Growth characteristics and pathogenecity of local Beauveria bassiana isolates against the Nyambene tea weevil (Sphrigodes mixtous) in Kenya

Twenty two local isolates of Beauveria bassiana were characterised on their radial growth, sporulation, germination, and assayed for insecticidal activities against adult Nyambene tea weevils. The isolates were coded according to the different places they were isolated from. They include Bb Ke 1-5 recently isolated from Kericho County, Tea Research Foundation of Kenya (TRFK), Timbilil Tea Fields and isolates Bb Ke 6a and Bb Ke 6b isolated in 2006. Isolates Bb Gi7 series were isolated from soils and assorted weevils from Giciaro farm, Meru County, Isolates Bb Ch 8 and Bb Ch c (1-5) isolated from Chepkoilel soils in Uasin Gishu County and Bb Mu 9 series was isolated from Mununga, Kirinyaga County. The growth characteristics were determined by measuring the radial diameter in mm for seven days of the isolates growing on Potato Dextrose Agar (PDA), sporulation was determined after 2 weeks using a Neubauerheamacytometer under a light microscope (40X) and germination of conidia after 16 h of inoculation on freshly prepared PDA plates. A few representatives of the isolates were chosen to determine their efficacy against the Sphrigodes mixtous by dipping in conidia concentration of 10 5 , 10 4 and 10 3 . The radial growth, germination and sporulation varied in the different isolates. High negative correlation was noted between some variables like radial growth and spore count. A positive correlation between the colony forming unit and germinating conidial count was noted. Isolate Bb Gi 7a showed higher insecticidal activity against the tea weevils compared to the other few selected isolates.