Natural occurrence of Bacillus thuringiensis on cabbage foliage and in insects associated with cabbage crops.

Bacillus thuringiensis was isolated from the phylloplane of organically grown cabbage in one field during two growth seasons (1992-93). The frequency of B. thuringiensis varied between 0.02 and 0.67 of the total B. cereus/B. thuringiensis population, with an average of 0.11. Characterization of the B. thuringiensis isolates from foliage showed that the majority (64% of 150 isolates) belonged to serovar kurstaki, had bipyramidal crystals and toxicity towards Pieris brassicae and/or Trichoplusia ni. Other serovars were also found on the foliage but occurred at very low frequencies (one to three isolates of each serovar). Bacillus thuringiensis was also isolated from insects associated with the cabbage crop (Pieris rapae (Lep.), Delia radicum (Dip.), Syrphidae ribesii (Dip.) and Aleochara bilineata (Col.)), which were collected alive at different developmental stages in the same field. Serologically these isolates were assigned to the serovars kurstaki, aizawai, tochigiensis, colmeri and indiana/colmeri.