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Changes in the survival rate and fecundity of the rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), at different magnesium concentrations in hydroponic culture
Crop Protection 30 (2011) 765e769
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Changes in the survival rate and fecundity of the rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), at different magnesium concentrations in hydroponic culture Jie Xu, Li-Ben Jiang, Jin-Cai Wu* School of Plant Protection, Yangzhou University, Yangzhou 225009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2010 Received in revised form 1 March 2011 Accepted 6 March 2011
The rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), is a leaf-feeding pest and the physical and chemical nature of the rice leaf affects its feeding, survival and fecundity. We examined changes in the survival rates of larvae and pupae, pupal weight and the number of eggs laid by adult females developed from a resistant variety (Huaidao 9) and a susceptible variety (Yangjing 9538) in hydroponic solutions containing different concentrations of magnesium (Mg). The concentration of Mg in the hydroponic solution significantly affected the survival rates of the larvae and pupae, the pupal weights and the numbers of eggs laid. Larval survival rates on the resistant variety were significantly higher for fourth instars exposed to 60 and 80 ppm Mg and pupae exposed to 20, 40, 60 and 80 ppm Mg than those for the control treatment (without Mg). For the susceptible variety, survival rates of first, second and third instars were significantly higher at high concentrations of Mg than those under control conditions. In addition, the number of eggs laid by adult females increased with increasing Mg concentrations, though there was a slight decrease at 80 ppm compared to 60 ppm. These results indicate that Mg is beneficial to the development, survival and fecundity of C. medinalis. They further suggest that high levels of nitrogen fertilizer in rice fields favour outbreaks of this pest because application of nitrogen elevates the chlorophyll content and Mg levels in the rice leaf. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Elemental magnesium Cnaphalocrocis medinalis Fecundity Survival
1. Introduction The rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), is one of the most notorious leaf-feeding pests of rice, Oryza sativa (L.) (Graminales, Poaceae), in subtropical countries (Heinrichs et al., 1985; Khan et al., 1988). The insect is a pest particularly in areas where modern varieties of rice are grown extensively (Mohandas, 1975). In China, outbreaks of the insect have occurred in recent years in the rice producing regions of southern China and in the middle and lower reaches of the Yangtze River (Zhu and Chen 1999; Yang et al., 2004). C. medinalis larvae feed on rice leaf blades, which results in a loss of economic output, particularly during the grain-filling stage (Jin, 1984). Over a typical crop cycle, the damage incidence forms a bell-shaped curve with the maximum number of damaged rice plants observed in the late reproductive and early ripening stages (Litsinger et al., 2006). Several cultivars or lines, including several wild rice species, have been screened since the 1970s for resistance to the rice-leaf * Corresponding author. Tel.: þ86 514 87979246; fax: þ86 517 87349817. E-mail address: [email protected] (J.-C. Wu). 0261-2194/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2011.03.008
roller damage (Heinrichs et al., 1985; Ramachandran and Khan, 1991). Investigations into the resistance mechanisms of wild rice have shown that volatile substances in plants and the arrangement of silica cells on leaf surfaces are related to resistance (Ramachandran and Khan, 1991). Wang et al. (2008) demonstrated that the resistance of rice to C. medinalis was significantly related to the silica and wax content of leaf surfaces. The population growth of C. medinalis is associated with rice varieties and nitrogen fertilizers (Zhang et al., 1995). Population trend indices between resistant and susceptible varieties were found to be significantly different (Xu et al., 2007). Populations of pests in rice fields with higher nitrogen levels were significantly higher than those in fields with low nitrogen levels (Zhang et al., 1995; Cheng et al., 2008). Nitrogen fertilizer can increase the chlorophyll content in rice leaves, which in turn increases magnesium (Mg) content because of the positive correlation between chlorophyll and Mg content. In addition, the Mg content of leaves is positively correlated with the level of damage to rice leaves (Xu et al., unpublished data). Mg is a major element in the chlorophyll pigments of leaves and plays a key role in plant development. Mg deficiency leads to a decline in chlorophyll content and the rate of photosynthesis (Wang et al.,
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2004; Yang et al., 2005). Therefore, Mg levels in soil affect the growth and health of plants. However, the relationship between the Mg content in leaves and the growth and survival of immature and subsequent fecundity of adult C. medinalis is not well understood. An investigation into the effect of Mg on these life-history traits of C. medinalis is important for understanding the population growth and resistance mechanisms of rice. The objective of the present study was to examine changes in the development and fecundity of C. medinalis by manipulating Mg concentrations in hydroponically cultured rice. 2. Materials and methods 2.1. Rice varieties and insects Two varieties of japonica rice, Huaidao 9 and Yangjing 9538 were used in these experiments. Huaidao 9 is known to be resistant to C. medinalis and Yangjing 9538 is known to be susceptible (Xu et al., 2007). Seeds were sown into cement tanks at an experimental farm of Yangzhou University. Six-leaf seedlings were selected from the tanks and the soil of the plant roots was then washed off with tap water. The seedlings were immediately transplanted into porcelain pots (13 cm high and 10 cm in diameter) containing Espino hydroponic solution (Mae and Ohira, 1981) with NH4NO3, NaH2PO4.2H2O, K2SO4, CaCl2, MgSO4.7H2O, MnCl2.4H2O, (NH4)6Mo7O24.4H2O, H3BO3, ZnSO4.5H2O, FeCl3, CuSO4.5H2O and citric acid. Six seedlings were planted in a pot as a hill. The hydroponic solution was replaced once a week and its pH value was adjusted daily to 5.0 with 1 N HCl or 1 N NaOH. Details of the cultivation method are described in Wu et al. (2003). Mg concentration gradient experiments were conducted seven days after seedlings were cultured in the standard hydroponic solution. Five gradient concentrations of Mg were set up using the following concentrations of MgSO4.7H2O: 0, 20, 40, 60 and 80 ppm. Other elements in the hydroponic solution were not changed. First instar larvae that had hatched from eggs laid by field-caught moths at the experimental farm of Yangzhou University were used for experiments. 2.2. Experiment For each rice variety, four potted plants at the tillering stage were covered with a cage (80-mesh) and served as one replicate for each Mg concentration. Ten first instar larvae were placed onto the leaves of potted rice using a small writing brush (brush5; Yangzhou Wantou Hubi Ltd. Co., Yangzhou, China). Each treatment and control (hydroponic solution without Mg) experiment was
repeated three times (i.e., 12 pots). Treated and control plants were placed in a greenhouse at a natural temperature (24e32 C) and a natural photoperiod (about 14 h of light and 10 h of darkness). The growth and development of the larvae was observed every two days by carefully opening folded leaves and counting the number of live larvae. The live larvae were then left on the folded leaves after the number was recorded. Pupae in the whole plants (sheaths or leaves) were collected after the larvae had developed to the pupal stage. Collected pupae were weighed and then put into Petri dishes with wet filter paper. Adult emergence was recorded daily. Two adults (a male and a female) were put into a glass jar (8 cm high and 5 cm in diameter) at a constant temperature of 24 2 C to lay eggs. The jars were covered with a black cloth during egg laying. A piece of absorbent cotton was dipped into 5% honeyewater and was put into the jar to provide food for the adults. The cotton was changed daily. Adult females laid eggs on the glass jar and the number of eggs laid per female was counted. Fecundity was measured from 14, 14, 27, 32 and 21 female moths that had been exposed to 0, 20, 40, 60 and 80 ppm Mg, respectively. 2.3. Statistical analysis Normal distribution and homogeneity of variance (using a Bartlett test) were tested before running analyses of variance (ANOVAs). A two-way ANOVA (using rice variety and Mg concentration as factors) was performed to analyse differences in the fecundity of adult females, the survival rates of larvae and pupae and the pupal weight of C. medinalis on two varieties of rice at different Mg concentrations. Multiple comparisons of means were conducted based on Fisher’s protected least significant difference (PLSD) test. In addition, regression equations were established to examine the relationship between the Mg concentration and the fecundity, survival rate and pupal weight. All analyses were conducted using the data processing system (DPS) of Tang and Feng (2002). 3. Results 3.1. Survival rates of larvae and pupae The effect of Mg on the survival rate of larvae varied, depending on the instars, Mg concentration and rice variety (Table 1). The data in Table 1 were analysed using a two-way ANOVA and showed that Mg concentrations significantly affected the survival rates of first through fifth instars but did not affect the survival rate of pupae (Table 2). The rice variety also affected the survival rates of the first and the fourth instars. The grand mean (the pooled mean of all Mg concentrations for each rice variety) of the survival rate of the first
Table 1 Survival rates of larvae and pupae on two rice varieties at different Mg concentrations in hydroponic solutions. Rice variety
Insect stadium
Mg concentration (ppm) 0
20
40
60
80
Mean
SE
Mean
SE
Mean
SE
Mean
SE
Mean
SE
Huaidao 9
First instar Secondethird instar Fourth instar Fifth instar Pupa
0.66a 0.91a 0.61a 0.84ab 0.63c
0.01 0.05 0.10 0.01 0.04
0.72a 0.86a 0.67b 0.83ab 0.74b
0.08 0.05 0.10 0.07 0.03
0.74a 0.86a 0.83ab 0.92a 0.84a
0.05 0.05 0.12 0.05 0.05
0.77a 0.95a 0.98a 0.91a 0.79ab
0.02 0.03 0.01 0.06 0.02
0.78a 0.81a 0.93a 0.73b 0.78ab
0.05 0.10 0.01 0.03 0.03
Yangjing 9538
First instar Secondethird instar Fourth instar Fifth instar Pupa
0.62b 0.78c 0.81a 0.83a 0.75a
0.02 0.01 0.05 0.12 0.07
0.60b 0.82bc 0.84a 0.94a 0.82a
0.05 0.02 0.05 0.05 0.01
0.65b 0.89ab 0.94a 0.95a 0.80a
0.02 0.06 0.06 0.004 0.05
0.80a 0.95a 0.95a 0.85a 0.73a
0.02 0.01 0.03 0.04 0.15
0.64b 0.94a 0.91a 0.80a 0.77a
0.03 0.02 0.07 0.03 0.06
Means SE followed by different letters within the same row are significantly different at the 5% significance level.
J. Xu et al. / Crop Protection 30 (2011) 765e769
28
Table 2 ANOVA of data of survival rates, pupae weights and the number of eggs laid (see Table 1 and Figs. 1 and 2). Source of variance
Df
F-value
P-value
Survival rate of first instar
Rice variety (A) Mg concentration (B) AB
1 4 4
18.7 8.6 3.1
0.0003 0.0003 0.03
Survival rate of secondethird instar
A B AB
1 4 4
Survival rate of fourth instar
A B AB
1 4 4
10.3 13.3 3.2
0.004 0.0001 0.03
Survival rate of fifth instar
A B AB
1 4 4
1.5 6.5 1.7
0.2 0.001 0.1
Survival rate of pupae
A B AB
1 4 4
0.4 2.4 1.8
0.5 0.08 0.1
Number of eggs laid
A B AB
1 4 4
101.3 36.1 11.6
0.0001 0.0001 0.001
Pupal weight
A B AB
1 4 4
17.9 40.6 4.9
0.0004 0.0001 0.006
0.0004 4.3 4.8
0.9 0.01 0.006
instars on Huaidao 9 was significantly higher than on Yangjing 9538. However, the grand mean of survival rate of the fourth instars on Yangjing 9538 was significantly higher than on Huaidao 9. In addition, there was a significant interaction effect between the Mg concentration and the rice variety on the survival rates of the first, second-third and fourth instars. For Huaidao 9, multiple comparisons of means for different Mg concentrations showed that the survival rate of fourth instars at Mg levels of 60 and 80 ppm was significantly higher than for those under the control treatment (without Mg) or at 20 ppm Mg (Table 1). Furthermore, the survival rate of the pupae at the four different Mg concentrations was significantly higher than that of the control. For Yangjing 9538, the survival rates of the first and second-third instars in the presence of high Mg concentrations were significantly higher than those observed under the control, but no significant differences were observed for the survival rates of the fourth and the fifth instars.
Huaidao 9 Yangjing 9538 Huaidao 9 Yangjing 9538
27
Pupa weight (mg)
Insect stadium
767
26 25 24 23 22 21
0
20
40
60
80
Mg concentrations (ppm) in the hydroponic solution Fig. 1. Weight of pupae developed from rice plants at different Mg concentrations. P ¼ 22.3742 þ 0.0684M 0.0005M2, R2 ¼ 0.7169, P > 0.05 for Huaidao 9; P ¼ 21.5338 þ 0.1684M 0.0015M2, R2 ¼ 0.8711, P < 0.05 for Yangjing 9538.
the Mg concentration was fitted to two quadratic equations (Fig. 2), which indicates that the number of eggs laid under exposure to 80 ppm Mg decreased to a certain extent in comparison with the number laid under exposure to 60 ppm Mg. 3.4. Development duration of larvae and pupae Mg levels in the hydroponic solution did not significantly influence the development of larvae and pupae (P > 0.05 for all effects, Table 3). 4. Discussion The present findings indicate that Mg has a significant effect on the survival and fecundity of C. medinalis. However, no significant differences were observed in the development of larvae and pupae. C. medinalis larvae tie the edges of the tips of rice leaves together with silken threads, thereby forming a protective cavity within which they feed by scraping mesophyll cells and epidermal tissue from leaf blades. Therefore, mesophyll cells are the nutrient source for the feeding insect. Mesophyll cells are mainly composed of
3.2. Pupal weight
3.3. Fecundity The Mg concentration, rice variety and the interaction of these two factors significantly influenced the number of eggs laid (Table 2, Fig. 2). Multiple comparisons indicated that the number of eggs laid by adult females on the two rice varieties at 40, 60 and 80 ppm Mg was significantly higher than it was for the control. At these concentrations, the number of eggs laid increased by 105, 141 and 108% for Huaidao 9 and by 128, 259 and 91% for Yangjing 9538, respectively. The relationship between the number of eggs laid and
Fecundity rate (egg numbers)
100
An ANOVA of the data in Fig. 1 showed that Mg concentration, rice variety and their interactions significantly affected pupal weight (Fig. 1, Table 2). Pupal weight increased with increasing Mg concentrations but had a slight decline at 80 ppm Mg; thus the relationship between the weight of pupae and the Mg concentration is apparently parabolic. On the two varieties of rice, the weights of the pupae were significantly greater at all Mg concentrations than they were in the control.
Huaidao 9 Yangjing 9538 Huaidao 9 Yangjing 9538
80
60
40
20
0
0
20
40
60
80
Mg concentrations (ppm) in the hydroponic solution Fig. 2. The number of eggs laid by adult female Cnaphalocrocis medinalis grown on rice plants at different Mg concentrations. F ¼ 12.4096 þ 0.5661M 0.0045M2, R2 ¼ 0.9718, P < 0.05; F ¼ 15.5333 þ 1.6319M 0.0148M2, R2 ¼ 0.6524, P > 0.05 for Yangjing 9538.
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Table 3 Developmental duration (days) of larvae and pupae on two rice varieties at different Mg concentrations in hydroponic solutions. Rice variety
Insect stadium
Mg concentration (ppm) 0
20
40
60
80
Mean
SE
Mean
SE
Mean
SE
Mean
SE
Mean
SE
Huaidao 9
First instar Second instar Third instar Fourth instar Fifth instar Pupa
2a 3.3a 2.6a 4.6a 6a 10a
0.0 1.1 1.1 1.1 2.0 2.0
2a 3.3a 4a 5.3a 6.6a 8.6a
0.0 1.1 0.0 1.8 1.1 2.3
2a 4a 3.3a 4.6a 6.6a 9.3a
0.0 0.0 1.1 1.1 1.1 2.3
2a 4a 2.6a 4.6a 5.3a 9.3a
0.0 2.0 1.1 1.1 1.1 1.1
2.6a 3.3a 2.6a 4.6a 5.3a 9.3a
1.1 1.1 1.1 1.1 2.3 1.1
Yangjing 9538
First instar Second instar Third instar Fourth instar Fifth instar Pupa
2.6a 3.3a 2.6a 3.3a 6.0a 10.0a
1.1 1.1 1.1 1.1 2.0 2.0
2.0a 2.6a 2.0a 3.3a 5.3a 10.0a
0.0 1.1 0.0 1.1 1.1 2.0
2.0a 2.0a 2.6a 2.0a 4.6a 10.6a
0.0 0.0 1.1 0.0 1.1 1.1
2.0a 3.3a 3.3a 2.6a 4.6a 8.6a
0.0 1.1 1.1 1.1 1.1 1.1
2.0a 3.3a 2.0a 2.0a 5.3a 9.3a
0.0 1.1 0.0 0.0 1.1 1.1
Means SE followed by different letters within the same row are significantly different at the 5% significance level.
chloroplasts, which are largely made up of elemental Mg (Wang, 2000). Previous studies have demonstrated that the chlorophyll content of plant leaves increases with increasing concentrations of Mg in the hydroponic solution (Yang et al., 2005). Magnesium deficiency leads to a decline in the chlorophyll content and a decline in the rate of photosynthesis (Yang et al., 2002, 2005; Wang et al., 2004). However, excessive Mg levels in a hydroponic solution or in soil have been shown to result in a decline in the content of chlorophyll in soybean leaves (Wang et al., 2004). Our findings indicate that a Mg concentration of 80 ppm results in a decrease in the number of eggs laid by adult females (Fig. 2). These results demonstrate that the concentration of Mg influences the chlorophyll content of plant leaves, which in turn affects the survival and fecundity of C. medinalis. Previous investigations have suggested that the chlorophyll content of the rice leaf is related to the amount of damage suffered by a particular rice variety; the higher the chlorophyll content, the greater the damage (Xu et al., unpublished data). However, the number of eggs laid on the two rice varieties at each of the Mg concentrations was significantly higher than what was observed in the controls. This demonstrates that elevated levels of Mg promote fecundity of the adult female. In natural populations, the amount of nitrogen fertilizer influences the occurrence of C. medinalis. Higher levels of nitrogen fertilizer lead to increased damage by leaf-folding insects (Zhang et al., 1995). The larval density of C. medinalis increased 6.2e9.4 times when high nitrogen fertilizers were used in comparison to the density observed when low nitrogen fertilizers were used (Zhang et al., 1995). The use of nitrogen fertilizer affects several bionomic characteristics of rice leaffolders, including an increased larval survival rate, leaf area consumed, pupal weight, fecundity of the moth and its preference of oviposition (de Kraker et al., 2000; Dan and Chen, 1990; Liang et al., 1984; Swaminathan et al., 1985; Zhang et al., 1995). The present findings are consistent with observations from natural populations. In fact, increased levels of nitrogen fertilizer typically result in an increase in the chlorophyll content of rice leaves because the renewal of chloroplasts in leaves relies on the supply of nitrogen (Wang, 2000). There is a positive correlation between chlorophyll content and Mg content in rice leaves (Wang, 2000). Thus, these results may provide valuable information on the relationship between high levels of nitrogen fertilizer and the outbreak of C. medinalis populations because application of nitrogen elevates the levels of chlorophyll and Mg in the rice leaf (Wang et al., 2004). Rice has developed both chemical and physical mechanisms to resist C. medinalis. Antibiosis has resulted in the presence of substances in the rice that are detrimental to the pest; physical
resistance is offered by the close arrangement of silica cells and the silica and wax content on the epidermal layer of rice leaves (Ramachandran and Khan, 1991; Wang et al., 2008). Physiological and behavioural responses of C. medinalis to resistant varieties of rice include orientation, settling, feeding, metabolism of ingested food, growth, survival and oviposition. These responses vary with the type of varietal resistance. For example, in the two varieties tested here, the fecundity and population growth of C. medinalis feeding on Huaidao 9 decreased significantly but no significant decrease of folded leaves was found. In contrast, while the number of folded leaves on Yangjing 9538 decreased significantly, fecundity and population growth were comparable to the susceptible variety (Xu et al., 2007). Our previous study showed that the resistance mechanisms of rice to C. medinalis are complex because they involve two types of resistance (Xu et al., 2007). For example, these responses can include resistance to feeding damage, which is mainly caused by a physical mechanism, and resistance to fecundity. Resistance to fecundity (the number of eggs laid) is mainly due to antibiosis because fecundity is related to the nutrients that larvae take up from rice plants. Thus the survival rate of larvae varies with the rice variety. Previously, the direct role of Mg in the resistance of rice plants to C. medinalis had not been studied in detail. However, as an essential element, Mg plays a key role in plant development, leaf photosynthesis, the activity of enzymes, the metabolism of lipids, and the syntheses and transport of assimilate and proteins (Walker and Weinstein, 1991; Peters and Berkowitz, 1991; Fischer and Bremer, 1993). Mg also affects chlorophyll content, the rate of photosynthesis and root vigour and causes a series of physiological and biochemical changes in plants (Yang et al., 2002, 2005; Wang et al., 2004). Therefore, Mg may have an indirect role in plant resistance to C. medinalis, as Mg is closely associated with photosynthesis. However, the present findings demonstrate that the elevation of the concentration of Mg in the hydroponic culture solution significantly increased the number of eggs laid by female moths. This may be due to an improvement in the nutrient conditions for larvae feeding rather than to resistance. No significant influence on the development of larvae and pupae was found. However, the direct effect of Mg on survival and fecundity of C. medinalis needs to be further investigated.
Acknowledgments This research was partially funded by The Industry Project of Ministry of Agriculture of the People’s Republic of China (200903051).
J. Xu et al. / Crop Protection 30 (2011) 765e769
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Changes in the survival rate and fecundity of the rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), at different magnesium concentrations in hydroponic culture Jie Xu, Li-Ben Jiang, Jin-Cai Wu* School of Plant Protection, Yangzhou University, Yangzhou 225009, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 March 2010 Received in revised form 1 March 2011 Accepted 6 March 2011
The rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), is a leaf-feeding pest and the physical and chemical nature of the rice leaf affects its feeding, survival and fecundity. We examined changes in the survival rates of larvae and pupae, pupal weight and the number of eggs laid by adult females developed from a resistant variety (Huaidao 9) and a susceptible variety (Yangjing 9538) in hydroponic solutions containing different concentrations of magnesium (Mg). The concentration of Mg in the hydroponic solution significantly affected the survival rates of the larvae and pupae, the pupal weights and the numbers of eggs laid. Larval survival rates on the resistant variety were significantly higher for fourth instars exposed to 60 and 80 ppm Mg and pupae exposed to 20, 40, 60 and 80 ppm Mg than those for the control treatment (without Mg). For the susceptible variety, survival rates of first, second and third instars were significantly higher at high concentrations of Mg than those under control conditions. In addition, the number of eggs laid by adult females increased with increasing Mg concentrations, though there was a slight decrease at 80 ppm compared to 60 ppm. These results indicate that Mg is beneficial to the development, survival and fecundity of C. medinalis. They further suggest that high levels of nitrogen fertilizer in rice fields favour outbreaks of this pest because application of nitrogen elevates the chlorophyll content and Mg levels in the rice leaf. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Elemental magnesium Cnaphalocrocis medinalis Fecundity Survival
1. Introduction The rice leaffolder, Cnaphalocrocis medinalis Guenée (Lepidoptera: Pyralidae), is one of the most notorious leaf-feeding pests of rice, Oryza sativa (L.) (Graminales, Poaceae), in subtropical countries (Heinrichs et al., 1985; Khan et al., 1988). The insect is a pest particularly in areas where modern varieties of rice are grown extensively (Mohandas, 1975). In China, outbreaks of the insect have occurred in recent years in the rice producing regions of southern China and in the middle and lower reaches of the Yangtze River (Zhu and Chen 1999; Yang et al., 2004). C. medinalis larvae feed on rice leaf blades, which results in a loss of economic output, particularly during the grain-filling stage (Jin, 1984). Over a typical crop cycle, the damage incidence forms a bell-shaped curve with the maximum number of damaged rice plants observed in the late reproductive and early ripening stages (Litsinger et al., 2006). Several cultivars or lines, including several wild rice species, have been screened since the 1970s for resistance to the rice-leaf * Corresponding author. Tel.: þ86 514 87979246; fax: þ86 517 87349817. E-mail address: [email protected] (J.-C. Wu). 0261-2194/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.cropro.2011.03.008
roller damage (Heinrichs et al., 1985; Ramachandran and Khan, 1991). Investigations into the resistance mechanisms of wild rice have shown that volatile substances in plants and the arrangement of silica cells on leaf surfaces are related to resistance (Ramachandran and Khan, 1991). Wang et al. (2008) demonstrated that the resistance of rice to C. medinalis was significantly related to the silica and wax content of leaf surfaces. The population growth of C. medinalis is associated with rice varieties and nitrogen fertilizers (Zhang et al., 1995). Population trend indices between resistant and susceptible varieties were found to be significantly different (Xu et al., 2007). Populations of pests in rice fields with higher nitrogen levels were significantly higher than those in fields with low nitrogen levels (Zhang et al., 1995; Cheng et al., 2008). Nitrogen fertilizer can increase the chlorophyll content in rice leaves, which in turn increases magnesium (Mg) content because of the positive correlation between chlorophyll and Mg content. In addition, the Mg content of leaves is positively correlated with the level of damage to rice leaves (Xu et al., unpublished data). Mg is a major element in the chlorophyll pigments of leaves and plays a key role in plant development. Mg deficiency leads to a decline in chlorophyll content and the rate of photosynthesis (Wang et al.,
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2004; Yang et al., 2005). Therefore, Mg levels in soil affect the growth and health of plants. However, the relationship between the Mg content in leaves and the growth and survival of immature and subsequent fecundity of adult C. medinalis is not well understood. An investigation into the effect of Mg on these life-history traits of C. medinalis is important for understanding the population growth and resistance mechanisms of rice. The objective of the present study was to examine changes in the development and fecundity of C. medinalis by manipulating Mg concentrations in hydroponically cultured rice. 2. Materials and methods 2.1. Rice varieties and insects Two varieties of japonica rice, Huaidao 9 and Yangjing 9538 were used in these experiments. Huaidao 9 is known to be resistant to C. medinalis and Yangjing 9538 is known to be susceptible (Xu et al., 2007). Seeds were sown into cement tanks at an experimental farm of Yangzhou University. Six-leaf seedlings were selected from the tanks and the soil of the plant roots was then washed off with tap water. The seedlings were immediately transplanted into porcelain pots (13 cm high and 10 cm in diameter) containing Espino hydroponic solution (Mae and Ohira, 1981) with NH4NO3, NaH2PO4.2H2O, K2SO4, CaCl2, MgSO4.7H2O, MnCl2.4H2O, (NH4)6Mo7O24.4H2O, H3BO3, ZnSO4.5H2O, FeCl3, CuSO4.5H2O and citric acid. Six seedlings were planted in a pot as a hill. The hydroponic solution was replaced once a week and its pH value was adjusted daily to 5.0 with 1 N HCl or 1 N NaOH. Details of the cultivation method are described in Wu et al. (2003). Mg concentration gradient experiments were conducted seven days after seedlings were cultured in the standard hydroponic solution. Five gradient concentrations of Mg were set up using the following concentrations of MgSO4.7H2O: 0, 20, 40, 60 and 80 ppm. Other elements in the hydroponic solution were not changed. First instar larvae that had hatched from eggs laid by field-caught moths at the experimental farm of Yangzhou University were used for experiments. 2.2. Experiment For each rice variety, four potted plants at the tillering stage were covered with a cage (80-mesh) and served as one replicate for each Mg concentration. Ten first instar larvae were placed onto the leaves of potted rice using a small writing brush (brush5; Yangzhou Wantou Hubi Ltd. Co., Yangzhou, China). Each treatment and control (hydroponic solution without Mg) experiment was
repeated three times (i.e., 12 pots). Treated and control plants were placed in a greenhouse at a natural temperature (24e32 C) and a natural photoperiod (about 14 h of light and 10 h of darkness). The growth and development of the larvae was observed every two days by carefully opening folded leaves and counting the number of live larvae. The live larvae were then left on the folded leaves after the number was recorded. Pupae in the whole plants (sheaths or leaves) were collected after the larvae had developed to the pupal stage. Collected pupae were weighed and then put into Petri dishes with wet filter paper. Adult emergence was recorded daily. Two adults (a male and a female) were put into a glass jar (8 cm high and 5 cm in diameter) at a constant temperature of 24 2 C to lay eggs. The jars were covered with a black cloth during egg laying. A piece of absorbent cotton was dipped into 5% honeyewater and was put into the jar to provide food for the adults. The cotton was changed daily. Adult females laid eggs on the glass jar and the number of eggs laid per female was counted. Fecundity was measured from 14, 14, 27, 32 and 21 female moths that had been exposed to 0, 20, 40, 60 and 80 ppm Mg, respectively. 2.3. Statistical analysis Normal distribution and homogeneity of variance (using a Bartlett test) were tested before running analyses of variance (ANOVAs). A two-way ANOVA (using rice variety and Mg concentration as factors) was performed to analyse differences in the fecundity of adult females, the survival rates of larvae and pupae and the pupal weight of C. medinalis on two varieties of rice at different Mg concentrations. Multiple comparisons of means were conducted based on Fisher’s protected least significant difference (PLSD) test. In addition, regression equations were established to examine the relationship between the Mg concentration and the fecundity, survival rate and pupal weight. All analyses were conducted using the data processing system (DPS) of Tang and Feng (2002). 3. Results 3.1. Survival rates of larvae and pupae The effect of Mg on the survival rate of larvae varied, depending on the instars, Mg concentration and rice variety (Table 1). The data in Table 1 were analysed using a two-way ANOVA and showed that Mg concentrations significantly affected the survival rates of first through fifth instars but did not affect the survival rate of pupae (Table 2). The rice variety also affected the survival rates of the first and the fourth instars. The grand mean (the pooled mean of all Mg concentrations for each rice variety) of the survival rate of the first
Table 1 Survival rates of larvae and pupae on two rice varieties at different Mg concentrations in hydroponic solutions. Rice variety
Insect stadium
Mg concentration (ppm) 0
20
40
60
80
Mean
SE
Mean
SE
Mean
SE
Mean
SE
Mean
SE
Huaidao 9
First instar Secondethird instar Fourth instar Fifth instar Pupa
0.66a 0.91a 0.61a 0.84ab 0.63c
0.01 0.05 0.10 0.01 0.04
0.72a 0.86a 0.67b 0.83ab 0.74b
0.08 0.05 0.10 0.07 0.03
0.74a 0.86a 0.83ab 0.92a 0.84a
0.05 0.05 0.12 0.05 0.05
0.77a 0.95a 0.98a 0.91a 0.79ab
0.02 0.03 0.01 0.06 0.02
0.78a 0.81a 0.93a 0.73b 0.78ab
0.05 0.10 0.01 0.03 0.03
Yangjing 9538
First instar Secondethird instar Fourth instar Fifth instar Pupa
0.62b 0.78c 0.81a 0.83a 0.75a
0.02 0.01 0.05 0.12 0.07
0.60b 0.82bc 0.84a 0.94a 0.82a
0.05 0.02 0.05 0.05 0.01
0.65b 0.89ab 0.94a 0.95a 0.80a
0.02 0.06 0.06 0.004 0.05
0.80a 0.95a 0.95a 0.85a 0.73a
0.02 0.01 0.03 0.04 0.15
0.64b 0.94a 0.91a 0.80a 0.77a
0.03 0.02 0.07 0.03 0.06
Means SE followed by different letters within the same row are significantly different at the 5% significance level.
J. Xu et al. / Crop Protection 30 (2011) 765e769
28
Table 2 ANOVA of data of survival rates, pupae weights and the number of eggs laid (see Table 1 and Figs. 1 and 2). Source of variance
Df
F-value
P-value
Survival rate of first instar
Rice variety (A) Mg concentration (B) AB
1 4 4
18.7 8.6 3.1
0.0003 0.0003 0.03
Survival rate of secondethird instar
A B AB
1 4 4
Survival rate of fourth instar
A B AB
1 4 4
10.3 13.3 3.2
0.004 0.0001 0.03
Survival rate of fifth instar
A B AB
1 4 4
1.5 6.5 1.7
0.2 0.001 0.1
Survival rate of pupae
A B AB
1 4 4
0.4 2.4 1.8
0.5 0.08 0.1
Number of eggs laid
A B AB
1 4 4
101.3 36.1 11.6
0.0001 0.0001 0.001
Pupal weight
A B AB
1 4 4
17.9 40.6 4.9
0.0004 0.0001 0.006
0.0004 4.3 4.8
0.9 0.01 0.006
instars on Huaidao 9 was significantly higher than on Yangjing 9538. However, the grand mean of survival rate of the fourth instars on Yangjing 9538 was significantly higher than on Huaidao 9. In addition, there was a significant interaction effect between the Mg concentration and the rice variety on the survival rates of the first, second-third and fourth instars. For Huaidao 9, multiple comparisons of means for different Mg concentrations showed that the survival rate of fourth instars at Mg levels of 60 and 80 ppm was significantly higher than for those under the control treatment (without Mg) or at 20 ppm Mg (Table 1). Furthermore, the survival rate of the pupae at the four different Mg concentrations was significantly higher than that of the control. For Yangjing 9538, the survival rates of the first and second-third instars in the presence of high Mg concentrations were significantly higher than those observed under the control, but no significant differences were observed for the survival rates of the fourth and the fifth instars.
Huaidao 9 Yangjing 9538 Huaidao 9 Yangjing 9538
27
Pupa weight (mg)
Insect stadium
767
26 25 24 23 22 21
0
20
40
60
80
Mg concentrations (ppm) in the hydroponic solution Fig. 1. Weight of pupae developed from rice plants at different Mg concentrations. P ¼ 22.3742 þ 0.0684M 0.0005M2, R2 ¼ 0.7169, P > 0.05 for Huaidao 9; P ¼ 21.5338 þ 0.1684M 0.0015M2, R2 ¼ 0.8711, P < 0.05 for Yangjing 9538.
the Mg concentration was fitted to two quadratic equations (Fig. 2), which indicates that the number of eggs laid under exposure to 80 ppm Mg decreased to a certain extent in comparison with the number laid under exposure to 60 ppm Mg. 3.4. Development duration of larvae and pupae Mg levels in the hydroponic solution did not significantly influence the development of larvae and pupae (P > 0.05 for all effects, Table 3). 4. Discussion The present findings indicate that Mg has a significant effect on the survival and fecundity of C. medinalis. However, no significant differences were observed in the development of larvae and pupae. C. medinalis larvae tie the edges of the tips of rice leaves together with silken threads, thereby forming a protective cavity within which they feed by scraping mesophyll cells and epidermal tissue from leaf blades. Therefore, mesophyll cells are the nutrient source for the feeding insect. Mesophyll cells are mainly composed of
3.2. Pupal weight
3.3. Fecundity The Mg concentration, rice variety and the interaction of these two factors significantly influenced the number of eggs laid (Table 2, Fig. 2). Multiple comparisons indicated that the number of eggs laid by adult females on the two rice varieties at 40, 60 and 80 ppm Mg was significantly higher than it was for the control. At these concentrations, the number of eggs laid increased by 105, 141 and 108% for Huaidao 9 and by 128, 259 and 91% for Yangjing 9538, respectively. The relationship between the number of eggs laid and
Fecundity rate (egg numbers)
100
An ANOVA of the data in Fig. 1 showed that Mg concentration, rice variety and their interactions significantly affected pupal weight (Fig. 1, Table 2). Pupal weight increased with increasing Mg concentrations but had a slight decline at 80 ppm Mg; thus the relationship between the weight of pupae and the Mg concentration is apparently parabolic. On the two varieties of rice, the weights of the pupae were significantly greater at all Mg concentrations than they were in the control.
Huaidao 9 Yangjing 9538 Huaidao 9 Yangjing 9538
80
60
40
20
0
0
20
40
60
80
Mg concentrations (ppm) in the hydroponic solution Fig. 2. The number of eggs laid by adult female Cnaphalocrocis medinalis grown on rice plants at different Mg concentrations. F ¼ 12.4096 þ 0.5661M 0.0045M2, R2 ¼ 0.9718, P < 0.05; F ¼ 15.5333 þ 1.6319M 0.0148M2, R2 ¼ 0.6524, P > 0.05 for Yangjing 9538.
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Table 3 Developmental duration (days) of larvae and pupae on two rice varieties at different Mg concentrations in hydroponic solutions. Rice variety
Insect stadium
Mg concentration (ppm) 0
20
40
60
80
Mean
SE
Mean
SE
Mean
SE
Mean
SE
Mean
SE
Huaidao 9
First instar Second instar Third instar Fourth instar Fifth instar Pupa
2a 3.3a 2.6a 4.6a 6a 10a
0.0 1.1 1.1 1.1 2.0 2.0
2a 3.3a 4a 5.3a 6.6a 8.6a
0.0 1.1 0.0 1.8 1.1 2.3
2a 4a 3.3a 4.6a 6.6a 9.3a
0.0 0.0 1.1 1.1 1.1 2.3
2a 4a 2.6a 4.6a 5.3a 9.3a
0.0 2.0 1.1 1.1 1.1 1.1
2.6a 3.3a 2.6a 4.6a 5.3a 9.3a
1.1 1.1 1.1 1.1 2.3 1.1
Yangjing 9538
First instar Second instar Third instar Fourth instar Fifth instar Pupa
2.6a 3.3a 2.6a 3.3a 6.0a 10.0a
1.1 1.1 1.1 1.1 2.0 2.0
2.0a 2.6a 2.0a 3.3a 5.3a 10.0a
0.0 1.1 0.0 1.1 1.1 2.0
2.0a 2.0a 2.6a 2.0a 4.6a 10.6a
0.0 0.0 1.1 0.0 1.1 1.1
2.0a 3.3a 3.3a 2.6a 4.6a 8.6a
0.0 1.1 1.1 1.1 1.1 1.1
2.0a 3.3a 2.0a 2.0a 5.3a 9.3a
0.0 1.1 0.0 0.0 1.1 1.1
Means SE followed by different letters within the same row are significantly different at the 5% significance level.
chloroplasts, which are largely made up of elemental Mg (Wang, 2000). Previous studies have demonstrated that the chlorophyll content of plant leaves increases with increasing concentrations of Mg in the hydroponic solution (Yang et al., 2005). Magnesium deficiency leads to a decline in the chlorophyll content and a decline in the rate of photosynthesis (Yang et al., 2002, 2005; Wang et al., 2004). However, excessive Mg levels in a hydroponic solution or in soil have been shown to result in a decline in the content of chlorophyll in soybean leaves (Wang et al., 2004). Our findings indicate that a Mg concentration of 80 ppm results in a decrease in the number of eggs laid by adult females (Fig. 2). These results demonstrate that the concentration of Mg influences the chlorophyll content of plant leaves, which in turn affects the survival and fecundity of C. medinalis. Previous investigations have suggested that the chlorophyll content of the rice leaf is related to the amount of damage suffered by a particular rice variety; the higher the chlorophyll content, the greater the damage (Xu et al., unpublished data). However, the number of eggs laid on the two rice varieties at each of the Mg concentrations was significantly higher than what was observed in the controls. This demonstrates that elevated levels of Mg promote fecundity of the adult female. In natural populations, the amount of nitrogen fertilizer influences the occurrence of C. medinalis. Higher levels of nitrogen fertilizer lead to increased damage by leaf-folding insects (Zhang et al., 1995). The larval density of C. medinalis increased 6.2e9.4 times when high nitrogen fertilizers were used in comparison to the density observed when low nitrogen fertilizers were used (Zhang et al., 1995). The use of nitrogen fertilizer affects several bionomic characteristics of rice leaffolders, including an increased larval survival rate, leaf area consumed, pupal weight, fecundity of the moth and its preference of oviposition (de Kraker et al., 2000; Dan and Chen, 1990; Liang et al., 1984; Swaminathan et al., 1985; Zhang et al., 1995). The present findings are consistent with observations from natural populations. In fact, increased levels of nitrogen fertilizer typically result in an increase in the chlorophyll content of rice leaves because the renewal of chloroplasts in leaves relies on the supply of nitrogen (Wang, 2000). There is a positive correlation between chlorophyll content and Mg content in rice leaves (Wang, 2000). Thus, these results may provide valuable information on the relationship between high levels of nitrogen fertilizer and the outbreak of C. medinalis populations because application of nitrogen elevates the levels of chlorophyll and Mg in the rice leaf (Wang et al., 2004). Rice has developed both chemical and physical mechanisms to resist C. medinalis. Antibiosis has resulted in the presence of substances in the rice that are detrimental to the pest; physical
resistance is offered by the close arrangement of silica cells and the silica and wax content on the epidermal layer of rice leaves (Ramachandran and Khan, 1991; Wang et al., 2008). Physiological and behavioural responses of C. medinalis to resistant varieties of rice include orientation, settling, feeding, metabolism of ingested food, growth, survival and oviposition. These responses vary with the type of varietal resistance. For example, in the two varieties tested here, the fecundity and population growth of C. medinalis feeding on Huaidao 9 decreased significantly but no significant decrease of folded leaves was found. In contrast, while the number of folded leaves on Yangjing 9538 decreased significantly, fecundity and population growth were comparable to the susceptible variety (Xu et al., 2007). Our previous study showed that the resistance mechanisms of rice to C. medinalis are complex because they involve two types of resistance (Xu et al., 2007). For example, these responses can include resistance to feeding damage, which is mainly caused by a physical mechanism, and resistance to fecundity. Resistance to fecundity (the number of eggs laid) is mainly due to antibiosis because fecundity is related to the nutrients that larvae take up from rice plants. Thus the survival rate of larvae varies with the rice variety. Previously, the direct role of Mg in the resistance of rice plants to C. medinalis had not been studied in detail. However, as an essential element, Mg plays a key role in plant development, leaf photosynthesis, the activity of enzymes, the metabolism of lipids, and the syntheses and transport of assimilate and proteins (Walker and Weinstein, 1991; Peters and Berkowitz, 1991; Fischer and Bremer, 1993). Mg also affects chlorophyll content, the rate of photosynthesis and root vigour and causes a series of physiological and biochemical changes in plants (Yang et al., 2002, 2005; Wang et al., 2004). Therefore, Mg may have an indirect role in plant resistance to C. medinalis, as Mg is closely associated with photosynthesis. However, the present findings demonstrate that the elevation of the concentration of Mg in the hydroponic culture solution significantly increased the number of eggs laid by female moths. This may be due to an improvement in the nutrient conditions for larvae feeding rather than to resistance. No significant influence on the development of larvae and pupae was found. However, the direct effect of Mg on survival and fecundity of C. medinalis needs to be further investigated.
Acknowledgments This research was partially funded by The Industry Project of Ministry of Agriculture of the People’s Republic of China (200903051).
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