Control Efficiency And Its Influence Factors Of Entomopathogenic Fungi Against Stephanitis Nashi

Stephanitis nashi is one of the most destructive sap-sucking insect pests on various fruit and garden plants,especially in the regions of the Yangtze valley and Northern China.The pest with many generations one year easily produces resistance to chemical pesticides.Biological control might be helpful in reducing the selection pressure for resistance to chemical insecticides and posing a sustainable control on the pest S.nashi.The potential of entomopathogenic fungi as a biological control agent against the pest was evaluated in the laboratory assays.Fifteen strains of entomopathogenic fungi with good sporulation ability were selected to test their pathogenicity both on S.nashi adults and the 2nd instar nymphs.The cumulative mortalities of 6 Beauveria strains and 1 Materhazium strain reached 100% both on S.nashi adults and the 2nd instar nymphs at an inoculation concentration of 1×106conidia·ml-1.The LT50(half lethal time)of the strains of Bb2352、Bb2359、Bb2372against the adult were less than 4 days,while the LT50 of the strains of Bb2352、Bb2359、Ma67 against the 2nd instar nymphs were less than 5 days.The LT50 difference between the adults and the nymphs showed the adults were knocked down more quickly.In the time-dose-mortality model,the cumulative mortality of the lace bug rose up sharply from the 3rd to the 7th day after the fungal innoculation,while gently changed before and after that period.The mortalities rose up along with the increase of inoculation dosage,which presenting an obvious dose-response.The strain Bb2359 showed the highest virulence against both S.nash adults and its nymphs with a LC50 of 6.466×104 conidia·ml-1,and of 4.747×104 conidia·ml-1,respectively.The LT50 for the adult lace bug and its nymph was 2.89 days and 3.54 days,respectively,at an inoculated concentration of 1×107 conidia·ml-1.Conidia suspensions of the strain Bb2359 at the 3 different concentrations of 1 x108 spores·ml-1,1 x 107 spores·ml-1,1 x 106 spores·ml-1 were used by ultra low volume spraying technique in field trials to control the adult and nymph of S.nash on pyracantha,with a control treatment of conidia-free spray.Mortality of each treatment checked everyday,and dead lace bug were collected for indentification of fungal infection.At the 10 th day after field application of conidia suspension spray,corrected mortality of the adult and nymph of S.nash was 76.25% and 91.25%,respectively,meanwhile,infection ratio of the adult and the nymph was 75.00% and 86.25%,respectively,at the application concentration of 1 x 108 spores / mL.when theapplication concentration decreased to 1 x 107 spores / mL,no significant diffence on mortality and infecton fatio appeared between the concentration and the higher concentration of 1 x 108 spores / mL.the corrected mortalities of the adult and nymph of S.nashi were 80.00% and 88.75%,and the infection ratio was 75.00% and 86.25%,respectively.At each same application concentration,significant higher mortality and infection ratio appeared in the field control of nymph of S.nashi,which revealed the optimal control timing was to control the nymph of S.nashi of the first generation in the spring season by entomophathogenic fungi.Both bioassay in labarotory and field trial showed the entomophathogenic fungus had great potential to control the hazardous lace bug,S.nash as an efficicent biological control agent.