Research On Virulence And Control Effect Of Beauveria Bassiana Against Onion Maggot(Diptera: Anthomyiidae)

The onion maggot,Delia antiqua?Meigen??Diptera:Anthomyiidae?,is a devastating pest of spring-grown onions in northern temperate zones,and Beauveria bassiana is an important biological agent for control of onion maggot.High-efficient B.bassiana isolates were selected for control of the onion maggot on the basis of their virulence under laboratory conditions.We definituded the fungal infection process and what parts of the adult body were most susceptible to conidia attachment and penetration used SEM and fluorescence microscopy observations.We assess the changes of important physiological indicatorsproteins of onion maggot and protein,revertose and free amino acid in haemolymph of onion maggot adult infected by B.bassiana at different times,understand the relationship between the contents of important substanceand the infection of B.bassiana.The changes of the activities of some protection enzymes and detoxification enzymes of onion maggot adult infected by B.bassiana at different stages were determined and the changes of hemocyte after infected were recorded by taking a photograph.The sublethal effect of B.bassiana on subsequent generations of onion maggot was analysed using comparative life table method.We improved the method of Improvement of the method of indoor bioanalysis and screening of the strains against larvae.We carried out the indoor cage experiment,looking for the best application of B.bassiana in field conditions.The major results and conclusions are listed as follows:1.Selection of B.bassiana isolates for control of the onion maggotThe susceptibility of adults to 12 isolates,at a concentration of 1×10⁷conidia/mL,was tested in laboratory experiments.Eight of the more potent strains caused in excess of 85%adult mortality 8 d post inoculation,while the median lethal time(LT₅₀)of these strains was<6 d.The virulence of the more effective strains was further tested,and the median lethal concentrations(LC₅₀)were calculated by exposing adults to doses ranging from 10³–10⁷ conidia/mL.The lowest LC₅₀ value,found in the isolate XJWLMQ-32,GZGY-1-3 and SDDZ-9 for the adults were 3.87×10³,6.64×10³,4.95×10³ conidia/mL,respectively.2.Infection observations of B.bassiana infected of the onion maggotWe observed the fungal infection process in onion maggot by scanning electron microscopy.The results revealed that B.bassiana successfully infected onion maggot within 72h.We observed almost no conidia in the vicinity of either the abdomen or the thorax where there was an abundance of microchaetae that were likely to prevent conidia from attaching and,in so doing,prevent infection in these areas.The superficial area and structure of the wings and macrochaetae may facilitate the attachment of conidia and enable effective penetration.3.The changes of physical pathology of onion maggot infected by B.bassianaThe content of water content,body weight and fat content were declined.The content of proteins declined first and then increased the treatment group and control group showed similar changes.The content of free amino acids decreased in 12-36 h,increased in 48-60 h and decreased in 72 h again after inoculation.After infection,there was no change in the type of free amino acids in the haemolymph,free amino acid content of various fluctuations was quite different.The infection of B.bassiana had significant influence on the contents of important amino acids in the haemolymph.the content of revertose were declined.4.The activity of defense enzymes and cellular defense response of adult of onion maggot infected B.bassianaThe activities of SOD,CAT,POD,GSTs and AChE increased firstly,then decreased.The activities of CarE had almost the same changes as those of control.The activities of CarE were lower than the control always.Mass hemocytes were gathered around the mycelium.Nucleation and encapsulation were occurred to impede and inhibit the proliferation of mycelium after infected 36-48 h.After infection54 h,mycelium began to break through the encapsulation block,proliferate in haemolymph,blood cell immune responses were destroyed.The study concluded that the infection of B.bassiana will trigger the immune system of onion maggot adult.Immune system was collapsed due to the large proliferation of B.bassiana,leading the adults to dead.5.Sublethal effects of B.bassiana on life table parameters of onion maggotWe assessed effects of parental exposure to B.bassiana on life history traits of subsequent generations of onion maggot.Progeny from individuals that survived fungal-exposure,had not egg and pupa stages,but shorter larval development times than corresponding untreated controls.Fecundities of the parental types were significantly decline.The generation time reduced,the intrinsic rate of increase?r?,finite rate of increase???,net reproductive rate?R₀?increased and the population more increase slowly.6.Susceptibility of onion maggot larvae to entomopathogenic fungiThe pathogenicity of eight B.bassiana strains fungal pathogens exposed to third-instar onion maggot larvae was evaluated in laboratory bioassays.Results show that all B.bassiana isolates consequently reduced adult eclosion.The B.bassiana isolates NMTL-1,NMTL-27 and XJWLMQ-32produced the eclosion reduction of 73.91%,67.39%and 39.13%,respectively.7.Evaluation of biological control potential of B.bassiana on onion maggot.In the experiment,the wettable powder,the spore suspension and the soil were sprayed with wheat bran powder,and the pathogenicity of adults and larvae were continuously observed.The results showed that the effect of sudden sprinkle powder on the control of larvae and adults was significant.