| Aphids seriously damage many agricultural and forestry plants,have strong fertility,wide distribution and many generations one year.Inundative chemical control of the pests led to serious chemical resistance and environmental pollution.Entomopathogenic fungi as well as natural Insect enemies are considered as effective alternatives of chemical insecticides in aphids control.Fungal strains Isaria fumisorosea Ifu13a and Beauveria bassiana Bb202 showed high virulence against aphids in previous studies.Through laboratory bioassays,pathogenicity of the two fungal strains against the ladybug Harmonia axyridi and their joint control on the cabbage aphid Brevicoryne brassicae were well studied with control experiments under artificial simulated climate of natural condition,and field trials in different seasons.Compared experiments were carried out to evaluate control efficiency of the cabbage aphid B.brassicae between the single control by the fungus I.fumisorosea or by the ladybug H.axyridi and a joint control of them.Pathogenicity of the two strains I.fumisorosea Ifu13a and B.bassiana Bb202 was tested to different developing stages of H.axyridi,the egg,larva,pupa,and adult.The results indicated that both strains didn't pose any detrimetal effect on larvae and adults of H.axyridis at a temperature of 25±1°C,a relative humidity of 95±5%and a ranged concentrations of 1.0×106,1.0×107and 1.0×108conidia/ml.However,with the increase of the inoculated concentration of the conidia liquid of Bb202 from the lowest to the highest,the corrected egg hatching rate and eclosion rate of H.axyridis decreased by 1.9%,43.4%,62.2%,and 5.3%,17.7%and 25.8%,respectively.The fungal strain Ifu13a showed the same tendency that,from lower to higher inoculated conidia concentration,the corrected egg hatching rate of H.axyridis decreased by 41.2%,58.5%and 70.5%,respectively,whille the corrective eclosion rate decreased by 17.7%,22.1%and 27.9%,respectively.The results confirmed that the Bb202 and Ifu13a fungal strains showing high virulence to aphids,were safe to larvae and adults of H.axyridis,whereas,the fungi showed weak pathogenicity to pupae and considerable infection on eggs,in where high relative humidity and suitable temperature existed.The fungal infection was positively related to the inoculation dosage of entomopathogenic fungi,and showed density-dependant.By artificially simulating climate of natural condition in a temperature,relatively humidity,as well as illuminating system automatically controlled room,the joint control of the cabbage aphid B.brassicae by second instar larvae of the ladybug H.axyridi and the entomopathogenic fungus I.fumosorosea was studied.The experiments showed that one second instar ladybug larva alone could not control the aphid B.brassicae population growth on the cabbages,with a population increase of 4.0%on the 9th day.The control of the aphid B.brassicae by I.fumosorosea alone showed some good results.The corrected control effeciency of the B.brassicae by Ifu13a was 32.0%,58.0%and 82.8%,respectivily,at different conidia suspension of 1.0×106,1.0×107and 1.0×108 conidia/ml.Combined use of fungi and ladybugs under the same conditions could obtain better correction control,respectively 37.3%,100%and 100%.The combined control efficiency of the cabbage aphid B.brassicae was much better than that by the entomopathogenic fugus or the ladybug H.axyridis alone,which suggested the great potential of the joint application of entomopathogenic fungi and natural insect enemies.In the present combination appliction case,the optimal application concentration of the strain Ifu13a was 1.0×107 conidia/ml.Conidia suspension of the two fungi at three concentrations of 1.0×106,1.0×107and1.0×108 conidia/ml was sprayed to contorl the cotton aphid A.gossypii on Pittosporum tobira in the filed.The results showed that when the applied concentration of Bb202 strain increased from 1.0×106 conidia/ml to 1.0×107 and to 1.0×108 conidia/ml,the corrected control effeciency of A.gossypii increased from 19.95%to 39.64%and to 43.75%.Artificially maintaining environmental relative humidity at a high level of 90±5%by spraying distilled water 2 times a day in the initial two days after fungal application against A.gossypii resulted in an increased control efficiency of 64.24%and 75.96%at the conidia concentration of 1.0×107 and 1.0×108 conidia/ml,respectively.Humidity manipulation in the treatment of 1.0×106 conidia/ml didn't show significant change.At the same time,the median lethal times(LT50)was 10.09,5.50 and 4.49 days,respectively for each lethal concentration,and medium lethal concentration(LC50)for A.gossypii was 3.50×107conidia/ml.Compared with B.bassiana strain Bb202,Ifu13a showed better field control effeciency on A.gossypii.When the field application concentration of Ifu13a strain increased from1.0×106 conidia/ml to 1.0×107 and to 1.0×108 conidia/ml,the control effeciency increased from 27.24%to 46.36%and to 62.91%.The median lethal times(LT50)for A.gossypii obtained from a regression-probit value was 10.00,7.25 and 5.61 days,respectively.Medium lethal concentration(LC50)for A.gossypii was 4.86×107 conidia/ml.If environmental relative humidity was artificially maintained at a high level of 90±5%by spraying distilled water in the initial 2 days,the corrected control effeciency would increased to 50.00%,84.63%and 85.45%,respectivily,at the above three application concentrations.The median lethal times(LT50)for A.gossypii were obtained from a regression-probit value was 7.12,4.51 and 4.35 days,respectively,while the medium lethal concentration(LC50)was 9.93x106 conidia/ml.The field trails showed that the B.bassiana Bb202 strain and the I.fumosorosea Ifu13a strain had good field control on A.gossypii population.The control effeciency was closely related to the application dosage of the fungi.Humidity manipulation will significantly improve the control effeciency of entomopathogenic fungi against ahpids in the field control,and definitly develop its potential on the pests control. |
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