Role of Photoperiod and Temperature in Diapause Induction of Endoparasitoid Wasp Microplitis mediator (Hymenoptera: Braconidae)

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1 PHYSIOLOGY, BIOCHEMISTRY, AND TOXICOLOGY Role of Photoperiod and Temperature in Diapause Induction of Endoparasitoid Wasp Microplitis mediator (Hymenoptera: Braconidae) WENXIANG LI, 1,2 JIANCHENG LI, 2 THOMAS A. COUDRON, 3 ZIYUN LU, 2 WENLIANG PAN, 2 XIAOXIA LIU, 1 AND QINGWEN ZHANG 1,4 Ann. Entomol. Soc. Am. 101(3): 613Ð618 (2008) ABSTRACT Diapause in Microplitis mediator Haliday (Hymenoptera: Braconidae) is manifested during the pupal stage and normally occurs during the winter season because of a photoperiodic response that is highly dependent on temperature. No diapause was observed at temperatures above 20 C regardless of the photoperiod. Only a small part of the population entered diapause at 20 C under short daylengths of light:dark 8:16Ð12:12. In contrast, when larvae were exposed to 16, 18, and 20 C combined with a photoperiod of 10:14, the percentages of parasitoids that entered pupal diapause was 97.9, 87.8, and 26.2%, respectively. Critical photoperiods were determined to be 7.03:16.97 and 12.21:11.79 at 16 C and 6.75:17.25 and 12.03:11.97 at 18 C. The second instar of the parasitoid was the most sensitive to diapause induction for the photoperiods and temperatures tested. KEY WORDS Microplitis mediator, diapause induction, photoperiod, temperature Control of development and synchronization are traits that can help in the mass production of insects for research and Þeld release efforts. Seasonal changes in temperature or photoperiod inßuence the ecologically relevant developmental processes in insects, such as continuous development or diapause, by affecting hormonal levels (Denlinger 1985, 2002). The arrestment of development that takes place during diapause serves two main purposes: to overcome a period of adverse conditions and to synchronize activity of individuals within the population (Tauber et al. 1986, Danks 1987). Diapause is a genetically determined pattern of response to environmental stimuli; thus, the expression of diapause is subject to both environmental and genetic factors. Among the environmental stimuli that inßuence diapause, photoperiod, and temperature, or an interaction of the two, are the most common diapause-inducing stimuli (Beck 1980, Saunders 1982, Tauber et al. 1986, Xue et al. 2002). In a few instances, diapause induction has been shown to be mediated solely by temperature (Tauber et al. 1986, Danks 1987). In such cases, temperature not only serves to induce diapause but also has an immediate effect on the rate of growth. Diapause within parasitoids has been shown to have varying degrees of dependence on stimuli from their hosts as 1 Department of Entomology, College of Agronomy and Biotechnology, China Agricultural University, Beijing , P. R. China. 2 Plant Protection Institute, Hebei Academy of Agriculture and Forestry Science, Integrated Pest Management Centre of Hebei Province, Baoding , P. R. China. 3 USDAÐARS, Biological Control of Insects Research Laboratory, Columbia, MO Corresponding author: zhangqinwen@263.com. well as temperature and photoperiod (Beck 1980, Saunders 1982, Tauber et al. 1986). The solitary larval endoparasitoid Microplitis mediator (Haliday) (Hymenoptera: Braconidae), a Palaearctic species widely distributed in Europe and Asia (Arthur and Mason 1986), parasitizes larvae of 40 lepidopteran species (Shenefelt 1973), and it was the dominant parasitoid reported to be associated with Helicoverpa armigera (Hübner) in China. M. mediator made up 58% of all larval parasitoids recovered from H. armigera in Þeld collections in 1982 (Wang et al. 1984), and it parasitized from 22.9 to 40% of the H. armigera in the Þeld. These Þndings indicate that M. mediator has good potential for biological control of H. armigera if mass rearing for augmentative release were possible (Wang et al. 1984, Li et al. 2004, 2006). Field investigations in Baoding, Hebei Province, China, showed that when the daily average temperature was below 17.9 C and the average daylength was shorter than h M. mediator entered diapause (Hun et al. 2005). A naturally occurring pupal diapause in M. mediator may be useful to synchronize large-scale production of the parasitoid if the onset and termination of diapause can be controlled through rearing conditions. Microplitis croceipes (Cresson) (Hymenotpera: Braconidae) is a larval endoparasitoid used as a biological control agent in cotton, Gossypium hirsutum L., in the southern United States. Previous work has shown that there was a signiþcant interaction between photoperiod and temperature in diapause induction for the M. croceipes and that the most sensitive time of development for induction of diapause was associated with the egg and larval stages (Brown and Phillips 1990). Preliminary studies with M. mediator suggested that dia /08/0613Ð0618$04.00/ Entomological Society of America

2 614 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 101, no. 3 pause occurred in Þeld settings in Hebei and results from simulated conditions suggested that a temperature and photoperiod induction of diapause was limited to the larval developmental stage (Hun et al. 2005). The purpose of this study was to evaluate more thoroughly the critical photoperiod and precisely deþne the developmental stage for maximizing diapause in M. mediator. The work reported here clearly documents the interaction of photoperiod and temperature and establishes the critical developmental period for the induction of diapause in the M. mediator and M. separata parasitoidð host complex. Understanding this interaction may improve the rearing capability and allow more accurate timing of parasitoid releases. Materials and Methods Origin and Maintenance of Insect Colonies. Mythimna separata (Walker) (Lepidoptera: Noctuidae) was used as a host for the mass rearing of M. mediator because H. armigera is cannibalistic and consequently more labor-intensive when used as a host. M. separata was obtained from a stock culture maintained by the Insect Natural Enemy Laboratory, Hebei Plant Protection Institute, Baoding, Hebei, China, and it was reared on an artiþcial diet (Bi 1981) composed of a mixture of wheat germ and macerated leaf material from seedling corn, Zea mays L. Moths were held at 26 1 C and 65% RH with a photoperiod of 14:10 (LD) h. The colony of M. mediator was initiated from parasitoids that emerged from H. armigera larvae collected in cotton Þelds in Baoding, Hebei, during the late summer in 1998, and it has been maintained continuously on larvae of M. separata. Parasitization was accomplished by placing to 4-d-old mated females of M. mediator in a cage (30 by 40 by 25 cm) containing 1,000 late Þrst or second instars of M. separata with diet. After 12 h of exposure, host larvae were transferred to individual rearing bags containing artiþcial diet until the parasitoid cocoons formed. For rearing purposes, the M. mediator colony was held at 26 1 C and 65% RH. Adult parasitoids were fed a solution of 10% commercial clover honey on a cotton wick. When reared in this manner, M. mediator achieved 85% parasitism on M. separata and a sex ratio of 1:1.2 (female:male). Voucher specimens of M. mediator were deposited in the entomological collection, Institute of Plant Protection, Hebei Academy of Agricultural and Forestry, Baoding, Hebei, China. Interaction of Temperature and Photoperiod in Diapause Induction. Experiments with temperature and photoperiod for inducing diapause were conducted with second instars of M. separata parasitized by M. mediator. The second instars were parasitized by placing two 2- to 4-d-old M. mediator females in each host-rearing chamber for 24 h. After parasitism, individual host larvae were isolated in 30-ml cups containing diet and placed in a randomized pattern within environmental chambers set at a selected temperature (16, 18, 20, 22, 24, and 26 C with a variation of 1 C at each temperature) and photoperiod (continuous light [LL]; LD 22:2, 20:4, 18:6, 16:8, 14:10, 12:12, 10:14, 8:16, 6:18, 4:20, 2:22; and continuous dark [DD]). The experiments were conducted in incubators (LRH- 250-G, Ningbo Jiangnan Equipment Factory, Ningbo, Zhejiang, China) equipped with eighteen 40-W ßuorescent bulbs controlled by an automatic timer. Light intensity at the level of the larvae was 7,000 lux. The cups were manually covered with an opaque black cloth to simulate scotophase. A minimum of 40 larvae were used for each treatment. M. mediator required 6 d to complete three larval instars at 26 C and LD 14:10 (nondiapause inducing conditions) and 20Ð25 d at 16 C and LD 10:14 (diapause inducing conditions) before cocoon spinning. Under nondiapause inducing conditions, parasitoid egg hatch occurred within 48 h after oviposition within a second instar host, the parasitoid molted from second to third instar as the host molted to the third instar and the parasitoid emerged from the third instar host before spinning a cocoon. Cocoons of diapausing M. mediator were easily identiþed by their brown color in contrast to the green color of the cocoons of normally developing M. mediator (Hun et al. 2005). Critical Photoperiod for Diapause Induction. The critical photoperiod, deþned as the photoperiod that elicits diapause in 50% of the population (Saunders 1981) was determined by examining the incidence of diapause at 16 and 18 C combined with various photoperiods. Diapause response curves were constructed from the results of these experiments. Determination of Diapause-Sensitive Developmental Stage. Two temperature and photoperiod regimes were used from the onset of parasitism to test the sensitivity of different developmental stages of the parasitoid to induction and disruption of diapause (Denlinger 1981, Hua et al. 2005). For this experiment, M. mediator while in situ was exposed to various combinations of diapause and nondiapause inducing temperature and photoperiod regimes. Based on our results of temperature and photoperiod interactions in diapause induction, we made use of the best diapause-inducing condition of 16 C with LD 10:14 as a diapause-inducing regime and the most suitable condition for host larvae to develop with none of the parasitoid entering diapause, which was 26 C with a photoperiod of LD 14:10, as the nondiapause regime. Parasitized larvae were cycled between different combinations of these two regimes throughout the developmental period of the parasitoid, using 2-d increments for the nondiapausing regime and 4-d increments for the diapauseinducing regime to allow for a slower developmental rate. The incidence of diapause was recorded for each combination used. Statistical Analysis. The results were subjected to analysis of variance for the incidence of diapause under the different temperatures and photoperiods compared by DuncanÕs test (P 0.05).

3 May 2008 LI ET AL.: DIAPAUSE INDUCTION OF M. mediator 615 Table 1. Incidence of pupal diapause in M. mediator exposed to continuous temp and photoperiod regimes Photoperiod (L:D) % entering diapause (no. in test/day to pupation) 16 C 18 C 20 C 22 C 24 C 26 C 0:24 0 (27/18) 0 (25/16) 0 (25/12) 0 (27/11) 0 (26/11) 0 (23/9) 2:22 0 (25/15) 0 (26/15) 0 (23/11) 0 (26/10) 0 (26/11) 0 (27/9) 4: (24/15) 2.4 (25/16) 0 (26/10) 0 (26/10) 0 (22/9) 0 (25/6) 6: (24/15) 36.1 (23/14) 0 (27/12) 0 (27/10) 0 (27/10) 0 (29/6) 8: (25/14) 73 (24/14) 12.8 (26/10) 0 (28/10) 0 (25/6) 0 (27/6) 10: (26/16) 87.8 (25/13) 26.2 (28/10) 0 (23/9) 0 (27/6) 0 (27/6) 12: (23/16) 50.8 (25/13) 3.9 (25/12) 0 (28/10) 0 (26/5) 0 (26/6) 14:10 0 (25/18) 0 (25/12) 0 (27/10) 0 (25/10) 0 (27/6) 0 (25/6) 16:8 0 (27/17) 0 (27/12) 0 (24/12) 0 (26/10) 0 (25/7) 0 (25/7) 18:6 0 (26/18) 0 (26/12) 0 (26/12) 0 (27/10) 0 (27/7) 0 (24/7) 20:4 0 (23/17) 0 (26/12) 0 (24/10) 0 (26/10) 0 (25/9) 0 (26/10) 22:2 0 (28/18) 0 (27/12) 0 (25/12) 0 (25/11) 0 (27/9) 0 (23/11) 24:0 0 (25/20) 0 (25/12) 0 (24/11) 0 (23/11) 0 (28/10) 0 (26/11) Photoperiod and temperature regimes that resulted in parasitoid diapause are indicated in bold. Results Interaction of Temperature and Photoperiod in Diapause Induction. The effect of 78 different combinations of constant temperature and photoperiod on the in situ induction of pupal diapause for M. mediator is shown in Table 1. There was an overall trend for the length of time to pupation to increase as the temperature decreased and as the light phase decreased for all temperatures and photoperiods tested, indicating a developmental responsiveness to both stimuli. Diapause was achieved at three different temperatures (e.g., 16, 18,and 20 C) and Þve different photoperiods (e.g., LD 4:20, 6:18, 8:16, 10:14, and LD 12:12). The two highest percentages of diapause were 97.9 and 87.8%, and they were achieved at 16 and 18 C with LD 10:14, respectively. There was no incidence of diapause at temperatures of 22, 24, and 26 C or photoperiods of LL, LD 2:22, 14:10, 16:8, 18:6, 20:4, 22:2, and DD. Greater than 2hto 14 h of light resulted in some diapause or termination of development at all temperatures tested, and development was terminated at all temperatures when exposed to 2 h of light. In contrast, development was observed at all temperatures tested when exposed to 14 h of light. Critical Photoperiod for Diapause Induction. The critical photoperiods at 16 and 18 C were extrapolated from a graphical depiction of the data in Table 1. At 16 C, 50% diapause was achieved at LD 7.03:16.97 and 12.21: At 18 C, 50% diapause was achieved at LD 6.75:17.25 and 12.03:11:97. The incidence of diapause occurred over a slightly narrower range of photoperiod at 16 C than that at 18 C. Determination of Diapause-Sensitive Developmental Stage. The occurrence of diapause in M. mediator after cycling between diapause-inducing and nondiapause-inducing temperature and photoperiod regimes is shown in Fig. 1. Both the duration and timing of the diapause-inducing conditions proved to be critical for the induction of diapause. In general, the longer the exposure to the diapause-inducing conditions the greater the incidence of diapause. However, it is also apparent that there is a critical window early in the developmental stage of M. mediator, at 4Ð8 d after parasitism, during which exposure to the diapausing condition is required to achieve 90% diapause. Induction of diapause was low when the duration of exposure to diapause inducing conditions was 4 d, regardless of when that exposure occurred and increased with additional time exposed to diapausing conditions. When the parasitized host larvae were initially exposed continuously to diapause-inducing conditions for 12, 16, and 20 d before being transferred to nondiapause conditions, the proportion of diapause increased to 50.6, 94.4, and 98.3%, respectively. In contrast, the percentage of diapause gradually declined (e.g., 100, 15.9, and 0%) with an increase in the number of days initially exposed to nondiapausing conditions (e.g., 2, 4, and 6 d, respectively). It is also noteworthy that induction of diapause was high when the time of exposure to diapause-inducing conditions exceeded the time of the exposure to nondiapausing conditions, regardless of whether the nondiapausing conditions occurred as one long interval or several short intervals. Induction of diapause was highest (e.g., 50%) when the timing of the diapause inducing conditions occurred within a window of time that included 4Ð10 d after parasitism. That window of time encompassed the period during which the host molted from the second to third instar and the parasitoid was in the second instar. Therefore, these results suggest the sensitivity to diapause induction for M. mediator occurs in the second instar. Discussion The recognition that there is an interaction between temperature and photoperiod that regulates diapause has been shown in many insect species (Lees and Giese 1968, Messenger 1969, Anderson and Kaya 1974, EskaÞ and Legner 1974, Wallner 1979, Beck 1980). The conþrmation of this interaction as critical to diapause induction in M. mediator was shown in this study. A photoperiod response also was expressed within a deþnitive temperature range of 16Ð20 C for M. mediator. Our results suggest that the diapause

4 616 ANNALS OF THE ENTOMOLOGICAL SOCIETY OF AMERICA Vol. 101, no. 3 Fig. 1. Incidence of pupal diapause in M. mediator exposed to changes in temperature and photoperiod regimes. The diapause-inducing condition used for this experiment was 16 C combined with a photoperiod of LD 10:14. The nondiapauseinducing condition was 26 C combined with a photoperiod of LD 14:10. Treatment initiated at the time of parasitization. 4, 4-d diapause condition; 2, 2-d nondiapause condition; P, pupa. response of M. mediator is determined by photoperiod and mediated by temperature, and it conforms to the deþnition of facultative diapause (Lees 1955), which frequently has been observed in multivoltine species (Beck 1980, Saunders 1982). The critical daylength (i.e., conditions resulting in 50% diapause induction) is important because it helps determine the calendar dates at which natural populations begin to enter diapause. The photoperiodic response curves in M. mediator showed two short day responses of 7.03 and 6.75 h at 16 and 18 C, respectively. Although the Þrst critical daylength of 7.03 h does not occur in the natural conditions in Hebei, it may have a physiological signiþcance. For M. mediator, the long day response at 16 C was LD 12.21:11.79 (measured graphically). When the temperature was increased to 18 C, the long day response shifted to LD 12.03: Equally important is that a critical daylength to stimulate diapause was not attained at 22 C. In most species, a number of photoperiod cycles are thought to be required for the induction of diapause (Saunders 1982, KoštÕál et al. 2000, Wei et al. 2001) and that speciþc developmental stages are more sensitive than other stages to the photoperiod programming effect (Xue et al. 2003, Wu et al. 2006). It is possible that development was too rapid at 20 C with the LD cycles of 6:18, 10:14, 12:12 and with more light than 12:12 for the parasitoid to be exposed to a sufþcient number of photoperiod cycles to induce diapause. Similarly, parasitoid development may have been too delayed at 16 C with less light than LD 4:20 for the parasitoid to be exposed to an appropriate number of photoperiod cycles to induce diapause. Furthermore, in continuous light or darkness there was no induction of diapause for M. mediator. Comparable data for diapause induction have been reported for Plodia interpunctella (Hübner) (Masaki and Kikukawa 1981). However, by comparison, long day responses at LD 11:13 and 18 C for M. croceipes (Brown and Phillips 1990) were slightly lower than for M. mediator. These comparisons suggest that M. croceipes enters diapause at lower temperatures and shorter photoperiods than M. mediator, which may enable M. croceipes to be a better parasitoid in cooler climates. Additionally, diapause was induced in M. croceipes at 18Ð27 C under continuous darkness, whereas diapause was inhibited in continuous light. These attributes also may enable a higher percentage of M. croceipes to survive in environments of more extreme light regimes. The second instar was the developmental stage most responsive to the induction of diapause for M. mediator. Prepupae, formed under nondiapause condi-

5 May 2008 LI ET AL.: DIAPAUSE INDUCTION OF M. mediator 617 tions, were not responsive to either photoperiod or temperature induction of diapause, suggesting the incidence of diapause cannot be induced during, or after the prepupa stage. Similarly, induction of diapause in Colaphellus bowringi (Baly) occurred in the larval stage at 25 C (Xue et al. 2002). Photoperiod and temperature have been shown to affect the milieu of carbohydrate, protein, and hormones in insects (Lee and Denlinger 1997, Christiansen-Weniger and Hardie 1999, TawÞk et al. 2002, Steppuhn and Wäckers 2004, Iwata et al. 2005). Consequently, the responses of M. mediator to photoperiod and temperature changes could be a direct effect of these parameters on the parasitoid or the responses could be the result of a secondary effect manifested via changes in the host ecophysiology. The role of the host ecophysiology in parasitoid diapause induction, especially host hormone effects, is the object of ongoing research. Acknowledgments We thank the staff and students of Natural Enemy Laboratory of Hebei Plant Protection Institute for their contributions to our research. We are grateful to Patrick De Clercq and Jian Song for critical reading of the manuscript and for helpful comments. The work was partially supported by a grant from National 973 Program (2006CB100204), National Key Technology R&D Program (2006BAD08A02), and Natural Science Foundation of Hebei (C ). References Cited Anderson, J. F., and H. K. Kaya Diapause induction by photoperiod and temperature in the elm spanworm egg parasitoid, Ooencyrtus spp. Ann. Entomol. Soc. Am. 67: 845Ð849. Arthur, A. P., and P. G. Mason Life history and immature stages of the parasitoid Microplitis mediator (Hymenoptera: Braconidae), reared on the bertha armyworm Mamestra configurata (Lepidoptera: Noctuidae). Can. Entomol. 118: 487Ð491. Beck, S. D Insect photoperiodism, 2nd ed. Academic, New York. Bi, F. 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