[Effect of multi-generation culture on virulence of Lecanicilliurn lecanii in different media]

The strain No. V3.4504 of Lecanicilliurn lecanii (Zimmermann), an entomopathogenic fungus, was studied on the effect of successive multi-generation culture in seven different media on its colony growth characteristics, extracellular enzyme activities and the virulence against scale insects.

The strain No. V3.4504 of L. lecanii was original isolated from a natural infected scale insect. The two species of scale insects used were Rhodococcus sariuoni Borchsenius and Ceroplastes japonicus Green. Seven media were used and fungus colony characteristics, growth rate and sporulation, extracellular protease and chitinase activity, and infective effect against the two species of scale insects were conducted.

The fungus cultured on PDA medium for successive nine generations showed the most fast in colony growth, the minimal in sporulation, straight decline of extracellular protease and chitinase activity with generation increasing, and the minimal mortality of the scale insects. There was no significant effect to promote virulence of the fungus by increasing peptone into medium. On the media D, E and F, that with the body materials of the two scale insects, although the fungus appeared lower in the colony growth rate, its sporulation was higher upward 8.83 x 10(6) - 9.13 x 10(6) spores/cm2, extracellular protease and chitinase activities averagely reached 2.16 - 2.13 U/g and 1.01 - 1.03 U/g respectively, and the mortalities of the two scale insects were 55% - 58% and 39% - 42% respectively. Cultured three generations in vitro of the two scale insects, the fungus exhibited the highest activities in its protease and chitinase that were 3.08 - 2.92 U/g and 1.45 - 1.42 U/g respectively and the best infection effect against the two scale insects with mortalities of 71.30% and 58.89% respectively. A linear correlation was found between extracellular protease and chitinase activities of the fungus and the mortalities of the scale insects.

Cultured on PDA medium successive multiple generations made retrogradation of the strain No. V3.4504 of of L. lecanii. It was significant effect on keeping the vigor and higher virulence of the fungus adding the body materials of the scale insects into the medium. The vitro by using live scale insects as medium materials was the best way for the rejuvenation of the entomopathogenic fungus and promoting its virulence.