Study On Characteristics Of Diapause In Laodelphax Striatellus (Fallen)

Diapause is extremely significant for the continuation of insect species.It can enhance the ability of insect to resist cold;drought and heat,helping insects live through the seasons unfavourable for population increase.Diapause makes the insect growth stage and generation time synchronization with the most favorable season,making full use of the food resources.Diapause also brings the uniformity of population growth increasing the rate of copulation.Therefore,more offspring can be produced to keep the population reproduction.Meanwhile,diapause makes the insects propagation at different times,avoiding unpredictable catastrophic climate damage the population.The small brown planthopper Laodelphax striatellus(Fallen)(SBPH)(Hemiptera:Delphacidae)is one of the important pest insects of rice,causing direct damage by sucking nutrients from vascular tissue of rice,and indirect damage by ovipositing in tissue.SBPH also transmits different viruses which results of the spread of disease and loss of the production of rice.3nd,4th instar nymph of SBPH overwinter in plants of grass family like wheat,farm land,wild land,ditch and roadside weeds.However,there has been controversy on diapauses of SBPH.In this study,we explicit the characteristic of diapauses of SBPH on the basis of the combination of the light-temperature simulation in lab and sample collection of field and the change regulation of juvenile hormone and ecdysteroids in nymphs.We also confirm the effect of light on the male wing type SBPH.Our results are listed as follows.1.Effects of temperature and photoperiod on diapauses of SBPHAccording to the effects of photoperiod on development time under 20??27?(higher than developmental threshold temperature of nymph),we reveal that SBPH is a insect of facultative larvae diapauses,belong to short day type among the 4 types of photoperiod response.Photoperiod is an important factor to induce diapauses of SBPH.Temperature regulates the nymph diapauses of SBPH on the basis of photoperiod.Low temperature and short photoperiod can induce the diapauses of SBPH most easily.2.Formulation of Laodelphax striatellus diapause standard:Under low temperature(20±1?)and suitable temperature(27±1?),nymphs development fastest under the long photoperiod 14 and 16 hour.We compare the development periods of nymphs under the two photoperiods.If there is no significant difference,the longest development period will act as the criterion of diapauses.On the contrary,the longest development period under photoperiod 16h will be used as the criterion of diapauses.3.Sensitive state of SBPH on photoperiodDifferent instar nymph were interconversed under the diapauses-inducing photoperiod(LD 10:14)and non-diapause inducing photoperiod(LD 16:8).We compared the rate of diapauses and development time under different treatments.The results indicate that the sensitive state of diapauses is 2nd instar nymph.In addition,we found that all instars nymphs are sensitive to photoperiod.4.Diapause of SBPH of different geographic populationsBy comparing the diapause characteristics of SBPH from Vietnam Hanoi(21.05°N,105.85°E),Jiangyan(32.51°N,120.14°E,Jiangsu)and Jilin(43.51°N,124.14°E,Changchun)of three different climatic conditions(tropical,subtropical,temperate zone),we found that diapause rate of population SBPH populations in high latitude regions was higher than that in low latitude area.Under different temperatures,the critical photoperiod in high latitude area changes quicker than in low latitude areas.According to the critical photoperiod and light sensitivity of nymph,we infer that SBPH in Jiangyan diapause in mid-late of September.And they begin to develop at mid-late of March in the next year.Diapause of SBPH in Changchun begins in early September.5.Over wintering properties of SBPHTaking SBPH of Jiangyan population as an example,the results related to over wintering properties indicate that on September 13th and September 16th,there were no diapause individualsin Jiangyan population.On September 28th more than 60%individuals began over wintering.And the rate of over wintering reached to 100%on November 15th.Over wintering state of SBPH is mainly at 4th and 5th instar nymphs.Very few individuals over winter at 2nd or 3th instar nymph.Based on the analysis of nymphal instars of the acquisition,on September 16th and September 28th the individuals were from different generations,and the younger nymphs were induced to enter diapause by photoperiod in the field.However,the older nymphs develop into adults for missing the light-sensitive period.We also carried out the over wintering release experiment of SBPH nymphs.Through the analysis of the light cycle changes in Jiangyan population,we found that on December 20 daylight time reduced to the shortest(10h)after prolonged;and from September 28th to December 10th in light culture box(27?,L:D 16:8)development period gradually extended.From December 26th it was gradually shortened.To sum up,change the nature of light cycle plays an important role on the development of SBPH nymph and over wintered growth.By comparing the development process at the three different temperatures(20?,27?,30?),we found that the lifting of SBPH over winter does not need low temperature,and there are high sensitive to temperature and photoperiod during the over wintering period.The eclosion time of the SBPH over wintering at 4th or 5th instar nymph is unified,which makes uniform development of the population of.Thus,we think that although lift over winter does not need low temperature,the low temperature further unified eclosion time of SBPH nymphs.6.Relative expression of hormone synthetase gene in diapause SBPHWe reveal that the diapauses of SBPH is related to juvenile hormone and ecdysing hormone by measuring expression of genes related to synthesis of juvenile JHAMT,CYP15A1,Cyp306A1,Cyp307A1 and Cyp314A1 using quantitative PCR.The results indicate that juvenile hormone in diapauses planthoppers is higher than the non diapauses planthoppers.However,ecdysing hormone is much lower in diapauses planthoppers is higher than then on diapauses ones.During the call-back of JH and ecdysing hormone,diapause larvae began molt successively.We conclude that both the juvenile hormone and ecdysing hormone control the diapauses of SBPH.Artificial feed liquid containing LsJHAMT and dsRNA,significantly reduced the expression of target gene mRNA,reducing the relative expression amount to about 30%.There is a slight short development period,however,it doesn't reduce to a level of nymph molting.As a result,there is no significant difference compared with the control.7.Effects of photoperiod on wing type SBPHAccording to analyze the wing types of females and males in the three populations(Hanoi,Vietnam;Jiangyan,Jiangsu;Changchun,Jilin),we found that wing type of SBPH and photoperiod are closely related.Short photoperiod induced the emergence of short wing males.And the percentage of short wing males changes consistently with diapause rate,reaching the summit at short photoperiod LD10:14.At long photoperiod,there is no short wing male.In the cycle of light conversion experiments,the percentage of short wing males was impacted by the photoperiod.Meanwhile,it is affected by the natural photoperiod and over wintering time in the field.Photoperiod has no obvious effects on the wing type of females.According to the above results and the relative expression of hormone synthetase gene in diapause SBPH,we infer that the genes regulating wing type are different in females and males.The increased amount of juvenile hormone expression is an important reason for ratio increases of diapause short wings of male SBPH.