 and 5 indicate the weight of sixth-instar larvae, pupa, female, and male adults of T. granarium when reared on various barley cultivars. According to the obtained results, the weight of sixth-instar larvae differed from 4.49 mg on Line30 to 5.96 mg on Line20 cultivar (F = 10.31; df = 9, 416; P < 0.0001). Moreover, pupal weight showed a significant difference among the tested barley cultivars (F = 26.12; df = 9, 385; P < 0.0001). The heaviest weights were measured on Makuyi cultivar and the lightest ones were recorded on Lokht11 (fig. 4). Also, female (F = 14.68; df = 9, 249; P < 0.0001) and male (F = 5.65; df = 9, 249; P < 0.0001) adult weights differed among the cultivars with female and adult weights being the highest on Line20 and Lisuei cultivars, respectively (fig. 5).
The weight (mean ± SE) of sixth-instar larvae and pupa of Trogoderma granarium reared on various barley cultivars (the number in parenthesis is sample size).
The weight (mean ± SE) of female and male adults of Trogoderma granarium reared on various barley cultivars (sample size on each cultivar was 25).
Adult life history parameters
The results of total fecundity (number of eggs laid per female) and daily fecundity per female of T. granarium are shown in table 1. The total fecundity significantly differed among the tested barley cultivars (F = 132.10; df = 9, 249; P < 0.0001). The highest fecundity was recorded for females developed from larvae fed on Lisuei cultivar, while the fecundity of T. granarium was not different among Abidar, Bahman, Line22, Line30, Lokht11, and Sahand cultivars. In addition, the mean number of eggs produced per female day−1 of T. granarium was affected by the tested cultivars (F = 44.42; df = 9, 249; P < 0.0001) (table 1). The egg-hatching rate was the highest on Lisuei cultivar (F = 2.94; df = 9, 249; P < 0.01). Age-specific fecundity rates of T. granarium on tested cultivars are shown in fig. 3. The width of the m x peak (i.e., the fecundity period) was higher on Lisuei, Line20, and Makuyi than the other cultivars.
Biological parameters of Trogoderma granarium adults reared on various barley cultivars.
Mean values in a column followed by different lowercase letters are significantly different on the basis of ANOVA with Tukey test (P < 0.05).
Various barley cultivars had a significant influence on the longevity of female (F = 17.46; df = 9, 249; P < 0.0001) and male (F = 8.23; df = 9, 249; P < 0.0001) adults of T. granarium. The shorter and longer longevity of female adults of T. granarium were observed on Bahman (7.60 days) and Makuyi (10.64 days), as well as Sahraa (10.28 days) cultivars, respectively. Also, a longer longevity of male adults was observed on Line20 cultivar (10.64 days), while the shorter longevity was on cultivar Line22 (7.92 days) (table 1).
Life table parameters
The population growth parameters of T. granarium reared on various barley cultivars are shown in table 2. The population reared on Lisuei had a higher net reproductive rate (R 0 value) and those reared on Line22 had a lower R 0 value (F = 133.05; df = 9, 249; P < 0.0001). The intrinsic rate of increase (r m ) of T. granarium was found to be significantly different (F = 24.49; df = 9, 249; P < 0.0001) depending on the barley cultivars on which they were reared. The r m values ranged from 0.035 to 0.059 female progenies per female day−1 on the tested barley cultivars (table 2). The value of the intrinsic rate of increase (r m) was highest when T. granarium was reared on Makuyi cultivar. The lower r m value resulted from rearing the T. granarium on Line22. The variations in finite rate of increase (λ) were similar to the intrinsic rate of increase and the former parameter was significantly influenced by different cultivars (F = 24.13; df = 9, 249; P < 0.0001). The mean generation time (T) of T. granarium was also different among the tested cultivars (F = 3.28; df = 9, 249; P = 0.0009) with the cultivar Makuyi promoting the fastest generation times. Furthermore, significant differences were observed for T. granarium doubling time (DT) when reared on the studied barley cultivars (F = 14.81; df = 9, 249; P < 0.0001). The DT values on Line22 cultivar were relatively higher than on Makuyi cultivar (table 2).
The mean (±SE) life table parameters of Trogoderma granarium reared on various barley cultivars.
Mean values in a column followed by different lowercase letters are significantly different on the basis of ANOVA with Tukey test (P < 0.05).
Nutritional indices of larvae
Nutritional indices of sixth-instar larvae of T. granarium were significantly different on various barley cultivars (table 3). The higher weight gain of larvae (F = 4.22; df = 9, 49; P < 0.0007) was on Makuyi cultivar and those on Lokht11 had a lower weight gain. A higher value of ECI (F = 5.41; df = 9, 49; P < 0.0001) came from larvae that were fed on Makuyi cultivar. Similarly, the larvae fed on Makuyi cultivar showed a higher value of RCR (F = 6.87; df = 9, 49; P < 0.0001). In addition, a higher value of RGR (F = 6.65; df = 9, 49; P < 0.0001) was found on Makuyi cultivar, while the lower value was observed on Lokht11 cultivar.
The mean (±SE) nutritional indices of Trogoderma granarium larvae reared on various barley cultivars.
The mean followed by different letters in the same column are significantly different (Tukey test, P < 0.05).
ECI, efficiency of conversion of ingested food; RCR, relative consumption rate; RGR, relative growth rate.
Percentage of humidity, grain hardiness, and protein content of barley cultivars
The results of the percentage of humidity, hardiness index, and protein content of various barley cultivars are shown in table 4. Significant difference was observed in moisture content among the examined cultivars (F = 3.35; df = 9, 29; P = 0117) and humidity of grain was the highest in Makuyi cultivar. The grain hardiness index (F = 3.89; df = 9, 29; P = 0.005) significantly differed among the barley cultivars and the highest and lowest values were observed in cultivars Line22 and Makuyi, respectively. Similarly, the highest protein content (F = 384.14; df = 9, 29; P < 0.0001) was measured in cultivar Makuyi, whereas the lowest content was detected in cultivar Line22. Significant correlations were observed between life history parameters and physical and biochemical traits of barley cultivars (grain humidity, hardiness, and protein content) (table 5). Fecundity, net reproductive rate, and RCR were not significantly correlated with grain protein content. Both r m and RGR were negatively correlated with grain hardiness and positively correlated with grain humidity and protein content. A dendrogram based on life table parameters and nutritional indices of T. granarium on different barley cultivars is shown in fig. 6. Two separate main clusters (labeled A and B) are apparent in the dendrogram. The main clusters (A and B) were divided to two sub-clusters (A1, A2 and B1, B2, respectively).
Dendrogram of different barley cultivars based on life table parameters and nutritional indices of Trogoderma granarium reared on barley cultivars (two distinct clusters A (including subclusters A1 and A2) and B (including subclusters B1 and B2)).
Percentage of grain humidity, hardiness index and protein (mean ± SE) of various barley cultivars used for feeding of Trogoderma granarium.
Mean values in a column followed by different lowercase letters are significantly different on the basis of ANOVA with Tukey test (P < 0.05).
Correlation coefficients (r) of some life history parameters of Trogoderma granarium fed on different barley cultivars with percentage of grain humidity, hardiness index, and protein of various barley cultivars.
Discussion
Host-plant resistance can be a useful component of an integrated pest management system compatible with other methods of control (Sedaratian et al., Reference Sedaratian, Fathipour and Moharramipour2009). Variation in host plant and the quality of varieties are known to affect the development, survivorship, reproduction, and population growth of insects (Tsai & Wang, Reference Tsai and Wang2001). Several authors have studied the susceptibility and resistance of this pest on various stored crops and different cultivars (Saliheen, Reference Saliheen2005; Musa & Dike, Reference Musa and Dike2009; Borzoui et al., Reference Borzoui, Naseri and Rahimi Namin2015; Al-Iraqi et al., Reference Al-Iraqi, Dallal-Bashi and Al-Safar2015; Naseri & Borzoui, Reference Naseri and Borzoui2016). Resistance of wheat cultivars against T. granarium has been previously reported by Mohamed (Reference Mohamed2003), Rao et al. (Reference Rao, Sharma, Samyal and Tomar2004), Sayed et al. (Reference Sayed, Hirad and Abro2006), Ahmedani et al. (Reference Ahmedani, Haque, Afzal, Aslam and Naz2009), and Golizadeh & Abedi (Reference Golizadeh and Abedi2016). This study was intended to improve the existing knowledge about the life table parameters of T. granarium and to examine them with various barley cultivars. The resistant cultivars can be recommended to be grown in areas where the damage of T. granarium is high for protecting or at least delaying the infestation of barley by this pest. This study shows that different barley cultivars have a significant effect not only on the life history and population growth parameters of T. granarium, but also on the nutritional indices of this insect.
There were six larval instars on all the tested barley cultivars, which was one instar more than that reported by Golizadeh & Abedi (Reference Golizadeh and Abedi2016) on wheat cultivars. Moreover, the larval periods on barley cultivars were longer than those on wheat cultivars. The number of larval instars and the duration of larval stages could be influenced by differences in the host species (Naseri et al., Reference Naseri, Golparvar, Razmjou and Golizadeh2014). These results are in agreement with the findings of Bernays & Chapman (Reference Bernays and Chapman1994), Naseri et al. (Reference Naseri, Golparvar, Razmjou and Golizadeh2014), and Borzoui et al. (Reference Borzoui, Naseri and Rahimi Namin2015). Based on Borzoui et al. (Reference Borzoui, Naseri and Rahimi Namin2015), the prolonged larval period of T. granarium on unsuitable cultivars is meant to gain nourishment.
Variations in the duration of immature stages of T. granarium might be attributed to differences in the macronutrients, inhibitors, grain hardiness, and humidity of the tested wheat cultivars (Borzoui & Naseri, Reference Borzoui and Naseri2016; Golizadeh & Abedi, Reference Golizadeh and Abedi2016). The short lifespan and high fecundity of pests on host plants indicate greater suitability of those plants (Lorenzen et al., Reference Lorenzen, Balbyshev, Lafta, Casper, Tian and Sagredo2001; Hemati et al., Reference Hemati, Naseri, Ganbalani, Dastjerdi and Golizadeh2012). The shorter development time of T. granarium on Makuyi cultivar could be justified by the higher protein value and grain humidity in this cultivar. Moreover, the grain hardiness could be another factor affecting the development time and immature survival rates. The grain hardiness in Makuyi cultivar was lower than the other cultivars. The results regarding the larval and pupal period of T. granarium were in agreement with those achieved by Borzoui et al. (Reference Borzoui, Naseri and Rahimi Namin2015), who reported incubation periods of 56.57 and 5.55 days for T. granarium reared on barley, respectively.
It has been reported that body weight is associated with the quality and quantity of food and is one of the main biological indices of insect population dynamics (Li et al., Reference Li, Hill and Hartman2004; Liu et al., Reference Liu, Li, Gong and Wu2004). The lowest pupal weight of the Khapra beetle on Lokht11 cultivar indicated that the larvae fed on Lokht11 had the lowest fitness compared with those fed on the other barley cultivars. Moreover, our findings suggest that the larvae fed on Makuyi cultivar achieved more nutrients than those fed on other barley cultivars. Also, the consequences of feeding on suitable cultivars are reflected in the weight of female and male adults (table 2). The females reared on Line20, Makuyi, and Lisuei cultivars were heavier. The weight range of pupal T. granarium on various barley cultivars tested in the current study was lower than that reported by Seifi et al. (Reference Seifi, Naseri and Razmjou2015). Such discrepancy might be attributed to either genetic differences in populations or variations in the experimental conditions and cultivars used for feeding of this pest.
Average fecundity of T. granarium on barley cultivars (30.4 eggs) was lower than that reported by Musa & Dike (Reference Musa and Dike2009) on stored groundnut kernels (80.2 eggs), as well as Golizadeh & Abedi (Reference Golizadeh and Abedi2016) report on wheat cultivars (40.8 eggs). Food quality can affect both the realized fecundity and egg fertility of female insects (Mebarkia et al., Reference Mebarkia, Rahbe, Guechi, Bouras and Makhlouf2010). Regarding the larval and pupal weights, it is evident that the fecundity values can be correlated with the weight of sixth instar larva and pupa on respective cultivars (Daryaei et al., Reference Daryaei, Darvishi, Etebari and Salehi2007; Naseri & Borzoui, Reference Naseri and Borzoui2016).
The intrinsic rate of increase (r m) is the most important population parameter for the study of population dynamics because it includes age, sex ratio, survivorship, and fecundity (Birch, Reference Birch1948; Carey, Reference Carey1993). Thus, this parameter could be the most appropriate index to evaluate the suitability of different host plants to an insect (Razmjou & Golizadeh, Reference Razmjou and Golizadeh2010). In this research, the highest r m value of T. granarium was found on Makuyi and Lisuei, which was mainly the result of relatively higher fecundity, immature survival rate, and fast development on respective cultivars (table 4). The relatively higher net reproductive rate of T. granarium on Lisuei may cause a relatively higher fecundity of T. granarium. However, the shorter mean generation time, which is a result of short development time on Makuyi, led to the highest r m value on this cultivar compared with Lisuei cultivar. This could be a function of higher protein content in Makuyi cultivar. The higher reproductive performance of T. granarium on Lisuei cultivar was found despite its lower protein content, which could be attributed to other factors such as fat content or secondary metabolites.
The food consumption and larval weight gain on barley cultivar was lower than that reported by Seifi et al. (Reference Seifi, Naseri and Razmjou2015) for T. granarium on the barley cultivars. This inconsistency can be due to differences in the barley varieties and experimental conditions or differences in T. granarium population. The larvae reared on Lokht11 cultivar had the lower values of ECI, indicating that the larvae feeding on this cultivar were less effective in converting the ingested and digested food to biomass. Similar results of ECI value were reported by Borzoui et al. (Reference Borzoui, Naseri and Rahimi Namin2015) with T. granarium reared on barley. The duration of the feeding period is an effective factor in RGR and RCR values (Hemati et al., Reference Hemati, Naseri, Ganbalani, Dastjerdi and Golizadeh2012). In this study, RCR and RGR values were higher on Makuyi cultivar and Lisuei had the second rank. Our results indicated that the Makuyi cultivar was a high-nutrient food for the larvae and a shorter period of development time was needed to complete immature stages on this cultivar. The higher protein value and lower grain hardiness in Makuyi cultivar could be a reason for the higher nutritional indices on this cultivar.
In the present study, cluster analysis revealed that with respect to life table parameters and nutritional indices of T. granarium, different barley cultivars can be divided in four distinct classes: cluster A1, A2, B1, and B2. The grouping within each class might be due to a high level of physiological similarity of barley cultivars. Cluster A1 consisted of the Abidar, Line30, Sahand, and Lokht11 cultivars (partially unsuitable group) and cluster A2 consisted only of Line22 (most relative unsuitable group). Resistance in Line22 could be a result of the higher grain hardiness and the lower protein value. Cluster B1 consisted of Line20 and Sahraa cultivars (partially susceptible group) and cluster B2 consisted of Makuyi and Lisuei cultivars (most relative susceptible group).
In conclusion, the results of the life table parameters and nutritional indices of T. granarium reared on ten barley cultivars revealed that the two cultivars of Makuyi and Lisuei were the most suitable (least resistant) diets for T. granarium among the tested barley cultivars. The relatively shorter development time and the higher fecundity rate are reflected in the higher intrinsic increase rate of T. granarium on these cultivars. The higher intrinsic rate of increase along with higher nutritional indices on respective cultivars would result in a higher population growth that in turn should lead to higher subsequent infestations. In contrast, Line22 cultivar was relatively the least suitable (most resistant) barley cultivar for T. granarium and was less preferred by this pest. Information about the quality of barley cultivars and the way in which demographic parameters of T. granarium are affected by barley cultivars can help us understand the population dynamics and may assist in the development of better management programs for this pest. In the future studies, T. granarium response to digestive enzymatic activity is necessarily recommended for the management of this pest.
Acknowledgement
The work received financial support by the University of Mohaghegh Ardabili, which is greatly appreciated.
