Insect Science (2011) 18, 697–702, DOI 10.1111/j.1744-7917.2010.01393.x
ORIGINAL ARTICLE
Effects of UV-A exposures on longevity and reproduction in Helicoverpa armigera, and on the development of its F1 generation Chang-Yu Zhang, Jian-Yu Meng, Xiao-Ping Wang, Fen Zhu and Chao-Liang Lei Hubei Insect Resources Utilization and Sustainable Pest Management Key Laboratory, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan, China
Abstract Helicoverpa armigera adults display a conspicuous positive phototactic behavior to light stimuli, and are especially sensitive to ultraviolet (UV) light. The effects of UV-A (longwave) exposures on adult longevity and reproduction in H. armigera were investigated, as well as the development of the F1 generation. Paired adults were exposed to UV-A for various time periods (0, 1, 5 and 9 h/day), until the end of adult life. The results showed that adult longevity decreased with increasing exposure time for both sexes, and a significant decrease was observed after exposure for 5 and 9 h/day. Fecundity increased when adults were exposed for 1 and 5 h/day, and a significant difference was observed in the 5 h/day group. Oviposition rates of females in all treatments were significantly higher than in the control. Exposure to UV-A for longer periods (5 and 9 h/day) caused a decline in cumulative survival of F1 immature stages, but no significant differences were found in egg hatch, pupation and eclosion. The developmental periods of F1 larvae were significantly prolonged after exposure to UV-A for 5 and 9 h/day. UV-A radiation had no significant effects on F1 pupal period. Key words development, reproduction, UV-A
Introduction Ultraviolet (UV) radiation is an important environmental stress factor for various organisms (Meyer-Rochow, 2000; Schauen et al., 2007). Exposure to UV radiation induces the formation of photo-excited states of cellular photosensitizers with subsequent generation of reactive oxygen species (ROS). It has been suggested that UVinduced ROS can induce damage to nucleic acids, membrane lipids and proteins (Jurkiewicz & Buettner, 1994; Vile & Tyrrell, 1995). Various types of damage induced
Correspondence: Chao-Liang Lei, College of Plant Science and Technology, Huazhong Agricultural University, Wuhan 430070, China. Tel: +86 27 87287207; fax: +86 27 87287207; email:
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Helicoverpa
armigera,
longevity,
oxidative
stress,
in these molecules lead to significant biological effects, including cytotoxicity, mutations and alterations in cell signaling pathways (McMillan et al., 2008). Responses to UV radiation in insects have been investigated with regard to behavior (Mazza et al., 2002; Leech & Johnsen, 2003; Cowan & Gries, 2009), developmental physiology (Gunn, 1998) and biochemistry (Mackerness et al., 1999; Lopez-Martinez et al., 2008; Meng et al., 2009, 2010). Recent studies also show that exposure to UV radiation in insects can increase the level of oxidative stress (Lopez-Martinez et al., 2008; Meng et al., 2009) and cause photoreceptor damage (Meyer-Rochow & Mishra, 2007). The cotton bollworm, Helicoverpa armigera (H¨ubner) (Lepidoptera: Noctuidae), is a serious crop pest in China and neighboring countries and attacks more than 60 crops such as cotton, corn, tobacco and soybean (Fitt, 1989;
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Chen, 1999). The moths of this nocturnal insect display a conspicuous positive phototactic behavior to light and are especially sensitive to UV light (Ding et al., 1974; Wei et al., 2000). Ultraviolet light has been widely used in integrated pest management (IPM) to forecast and control various types of insect pests, including H. armigera (Jing & Lei, 2004; Jing et al., 2004). Exposure to UV radiation increases the levels of oxidative stress in H. armigera adults (Meng et al., 2009), and results in dramatic changes in protein synthesis (Meng et al., 2010). We assume that such UV-induced damage might affect the survival, reproductive capacity and development of H. armigera. In the present study, the effects of UV-A (longwave) exposures on longevity and reproduction in H. armigera adults were investigated, as well as the development of the F1 generation. To our knowledge, this study, along with the studies by Meng et al. (2009, 2010), are the first to assess eco-physiological effects of UV radiation in phototactic insects. Materials and methods
radiation for various time periods, 0 (control), 1, 5 and 9 h per day, until all of the males and females died. Under these conditions the temperature in the irradiated area was 27◦ C. After irradiation, the adults of each treatment were maintained under the same conditions as described above. Adult longevity and reproduction Thirty female–male pairs were used in each treatment of UV-A radiation as explained above. One female–male pair reared in one plastic cup (15 cm height, 9 cm diameter) was tested as one replicate. The open end of the plastic cup was closed with a net cover of degreased cotton yarn and served as the oviposition substrate. Cotton yarn covers were replaced daily. The number of eggs per female per day, the total number of eggs per female throughout the whole oviposition period, and survival of males and females were recorded daily. In addition, the dead females were dissected, and the numbers of unlaid eggs were recorded. The experiments were terminated when all of the males and females died.
Experimental insects Larvae of H. armigera were collected from cotton plants growing in a suburb of Wuhan, Hubei Province of China (114◦ 3 E, 30◦ 5 N), and subsequently reared in an insectary for many generations at 27 ± 1◦ C, 75% ± 5% RH and 14 : 10 h L : D photoperiod (L, 6:00–20:00; D, 20:00–6:00). The larvae were fed with an artificial diet as described in Wu & Gong (1997). Females and males were segregated based on the morphology of the abdominal terminal segments of the pupa. After emergence, moths were released in cages (20 × 20 × 30 cm) to mate within 3 days, then paired (one female and one male) and placed in a plastic container (15 cm height, 9 cm diameter). The adults used for the experiments were provided with a 10% honey solution. Experimental design A UV lamp (East-China Electronic Tube Factory, Jiangsu, China), which emits UV in the range of 320– 400 nm (UV-A), was used as the source to irradiate the adults of H. armigera. The irradiance was 300 μW/cm2 . Paired adults were used for the experiments, and are referred to as the P generation. Prior to use in our experiments, the adults were divided into four groups. Every day, the adults were exposed to UV-A radiation 1 h after the start of scotophase. Adults were exposed to UV-A
Development of the F1 generation Eggs laid by adults of the P generation are referred to as the F1 generation. Cotton yarn with eggs was sprinkled with water and then transferred to a separate plastic Petri dish (7 cm diameter). One hundred randomly sampled fresh eggs were incubated in one plastic Petri dish with three replications per treatment (total of 300 eggs per treatment). The numbers of newly hatched larvae were recorded daily, and unhatched eggs were counted at the end of the experiment. Sixty newly hatched larvae were reared individually in plastic containers (3 × 3 × 4 cm) with three replications per treatment (total of 180 larvae per treatment). Numbers of pupae and the timing of pupation were recorded. Fifty new pupae obtained under the above conditions were moved into new individual plastic containers with three replications per treatment (total of 150 pupae per treatment). Number of adults and the timing of eclosion were recorded. The experiments were conducted at 27 ± 1 ◦ C, 75% ± 5% RH and 14 : 10 h L : D photoperiod (L, 6:00–20:00; D, 20:00–6:00). Statistical analysis Statistical analyses were performed using the procedure of SPSS (SPSS Inc., Chicago, IL, US). Data were analyzed by one-way analysis of variance (ANOVA) and means C
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Effects of UV-A exposures in Helicoverpa armigera
Fig. 1 Effect of UV-A radiation on adult longevity of Helicoverpa armigera, for: (A) females, (B) males. Error bars indicate s.e.m. Bars with different letters are significantly different by Tukey’s test (P > 0.05). n = 30 for each treatment.
were compared using Tukey’s test at P < 0.05. Data of egg hatch and survival of larvae and pupae were arcsine square-root transformed prior to analysis. Results Effect of UV-A radiation on the adult longevity The effect of UV-A radiation on adult longevity of H. armigera is shown in Fig. 1. Female and male adult longevity decreased gradually with increasing exposure time, and a significant decrease was observed after exposure to UV-A radiation for 5 and 9 h/day (F = 14.643, P < 0.01, df = 3,116 for female; F = 5.357, P < 0.01, df = 3,116 for male). However, no significant difference was noted in longevity between control and the 1 h/day group (Fig. 1A, B). Effect of UV-A radiation on reproduction Fecundity was defined as the mean total number of eggs laid per female throughout the whole oviposition period. Exposure to UV-A radiation for 1 and 5 h/day resulted in a fecundity increase in comparison with controls. A C
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significant difference was found after exposure for 5 h/day (F = 8.875, P < 0.01, df = 3,116). When adults were exposed to UV-A radiation for 9 h/day, females laid fewer eggs than the controls in the oviposition period, but the difference was not statistically significant (Table 1). Oviposition rate was defined as the mean number of eggs laid per female and per day. The oviposition rate was significantly influenced by UV-A radiation (F = 11.976, P < 0.01, df = 5,116). Oviposition rates of the females increased with increasing exposure time to reach a maximum in the 5 h/day group. At the longest exposure time (9 h/day), the oviposition rate decreased somewhat when compared with the 5 h/day group; however, in the 9 h/day group it was still significantly higher than in the control group (Table 1). Unlaid eggs were defined as the eggs which were left in the dead body of a female. Relative to the control, no significant difference in the number of unlaid eggs was observed when the adults were exposed to UV-A (F = 0.431, P = 0.731, df = 3,116). The rate of eggs laid was the result of combining both fecundity data and number of unlaid eggs. It was the percentage of laid eggs of the amount of eggs carried. Rates of eggs laid in the control and all of the treated groups were above 98% (Table 1).
Effect of UV-A radiation on the development of the F1 generation Data indicated that UV-A radiation had no significant effects on F1 egg hatch (F = 1.834, P = 0.219, df = 3,8), F1 larval survival (F = 3.659, P = 0.063, df = 3,8) and F1 pupal survival (F = 2.456, P = 0.138, df = 3,8). When the adults were exposed to UV-A radiation for 1 h/day, cumulative survival of immature stages in F1 H. armigera was still at the control level. However, exposure to UV-A radiation for longer periods (5 and 9 h/day) resulted in significant reductions in cumulative survival in comparison with the control (Table 2). The effects of UV-A radiation on developmental periods of F1 larvae and pupae appear in Table 3. When the adults were exposed to UV-A radiation, the developmental period of F1 larvae increased in comparison with the control. A significant delay in the development of F1 larvae was observed in the 5 and 9 h/day groups (F = 28.671, P < 0.01, df = 3,633). At the longest exposure time (9 h/day), F1 larvae took longest to reach the pupal stage. UV-A radiation had no significant effect on the developmental period of F1 pupae (F = 0.425, P = 0.735, df = 3,491). F1 pupae in the control and all of the treated groups took a similarly long time to reach the adult stage.
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Table 1 Effect of UV-A radiation on reproduction of Helicoverpa armigera. Exposure time (h/day) 0 1 5 9
Fecundity (eggs/female)
Oviposition rate (eggs/female/day)
Unlaid eggs/female
% eggs laid
813.5 ± 31.0 bc 949.2 ± 44.6 ab 990.0 ± 46.3 a 681.0 ± 61.5 c
83.1 ± 3.7 b 112.9 ± 6.4 a 132.4 ± 5.2 a 120.1 ± 8.1 a
13.4 ± 4.8 a 14.4 ± 4.5 a 15.5 ± 5.8 a 8.8 ± 1.7 a
98.4 98.5 98.5 98.7
Values (mean ± s.e.m.) followed by different letters within a column are significantly different by Tukey’s test (P < 0.05). n = 30 for each treatment.
Discussion Recently, Meng et al. (2009) reported that one of the effects of UV radiation was to increase oxidative stress on H. armigera adults, thus threatening this insect. UV-A radiation may disturb the functional activity of proteins and intensify the activity of protein oxidation processes in H. armigera adults. Therefore, exposure to UV-A radiation is probably a critical limiting factor for the survival in insects generally and H. armigera in particular. Presumably adult longevity was shortened as a result of UV-induced damage in H. armigera adults after exposure to UV-A radiation. In this study, all reproduction-related parameters of the P generation were seriously affected by the exposure to UV-A radiation. Fecundity and oviposition rates of females subjected to UV-A followed a similar pattern. Both total amount of eggs per female and mean number of eggs laid per day increased sharply from the control to the insects exposed for 5 h/day. At the longest exposure time (9 h/day), fecundity was lower than in the control, but no significant difference was observed. Similarly, oviposition rates were somewhat reduced after exposure to UV-A for 9 h/day, but remained significantly higher than in the control. Radiation by UV-A had no obvious effects on the number of unlaid eggs and percentage of eggs laid. The
analysis showed that the increased fecundity and oviposition rates were nearly independent of percentage eggs laid. Why reproductive capacity of H. armigera was enhanced after exposure to UV-A remains unknown. Our earlier work showed that exposure to UV can induce oxidative stress in the adults of H. armigera (Meng et al., 2009). Life-history theory predicts that certain traits may trade off with one another, especially when each trait is energetically costly (Zera & Denno, 1997; Zera & Harshman, 2001). It is well known that a key prediction of general life-history theory is that reproduction incurs a survival cost (Williams, 1966; Roff, 1992). Therefore, we suspect that there is a trade-off in energy allocation between reproduction and longevity in H. armigera adults under environmental stress. Besides, the moths also protect themselves from the adverse effects of the exposure to UV by increasing their antioxidant defences (Meng et al., 2009), and this response is likely to be costly for the organism in terms of energy. As antioxidant defenses and reproductive costs are increased in response to UV-A exposure, life-history theory predicts that survival ability must be weakened to a certain extent (Holloway et al., 1990; de Jong & van Noordwijk, 1992), just as we have seen in the shortened lifespan of H. armigera adults. Under our experimental conditions, UV-A radiation did not significantly inhibit egg hatch, pupation and adult
Table 2 Effects of UV-A radiation on egg hatch and survival of immature stages of F1 Helicoverpa armigera. Hatch and survival (%)
Exposure time (h/day)
0 1 5 9
Cumulative survival (%)
Egg
Larva
Pupa
96.7 ± 0.7 a 98.3 ± 0.9 a 92.0 ± 4.4 a 89.7 ± 1.8 a
93.3 ± 1.7 a 95.6 ± 2.0 a 96.1 ± 1.5 a 87.8 ± 2.0 a
91.3 ± 1.8 a 89.3 ± 3.5 a 79.3 ± 5.7 a 81.3 ± 3.5 a
82.4 83.9 70.1 64.0
Values (mean ± s.e.m.) followed by the same letter within a column are not significantly different by Tukey’s test (P > 0.05). The total numbers of insects in each treatment were: 300, 180 and 150 for egg hatch, larval and pupal survival, respectively.
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Effects of UV-A exposures in Helicoverpa armigera Table 3 Effects of UV-A radiation on developmental periods of larvae and pupae of F1 Helicoverpa armigera. Developmental period (day)
Exposure time (h/day) 0 1 5 9
Larva
Pupa
17.5 ± 0.1 (168) c 17.8 ± 0.1 (163) c 18.4 ± 0.1 (151) b 18.7 ± 0.1 (155) a
11.4 ± 0.1 (144) a 11.4 ± 0.1 (128) a 11.4 ± 0.1 (105) a 11.5 ± 0.1 (118) a
Values (mean ± s.e.m.) followed by different letters within a column are significantly different by Tukey’s test (P < 0.05). Sample numbers are given in parentheses.
eclosion of the F1 H. armigera. Exposure to UV-A for a long time can cause a decline in cumulative survival of F1 immature stages. The results suggested that the damage caused by UV-A exposure still existed in the F1 generation of H. armigera; at least a partial effect on survival of immature stages has been observed. The developmental periods of F1 pupae did not differ, but the developmental periods of F1 larvae differed between UV-A exposed and control insects. When adults were exposed to UV-A, F1 larvae took longer to reach the pupal stage. This result suggests that UV has additive negative effects on the F1 larval period. Meng et al. (2009, 2010) showed that UV can be considered an environmental stress factor in H. armigera. Organisms may, to a certain extent, defend themselves against adverse environmental conditions by prolonging their developmental periods, evidently as a strategy to compensate for the detrimental effect on growth (Gintenreiter et al., 1993; van Ooik et al., 2007). In conclusion, depending on exposure time under UV stress, some of the life history parameters in H. armigera change. The changes include a decrease of adult longevity, but increases of fecundity and oviposition rate. Parental effects persisted into at least the F1 generation when adults were exposed to UV radiation of 9 h daily for x number of days.
Acknowledgments The research was supported by National Natural Science Foundation of China (No. 30871639). We thank Dr. Q.Y. Huang for the helpful comments and suggestions on the manuscript, Ms. S. Xu and X. Yuan for assistance with experiments. C
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