| Diapause is a state of dormancy widely observed in insects where metabolic processes are substantially reduced permitting individuals to persist in a quiescent state to temporally avoid unfavorable conditions for development and reproduction. However, diapause may incur a cost in the form of reduced metabolic energy reserves, reduced potential fecundity, and missed reproductive opportunities. Cotesia plutellae is one of important larval endoparasitoids of the destructive crucifer vegetable pest, Plutellae xylostella, and enters larval diapause as a mature third larval instar (prepupa) in a cocoon outside its host body. Studies were designed to elucidate the impact of environmental factors (temperature and photoperiod) on two different geographic populations of this braconid wasp. One population came from Changchun (43°52' N), Jilin Province, north China, and the other came from Hangzhou (30°14' N), Zhejiang Province, south China. The major results are summarized as follows.The results obtained from the experiment, which is carried out at the different combinations of two temperatures (13℃and 15℃) with 7 photoperiods, showed that both population could be induced to enter diapause, and northern population had higher diapause incidence than southern population at the same photo-thermal combination conditions; both photoperiod and temperature had significant impact upon the incidence of diapause in this parasitoid, and there existed significant interaction between photoperiod and temperature; diapause incidence in northern population decreased as the photoperiod prolonged from 14 and 10 hour light under the temperature of 13℃and 15℃, respectively, and decreased as the temperature increased; however, the diapause incidence in southern population under 15℃did not match the phenomena, and it increased, then decreased with increase of photoperiod, with a peak at 8 hour light of photoperiod; the critical photoperiods (under which 50% individuals enter diapause) were estimated for the northern population to be 15.5 h and 11.8 h light a day under 13℃and 15℃, respectively, and for the southern population to be 10.5 h light a day under 13℃; the critical photoperiod for the northern population under 15℃could not be estimated because the diapause incidence at the temperature never reached 50% at all tested combinations.The diapause sensitive stage was evaluated using only northern population by exposing different developmental stage of the parasitoid to diapause inducing conditions (13℃LD 8:16 h) or to diapause averting conditions (25℃LD 8:16 h). The result suggested that the second and third larval instars together are the sensitive stage to diapause inducing stimuli.Effects of maternal experienced photoperiod on the diapause trait of Cotesia plutellae in northern population were examined in the laboratory by exposing mother generation (GO) to 5 day-lengths (8, 10, 12, 14 and 16 h light a day) at 25℃, and exposing their offspring (G1) to 7 day-lengths (8 to 14 h light a day) at 15℃. The results showed that long day length in GO reduced the diapause incidence and shorten the critical photoperiod for diapause-induction in G1; the diapause incidence decreased with increasing photoperiod in G1, and it never reached 50% irrespective of the photoperiod experienced by G1 when GO were reared out at photoperiod of 16 h light a day.The cross experiment between female and male with different photoperiod experience revealed that diapause incidence was entirely under mother control and the father had no role in his progeny fate (either undergoing continuous development or enter diapause). When the progeny was reared under 11 h light a day at 15℃, almost all the larvae (>90%) produced by mothers with photoperiod experience of 8 h light a day entered diapause but only about 20% larvae produced by mothers with photoperiod experience of 8 h light a day entered diapause regardless of the experienced photoperiod by the fathers.Experiments by exposing the egg-larval, cocoon and adult stages in GO to different photoperiod combinations of LD 12: 12 and 16:8 h at 25℃indicated that the diapause incidence in the progeny was predominantly determined by the conditions experienced by the mothers in the adult stage although each developmental stage itself in GO might be sensitive in part and two consecutive developmental life stages (egg-larval and cocoon stage or cocoon and adult stages) could enhance diapause induction.The influence of cold storage on diapausing and non-diapausing larvae of Cotesia plutellae was studied for both populations by keeping them at 4℃in dark for different durations (from 40 to 320 days) and then exposing them to LD 16:8 h at 25℃temperature. The results indicated that long-term cold storage could reduce the survival of C. plutellae in both northern and southern populations, but diapausing larvae could be stored much longer than hon-diapausing ones and northern population could be stored much longer than southern population. For the non-diapausing larvae, the adult emergence after 20-day cold storage was reduced by 50% compared with that in the control. Contrarily, for the diapausing larvae, the survival percentage was not reduced until 160 days for the northern population and 80 days for the southern population of cold storage, respectively, but adult emergence at 25℃was reduced greatly after stored at 4℃for 80 days in both populations. Moreover, cold storage of diapausing larvae had no adverse effects on female percentage and longevity of resulted adults. Also it did not reduce adult fecundity after storage at 4℃for 120 days in northern population and 40 days in southern population, respectively. In northern population, eighty-day cold storage at 4℃shortened the duration for the adult emergence and synchronized its emergence. However, cold storage for more than 40 days increased male and female sterile percentages in both populations. Physiological parameters such as dry body weight and the contents of water, fat, total sugar, trehalose and glycogen during cold storage period (0 days up to 160 days sampled at 40 days interval) in diapausing larvae of C. plutellae for both populations were measured. Diapausing larvae of the northern population maintained statistically consistent dry body weight, water and fat contents during the whole period of storage whereas those of the southern population tended to reduce their dry body weight, fat and water contents with the extension of cold storage period. The diapausing larvae of the northern population (in contrast to those of southern population) had higher rate of trehalose accumulation and a significant slower rate of total sugar and glycogen depletion during storage at 4℃. All the measured physiological parameters suggested that the diapausing larvae of the northern population (in contrast to southern population) had a higher capacity for survival at subzero temperatures.Chilling diapause larvae under 4℃for 80 days could terminate diapause in both populations, exposing diapause larvae from northern population to the vapor of n-hexane for 30 or 60 minutes could also terminate diapause although 1 minute exposure to the vapor of n-hexane failed to terminate the diapause. Exposing northern population diapausing larvae to high temperature (33℃) could reduce the duration required for the development to adult emergence after the larvae had stored under 4℃for 40 days. However, exposing to the vapor of n-hexane and to the high temperature will reduce the adult emergence rate although the exposures synchronized the adult emergence.Diapause and its related traits are of very importance in the implication of this parasitoid for either biological or IPM programs. Thus, C. plutellae originated from the south can be transferred northward into a moderate climate for use in biological control programs and escapes, the risk of establishment will be low, because of the low incidence of diapause and very short critical day-length for diapause induction. The opposite transfer of northern population southward does not seem to be effective, because these parasitoids will enter diapause too early in the season and may suffer high mortality before winter. However, northern population can be helpful in solving mass rearing problem of this parasitoid due to its higher ability to stand cold storage regime more than the southern population.
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