Insect Science (2006) 13, 189-193
Effect of host plants on diapause induction 189
Host plant mediation of diapause induction in the cabbage beetle , Colaphellus bowringi Baly (Coleoptera: Chrysomelidae) X I A O - P I N G WA N G 1 , 2 , F E N G G E
a n d FA N G - S E N X U E 1
Institute of Entomology, Jiangxi Agricultural University, Nanchang, and 2State Key Laboratory of Integrated Management of Pest Insects and Rodents, Institute of Zoology, Chinese Academy of Sciences, Beijing, China Abstract The cabbage beetle, Colaphellus bowringi Baly (Coleoptera: Chrysomelidae), is a serious pest of crucifers in China, undergoing an imaginal summer and winter diapause in the soil. The effects of host plants on diapause incidence were tested in the beetle. The ratio of adults entering diapause was significantly low when they fed on the mature leaves of Chinese cabbage Shanghaiqin (Brassica chinesis var communis) than those feeding on Chinese cabbage Suzhouqin (Brassica chinesis var communis), radish (Raphanus sativus var longipinnatus) and stem mustard (Brassica juncea var tumida) at 25℃ combined with 13: 11 (L: D) h. Fewer adults entered diapause on young leaves compared to physiologically aged and mature radish leaves at 25℃ combined with 13: 11 (L: D) h. The effect of host plant species on diapause induction was also evident under continuously dark rearing conditions or at different photoperiods. These experimental results demonstrate that host plant mediation of diapause induction exists in the cabbage beetle. However, at temperatures ≤ 20℃ or photoperiods of 16: 8 (L: D) h combined with 25℃, all individuals entered diapause regardless of the host plants, indicating that the effects of host plants on diapause induction could be expressed only within a limited range of temperatures and photoperiods. Key words Colaphellus bowringi, diapause induction, host plant, photoperiod, temperature DOI 10.1111/j.1744-7917.2006.00081.x
Introduction Insects, along with other organisms, do not live in an environment that is always perfect for optimal growth. They must adapt to and survive seasons or conditions that do not support continuous development. As an adaptive strategy to escape seasonal adverse conditions, diapause plays an important role in the seasonal life cycle of many insects. When diapause is facultative rather than obligatory, insects have the potential to develop without interruption while conditions are favorable. They enter diapause when conditions are unfavorable, especially in the regions where
Correspondence: Fang-Sen Xue, Institute of Entomology, Jiangxi Agricultural University, Nanchang 330045, China. Tel: +86 791 3813351; fax: +86 791 3814107; e-mail: email@example.com
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daylength and temperature differ within a season (Tauber et al., 1986; Danks, 1987). Environmental factors affecting diapause in insects have received considerable attention. However, most studies have concentrated on the roles of photoperiod and temperature as the primary cues of diapause induction, whereas relatively few investigations have been carried out in the area of diet (Danilevskii, 1965; Beck, 1980; Tauber et al., 1986; Danks, 1987; Steinberg et al., 1992; Tanzubil et al., 2000). Diet usually modifies the degree of photoperiodic response and affects diapause induction in most insects. Diet as a major factor regulating diapause has been reported in only a relatively small number of insects with summer diapause, and insects in tropical areas (Tauber et al., 1986). For phytophagous insects, the proportion of insects that enter diapause can be affected by: feeding on different host plant species, for example the peach fruitwww.blackwellpublishing.com/ins 189
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borer (Carposina niponensis) (Chang et al., 1977), the Colorado beetle (Leptinotarsa decemlineata) (Hare, 1983), the obliquebanded leafroller (Choristoneura rosaceana) (Hunter & McNeil, 1997); different growth phases of the host plant, for example the collembolan (Sminthurus viridis) (Wallace, 1968), the mite (Halotydeus destructor) (Wallace, 1970), the maize stemborer (Busseola fusca) (Usua, 1970); or by feeding on different organs of the host plant, for example, the cotton boll weevil (Anthonomus grandis) (Lloyd et al., 1967), the pink bollworm (Pectinophora gossypiella) (Zhang et al., 1986). The cabbage beetle, Colaphellus bowringi Baly (Coleoptera: Chrysomelidae), is a serious pest of crucifers in the mountain areas of Jiangxi Province, China. There are two distinct infestation peaks in the field, one generation in spring and 1-3 generations in autumn. The beetles enter summer and winter diapause in soil as adults (Xue & Kallenborn, 1993; Xue et al., 2002a). Previous studies indicate that temperature and photoperiod are two major diapause-inducing factors in this beetle. The sensitive stage to photoperiod is the larval stage, whereas the sensitive stages to temperature encompass larval, pupal and adult stages. Its photoperiodic response is highly dependent upon temperature. All adults enter diapause at ≤ 20℃ regardless of the photoperiod. The diapause-averting influences of short daylengths are expressed only at temperatures above 20℃ (Xue et al., 2002b; Wang et al., 2004a,b). In the field, the cabbage beetles mainly feed on cruciferous vegetables, such as Chinese cabbage (Brassica chinensis var. communis), radish (Raphanus sativus var. longipinnatus) and mustard (Brassica juncea) (Xue et al., 2002a). In this study, we examined the effects of these host plants on diapause induction in the cabbage beetle under different temperatures and photoperiods.
Materials and methods Insect
(B. chinensis var. communis) with thick black green leaves, Chinese cabbage Shanghaiqin (B. chinensis var. communis) with thin yellow green leaves, radish (R. sativus var. longipinnatus), and stem mustard (B. juncea var. tumida), were identified for experiments and cultivated in the autumn of 2002. Leaves were collected daily for experiments during peduncle growth in the spring of 2003. In order to eliminate the influence of aging during the experiment, vegetables were sown four times with 5-day intervals. Effects of different host plants on diapause induction Previous studies showed that the critical nightlength was about 13 h at 25℃ in C. bowringi (Xue et al., 2004b; Wang et al., 2004a), that is, the condition of 25℃ combined with 13: 11 (L: D) h was one of the threshold conditions for diapause induction in this beetle. Changes of host plants may be sensitive for diapause induction under this condition. Therefore, the effects of different host plants on diapause induction were determined by feeding the beetles with four cruciferous vegetables, Chinese cabbage Suzhouqin, Chinese cabbage Shanghaiqin, radish, and stem mustard under the condition of 25℃ and 13: 11 (L: D) h. Effects of food quality on diapause induction The leaves from upper, intermediate and bottom parts of radish during peduncle growth represent the physiological state of young, mature and aged respectively, and indicate the difference in food quality. The effects of food quality on diapause induction were determined by rearing the beetles with different physiological states of young, mature and aged respectively, at 25℃ combined with 13: 11 (L: D) h. Relationship between host plant and temperature or photoperiod
Adults of Colaphellus bowringi were collected in the field (29°1′N, 114°4′E, Jiangxi Province, China) in late November 2002. The beetles were then transferred to large glass bottles containing soil to burrow for diapause under natural conditions. In the spring of 2003, the post-diapause adults emerging from the soil were moved in pairs into transparent plastic containers (7.5 cm wide at base and 15.0 cm deep) to lay eggs.
To determine whether the effects of host plant on diapause induction are related to temperature, the beetles were reared at constant temperatures of 20, 22, 25 and 28℃ under continuous darkness. In these cases, the influence of illumination was eliminated. To determine whether the effects of the host plant on diapause induction are related to photoperiod, the beetles were reared at constant photoperiods of 12: 12 (L: D) h, 13: 11 (L: D) h, 14: 10 (L: D) h and 16: 8 (L: D) h combined with a constant temperature of 25℃.
Insect rearing and diapause determination
Four cruciferous vegetables, Chinese cabbage Suzhouqin
Just after hatching, the larvae were transferred to transInsect Science 13, 189-193
Effect of host plants on diapause induction 191
parent plastic containers containing a layer of soil and different host plants and then were placed in different conditions until diapause was determined. Fresh leaves were provided daily. Each experimental treatment was tested with three replications, with at least 50 newly hatched larvae for each replication. All experiments were conducted in illuminating incubators (LRH-250-GS, Guangzhou, Medical Instrument Manufacturer of Guangzhou), equipped with four fluorescent 30 w tubes controlled by an automatic timer. The light intensity was about 700 lux, variation of temperatures was ± 1℃, and relative humidity was about 70%±10%. The criterion for diapause in the cabbage beetle is very simple because the diapausing adults have a digging behavior and burrow into the soil for dormancy after several days of feeding (Xue et al., 2002b).
lower than those feeding on the mature leaves of Chinese cabbage Suzhouqin, radish and stem mustard. Effects of food quality on diapause induction The incidences of diapause differed significantly when the beetles were fed with aged, or mature leaves compared to young leaves of radishes at 25℃, 13: 11 (L: D) h (Fig. 1). The incidence of diapause was highest when fed with aged leaves, followed by mature and young leaves (F = 65.156; df = 2,8; P < 0.001).
Statistical analysis Using SPSS (SPSS Inc., Chicago, Illinous, USA), all data were analyzed by one-way analysis of variance (ANOVA), and means were compared using Tukey’ s test at P = 0.05. The data were arcsine square root transformed prior to analysis.
Results Effects of different host plants on diapause induction Diapause incidences differed in both the offspring of post-diapause adults (F = 17.948; df = 3,11; P = 0.001) and the offspring of non-diapause adults (F = 20.161; df = 3,11; P < 0.001) when they fed on four cruciferous host plants at 25℃ and 13: 11 (L: D) h (Table 1). The incidence of diapause in both the offspring of post-diapause adults and the offspring of non-diapause adults feeding on the mature leaves of Chinese cabbage Shanghaiqin were significantly
Fig. 1 Incidence of diapause when the beetles Colaphellus bowringi fed on radish leaves with different maturity at 25℃, 13: 11 (L: D) h. Three replications for each treatment. Error bars represent SE and different letters are significantly different by Tukey’s test at P < 0.05. n =121, 129 and 111, respectively.
Effects of host plant on diapause induction at different constant temperatures The effects of host plants on diapause induction were highly correlated with the rearing temperature under continuous darkness (Fig. 2). All beetles entered diapause at 20℃ regardless of host plants. The effects of host plants on
Table 1 Incidence of diapause when the beetles Colaphellus bowringi fed on four cruciferous host plants at 25℃, 13: 11 (L: D) h. Diapause incidence (%)†
Offspring of post-diapause adults Radish Stem mustard Chinese cabbage Suzhouqin‡ Chinese cabbage Shanghaiqin‡
71.27 ± 0.36 (108) a 53.25 ± 4.04 (119) a 71.33 ± 4.72 (117) a 27.88 ± 6.71 (128) b
Offspring of non-diapause adults 44.01 ± 1.81 (180) a 33.86 ± 4.85 (179) a 44.03 ± 1.72 (120) a 18.44 ± 0.91 (133) b
Values (mean ± SE) followed by different letters are significantly different by Tukey’s test at P < 0.05. Three replications for each treatment. Sample numbers are given in the parentheses. ‡ Differences between Chinese cabbage Suzhouqin and Chinese cabbage Shanghaiqin are that leaves of the former are black green, but leaves of the latter are yellow green and thin. †
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0.002), 13: 11 (L: D) h (F = 8.282; df = 2,8; P = 0.019) and 14: 10 (L: D) h (F = 8.622; df = 2,8; P = 0.017). The incidence of diapause was lower in stem mustard than radish and Chinese cabbage Suzhouqin.
Fig. 2 The effects of host plants on diapause induction in Colaphellus bowringi at different temperatures under condition of continuous darkness. Three replications for each treatment. Error bars represent SE. Different letters at same temperature are significantly different by Tukey’s test at P < 0.05. n = 106-183 for each treatment.
diapause induction were detectable at temperatures above 20℃ (F = 2.252; df = 2,8; P = 0.186 at 22℃; F = 54.652; df = 2,8; P < 0.001 at 25℃; F = 3.735; df = 2,8; P = 0.088 at 28℃). However, the incidences of diapause were significantly different among different host plants only at 25℃. Effects of host plant on diapause induction at different photoperiods The effects of host plants on diapause induction at 25℃ were also correlated with the rearing photoperiod. All beetles entered diapause at 16: 8 (L: D) h regardless of host plants (Fig. 3). The effects of host plants on diapause induction appeared when the beetles were exposed to photoperiods of 12: 12 (L: D) h (F = 19.357; df = 2,8; P =
Fig. 3 The effects of host plants on diapause induction in Colaphellus bowringi at 25℃ under different photoperiods. Three replications for each treatment. Error bars represent SE. Different letters at same photoperiod are significantly different by Tukey’s test at P < 0.05. n = 95-128 for each treatment.
Our laboratory experiments indicated that the effect of host plants on diapause induction was obvious at 25℃ and 13: 11 (L: D) h in the cabbage beetle, Colaphellus bowringi. Fewer adults entered diapause when fed on Chinese cabbage Shanghaiqin and radish than those fed on stem mustard and Chinese cabbage Suzhouqin (Table 1). Most adults entered diapause on mature and aged leaves compared to young leaves of radishes (Fig. 1). Under continuously dark rearing conditions, the incidence of diapause was different among different host-plants when the temperatures exceeded 20℃ (Fig. 2). There were also significant differences in the incidence of diapause among host plants when the beetles were exposed to photoperiods of 12: 12 (L: D) h, 13: 11 (L: D) h and 14: 10 (L: D) h (Fig. 3). These results demonstrate that there is a direct effect of host plant species on diapause induction. However, all adults entered diapause at 20℃ or at 16: 8 (L: D) h combined with 25℃ regardless of host plants, indicating that the effect of host plants on diapause induction was expressed only within a certain range of photoperiods and temperatures. Our results are consistent with most previous studies (Tauber et al., 1986; Danks, 1987). These experimental results can be used to speculate the incidence of diapause in the field in Jiangxi province. In the spring generation lavae generally hatch from the end of March to early May. Most of them are exposed to low temperatures (≤ 20℃ ) and enter summer diapause without much involvement of host plants (Xue et al., 2002b). The larvae hatching in early May encounter temperatures higher than 20℃; however, they enter summer diapause mainly in response to long daylengths, probably to host plants by degrees. In autumn, the larvae hatching at the end of August are exposed to high temperatures (above 25℃) and gradually reducing daylengths; most of them develop without diapause (Xue et al., 2002b; Wang et al., 2004a). The larvae hatching in mid-September experience temperatures above 20℃ and some of them continue reproducing. In these cases, host plants may influence the incidence of diapause to a certain extent. Individuals hatching after mid-October are exposed to low autumn temperatures (below 20℃) and enter winter diapause without much involvement of host plants (Xue et al., 2002b; Wang et al., 2004b). Therefore, we concluded that host plants mediated on the diapause induction of the cabbage beetle in temperate regions. Insect Science 13, 189-193
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Acknowledgment The authors were grateful to Dr Andrew I. Hsiao for critical reading and helpful comments. Comments by two anonymous reviewers were relevant and helped clarify our effort. The research was supported by a grant from the National Natural Science Foundation of China (30460074).
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Accepted February 21, 2006