Evaluation of potency of native Bacillus thuringiensis against maize insect pests and aflatoxin producing fungi in Kenya

Abstract

Bacillus  thuringiensis  is  the most  used  biological  control  agent  to  date. Among major  constraints  to maize  production,  safety  and
hence  food  sufficiency  in Kenya  is  infestation, damage  and  contamination by  insect pests. Maize grains  are  adversely damaged by
Prostephanus  truncatus which occasionally paves way  for  the growth of  aflatoxin producing  fungi. The  focus of  this  study was to establish the toxicity of native Bt against adult P. truncatus, second instar larvae of Chilo partellus, Aspergillus flavus and Aspergillus parasiticus. Seven Bt isolates (i.e. KG 411, KG 12-0, KG 20, KG 420, KM 31, KM 12 and KM 24) caused over 50% mean mortality of P. truncatus at the first preliminary dose of 10mg/ml. With subsequent analysis of  the efficacy of Bt against P. truncatus, isolate KG 411 was significantly more  toxic  to  it at 95% confidence  limit  (p <0.001)  than all  the other Bt  isolates. Bt  isolate KG 411 had LD50 of 0.30mg/ml which caused 77.1% mean mortality of adult P. truncatus. Potency tests of the Bt isolates against the second instar larvae of C. partellus showed significant differences at 95% confidence limit (p<0.001), with isolate KM 12 causing the highest mean mortality of 76%. Evaluation of effects of spores and crystals produced by the Bt isolates on A. flavus and A. parasiticus showed that isolate KM 31 caused  the highest inhibition of fungal growth. Only  isolate KM 31 was potent against both P. truncatus and the two fungal  species. However  isolate KG  411 which was  highly  toxic  against  P.  truncatus  had  no  significant  growth  inhibition  effect against  the  two  fungal  strains. This  result  demonstrates  that  native Bt  could  constitute  an  alternative  biological  control  option  for management of adult P. truncatus, A. parasiticus and A. flavus in maize protection.

Key Words: Bacillus  thuringiensis;  Prostephanus  truncatus;  Chilo  partellus;  Aspergillus  spp.,  aflatoxins