Effects Of Low Temperature Stress On Bee Larval Development And SBV Infection

Honey bee sacbrood disease is caused by an enterovirus genus sacbrood virus(SBV).The disease is not harmful to western honeybee but fatal to eastern honeybee.The disease mainly breaks in weak honeybee colony in early spring with an unstable climate situation when the colony is in propagation.The sacbrood virus mainly infect the 2nd instar bee larvae,the infected larvae is not dead immediately,but die in 7th instar when the larvae is in process of metamorphosis.The SBV can be transmitted among different colony or apiary,leading to a large number of honeybee and colony death.Worse more,there is no safe measure to effectively control the disease that the disease happened intermittently.The present control method is to breed a strong bee colony and take measure to keeping the colony warmFor exploring the effect of low temperature on honeybee metamorphosis and its relationship with sacbrood disease,the lasting time of honeybee metamorphosis was determined by taking photo of the whole metamorphosis process under different temperature of 15?,20?,25?,28?,33? and 35?.The results showed that the larvae could not turn into pupae under 15?,20? and 25?,the time they lived were 68.62±1h,126.95±13.94 h,and 127.69±8.31 h respectively.About half of the larvae could finished their pupation under 28?,and the metamorphosis process last 144±15.32 h,all the larvae reared under 33? and 35? could turned into pupae,and the metamorphosis process last 11 1.42±1 h and 95.95±1.96 h respectively.The effect of temperature on honeybee pupation is clearly.Simultaneously,the 2nd and 7th instar honeybee larvae with SBV inoculated were subjected to 20?treatment for 6 h,and their death rate were determined.The result showed that the death rate of 2nd larvae was 64%,the death rate of 7th instar larvae was 76%,both of them were significantly high than the death rate of 45%from the control groups.When comparing the death rate of 2nd larvae and the 7th instar larvae,the death rate of 7th instar larvae was significantly high than the 2nd instar larvae.Such a result illuminated that the low temperature could increase the infecting activity of SBV and the death rate of bee larvae.The 7th instar larvae were more sensitive to low temperature than the 2nd instar larvae.For determining the infecting route of SBV in bee larvae,SBV were detected from the midgut and Haemolymphe of bee larvae with different development stage.The result showed that the virus could be detected both in the samples of larvae midgut and Haemolymphe in 24 h after feeding the food containing SBV,and from then on,it could be continuously detected from both the samples of midgut and Haemolymphe till 72 h.however,it could be detected only from part of the larvae midgut and Haemolymphe samples between 72?120 h.this may due to the strong resistance of the western honeybee to SBV.To illuminate the molecular mechanism of low temperature promoting the SBV growth in honeybee larvae,the gene expression profile of 2nd instar and 7th instar larvae suffering and not suffering low temperature treatment for 6 h were determined by De novo sequnecing.Total 5876940 contigs,and 45613 unigene were obtained.Among them,28083 were successfully annotate to the nr,KEGG,COG,GO library.Many of them were related to growth,immunization,and disease resistance.Low temperature delays the bee larvae development,and decreased the capability of immunization,and disease resistance,which leading to the outbreak of SBV growth in honeybee larvae.Summary,low temperature postponed the growth and development of honeybee larvae,which proving more time and opportunity for the SBV to infect it honeybee host,which leading more larvae to death.Gene expression profile certificated that many gene related to the growth,immunization,and disease resistance were significantly different expressed between the normal reared larvae and the low temperature treated larvae.Such results explained from the molecular level why the low temperature treated larvae were more sensitive to SBV,thus was theoretic important.And the same time,it could also provide a basement for practical SBV disease control.