Biological Characteristics Of Phenacoccus Solani Ferris And Roles Of Cytochrome P450 4G Gene In Cuticle Lipid Biosynthesis

Phenacoccus solani Ferris（Hemiptera:Pseudococcidae）is a seriously invasive insect pest on succulent plants and has spread all over the world.In recent years,P.solani has also been discovered and recorded in China.However,due to the domestic research deficiency on P.solani,we lack the knowledge on P.solani and effective control measures.In this study,seven representative succulents from different genera,including Aizoaceae,Crassulaceae,Liliaceae,and Portulacaceae,were chosen as host plants to cultivate P.solani in an artificial plant growth chamber（25±1°C,RH 70%±5%,14L:10D）.We observed and recorded its biological characteristics,built the experimental population life table,and analyzed the damage of P.solani to succulents.As we all know,the control difficulty for P.solani is due to a thick waxy layer covering the insect body,which not only increases water retention but also reduces insecticide penetration.It was hypothesized that CYP4G gene may be related to the cuticle lipid biosynthesis.In this study,therefore,we sequenced the complete open reading frame of a CYP4G gene,determined the expression profiles of the gene at different developmental stages,and applied RNA interference to study potential roles of the gene in cuticle lipid biosynthesis in P.solani.The host selection experiments showed the following results:a)P.solani could complete the whole life cycle on the first five out of seven succulents tested in this study,including Graptopetalum paraguayuense,Sempervivum tecrum,Aeonium arboreum,Cotyledon tomentosa,Lithops pseudotruncatella,Anacampseros telephiastrum and Haworthia cooperi var.truncata.The mortality rates of the first-and second-instar nymphs were relatively higher,but they can develop into adults and enter the reproduction phase after the third instar.b)G.paraguayuense and A.arboreum were the most suitable hosts for P.solani growth,development,and reproduction as the insects showed the lowest mortality rate,and the highest egg production and F₁egg hatching rates.S.tecrum was a reasonably suitable host but C.tomentosa and L.pseudotruncatella were the worst for P.solani growth and development.The experimental population life table further showed that P.solani had the highest net reproductive rate,intrinsic rate,and the finite rate of increase on G.paraguayuense and A.arboretum.Life table data on C.tomentosa and L.pseudotruncatella is the opposite.d).Field survey revealed that P.solani caused different levels of damage to 87 succulent species in 24 genus of 5 families（Crassulaceae,Aizoaceae,Cactaceae,Liliaceae and Portulacaceae）.These experimental results are consistent with the field survey data from the succulent plant production facilities in Zhejiang Province.Furthermore,we successfully sequenced the complete open reading frame of a CYP4G gene（Ps CYP4G）from P.solani.The expression of Ps CYP4G in P.solani nymphal and adult egg-laying stages increased as their age increased,but decreased rapidly during the molting process.The best results of RNA interference were obtained when insects were treated with ds RNA in early nymphal stages.Both feeding and injection ds RNA delivery methods could suppress the expression of Ps CYP4G;however,ds RNA feeding in third-instars provided a better suppression of Ps CYP4G than injection method.Further experiments under two different humidity environments（30%RH and 80%RH）were carried out on different stages of P.solani after the insects were fed with ds RNA.Results indicated that survival time of P.solani fed with ds RNA was greatly affected by humidity.Specifically,lower humidity decreased the average survival time.In addition,insect mortality increased rapidly after molting and exceeded 80%3-4 days after molting.P.solani has a wide host range but this has not been previously established in China.In this study,we confirmed that P.solani can invade many succulent plants.In view of the importance and large market demand of succulents,we must pay a particular attention to any potential risk in the trade of succulents.The first and second instars of P.solani are critical stages for chemical insecticide-based management as they are relatively more susceptible to insecticide.Finally,it appears that Ps CYP4G plays an important role in cuticle lipid biosynthesis,which provides proof-of-cencept evidence of developing new management strategies by targeting this gene for controlling mealybug pests.