Effects Of Juvenile Hormone And Low Temperature On Flight And Reproduction Of The Rice Leaf Roller, Cnaphalocrocis Medinalis

The rice leaf roller, Cnaphalocrocis medinalis (Guenée)(Lepidoptera: Pyralidae) is one of the mostimportant insect pests of rice in China and other Asian countries. It is a seasonal and long-distantmigrant that great yield losses had been caused by the outbreak populations of C. medinalis. The flightand reproductive behavior of the adult C. medinalis were investigated after they were treated by juvenilehormone analogue (JHA) methoprene and low temperatures in the laboratory to (i) illustrate the factorsthat may regulate its migratory flight behavior,(ii) deepen the understanding of migratory flightbehavior, and (iii) provide evidences that may improve the level of prediction and management of C.medinalis. The results were summarized as follows:1. Effects of JHA on the reproduction and flight of adult C. medinalis1) Roles of JHA played in the reproduction of adult C. medinalis were confirmed. It wasshowed that preoviposition period (POP) of the moths treated by juvenile hormone analog (JHA),methoprene at the dosage of15μg/adult was significantly curtailed among the dosage of1.5,15and150μg/adult on day1post-emergence while the POP of the moths treated by the1.5and150.150μg/adult was not significantly differed from that of the control. Second, the POP of the adults treatedby JHA at d1of the adult life was significantly less than that of the control and moths treated at d2, andd3while POP of the adults treated at d2, and d3after emergence, and control was insignificantlydiffered. However, the lifetime fecundity, oviposition period, and longevity of the adults among the4treatments were not significantly different. These results suggested that the roles of JHA played in C.medinalis is speeding the ovarian development and curtailed the POP, the effective dosage of JHA wasaround15μg/adult and effective period for the application of JHAwas at day1post-emergence.2) Effects of JHA application on the flight capacity of the adults were determined. Flightcapacity of the moths at1-4days after treated with JHAat day1after emergence was tested for12h byflight mill. The results obtained showed that the flight duration, speed, and distance of the JHA treatedfemale moths was generally less than that of the control moths although only the flight duration anddistance of3-d-old moths and flight speed of1-d-old moth was significantly less than that of the controlmoths at the same age. Besides, application of JHA also reduced the flight capacity of the male mothsbut only the flight duration of the3-d-old males was significantly less than that of the control moths.These results showed that the effects of JHA application mainly on the flight capacity of the females,although the flight capacity of both male and females could be affected.3) Effects of JHA application on the flight muscle weight and triglyceride (TAG) content wereinvestigated. The dry weight of flight muscle and the content of triglyceride of adult C. medinalis werealso affected by the application of JHA treatments. The weight of flight muscle of females at the2ndand3rdday after the JHA treatment was significantly less than that of the control moth at the same agesalthough the differences at the4thday between the treatments was insignificant. The weight of flightmuscle in the JHA treated moths declined significantly at the3rdday after the treatment while the significant decline of flight muscle weight in the control occurred at the4thday after the treatment.However, effect of application of JHA on the flight muscle weight of males was insignificant during the5-days investigation period. The triglyceride content of the treated adults was significantly greater thanthat of the control at the1stday after the treatment, but was significantly smaller than that of the controlfrom the2ndto4thday after the treatment. However, the triglyceride content of the adults between thetreatments was insignificantly different during the observation period.These first obtained results demonstrated that JH is an important endocrine regulating element forthe reproduction and migration of C. medinalis. The adults will be shifted from a migrant into residentwhen their JH level is increased at the first day of the adult life. Characteristics of this shifting areexhibited as the ovarian development, flight muscle degeneration and triglyceride metabolism werespeeded up. The first day of the adult life is the sensitive stages to the change of JH level because theapplication of JHA after the first day of the adult life has no any significant effects on the ovariandevelopment.2. Effect of low temperatures on the reproduction and flight of C. medinalis1) Roles of low temperature played in the reproduction of adult C. medinalis were confirmed.After moths were treated with cold stress10℃and15℃for24hours on day1post emergence, theresults indicated that cold (15℃) treatment could significantly accelerated female reproduction,shortened the preoviposition period and the period of first oviposition, obviously increased the totalfecundity, mating frequency and mating percentage. There was no significant influence on reproductioncharacteristics after adult C. medinalis were treated by10℃cold stress, but the adult longevity weresignificantly shortened. The results showed that15℃was the effective cold stress to acceleratereproduction.2) It was revealed that day1after emergence was the low temperature-sensitive stage toinfluence reproduction. On day1,2and3after emergence, moths were treated by15℃cold stress,respectively, and the results showed that only on day1post-emergence cold stress could significantlyaccelerated female reproduction, shortened the preoviposition period and the period of first oviposition,obviously increased the total fecundity, mating frequency and mating percentage. On other days,however, there were no significant effect on reproduction except significantly shortened the period offirst oviposition. Therefore we inferred that day1after emergence was the low temperature-sensitivestage to accelerate reproduction.3) Effects of low temperature on the flight capacity of the adults were determined. Aftertreated with cold stress at day1after emergence, flight capacity of adults from1to4days was tested byflight mill. The results showed that the flight duration, flight speed and flight distance of the treatmentfemale moths were generally less than that of the control moths. Especially on days2and3post-treatment, cold-treated females flew significantly less distance than the controls, and from day1to3post-treatment the cold-treated females flew significantly slower speed than the controls. In males,however, cold-treated have no significant influences on their flight capacity. 4) Effects of low temperature on the ovarian development and flight muscle weight wereinvestigated. After females were treated by15℃on day1after emergence, their ovarian grade werenot significantly higher than that of the control on day0to1after treatment. However, on day2to4after the treatment the ovarian grade of cold-treated moths was significantly greater than that of thecontrol moths at the same ages. As a consequence of that, preoviposition period of the treated mothswas significantly thorter than that of the control. Cold treatments accelerated flight muscle weightdecrease of females at day2to3after the treatment. However, the flight muscle weight of males wasnot significantly influenced by the cold stress of15℃. In addition; the ovarian grade was negativelycorrelated with flight muscle weight after moths were treated by cold stress.All of the results suggested that day1after emergence was the sensitive stage for temperature toregulate the reproduction and migration of C. medinalis. During this period, reproduction developmentand flight muscle histolysis of female adults could be accelerated by the cold stress of15℃. Adult C.medinalis can be shifted from migrants into residents when they are encountered the cold temperature of15℃at the first day of the adult life, which may increase its life history plastic or adaptivesignificances to the changing environment.